Boiler location in a basement closet?

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Nathan Davis

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I live in a 1928 vintage house with hot water radiators. My natural gas-fired boiler is located about 24" away from the poured concrete wall in my basement, and the air intake and controls are all located on boiler surfaces facing away from the concrete wall, easily accessible for maintenance. It is not enclosed, but open to the entire basement. I would like to finish the basement and would like to move the boiler to within 3-4 inches of the concrete wall, keeping the air intake and controls all located the same as presently located, away from the concrete wall and accessible.

I would also like to enclose the boiler in a closet, putting a door immediately in front of the controls to provide maintenance accessibility to the boiler.

1. What is the minimum distance I can move it away from the concrete wall?
2. If I enclose the boiler in a closet with a door, what clearances are required, and what are the material requirements (type x gypsum, fire-proof gypsum, cement board, wood door or fireproof door, etc.)?
3. What are the air intake requirements for combustion? I assume that some rather large air grills will be required if I enclose the boiler in a closet.
 

Jadnashua

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If the boiler is fairly old, rather than removing and reinstalling it, it might be wise to consider replacing it with a right-sized, more efficient unit. Most of those use outside air for combustion and obviously, exhaust outside, often with pvc pipe so you could also get rid of the chimney. Your existing boiler is probably 3-4x larger than required, and a new one would end up being lots more efficient.

Each boiler will have minimum clearance requirements - there is not a general rule. Many of the new ones are smaller and are designed to hang on the wall, and the exhaust and intake being plastic, don't have any real clearance issues, but they do have required slope. There is some clearance required on the sides and in front for maintenance, but it's minimal.

A general rule for fresh air on a burner is one square inch/1Kbtu at both the bottom and top of the door. There is often a minimum volume of open space that it can draw that air from as well, and if you wall things off making maybe smaller rooms, there may not be enough volume. This assumes a certain amount of leaking into the structure from outside, which is not great, which is why the newer units get their combustion air from outside. Why pull in outside air, then throw already conditioned air out the chimney? You've already paid to condition it, plus, it brings in what is usually drier air in the winter, making the house drier and more uncomfortable than needed.
 

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Take heed to jadnashua's suggestion to consider retiring the boiler, especially if it is ridiculously oversized (which is typical, not rare.) What vintage &/or model boiler are we talking about here? If it was first put in service during the Clinton administration or early it's not insane to consider replacing it with something newer, more efficient, and more right sized for the actual loads. A wall hung condensing boiler taking it's combustion air from the outdoors can usually be mounted directly on an insulated wall or concrete wall.

To get a handle on your oversizing factor and what size you would be more appropriate, run fuel-use based load calculations on some wintertime gas bills. Use only winter billing periods, to minimize the error introduced by other gas appliances in the house. In my neighborhood 3x oversizing is the median (as if people were expecting a cold snap that got down to -180F or colder), which already takes a real hit in as-used AFUE (especially in an uninsulated basement), but I've seen oversizing factors twice that bad.

Finishing the basement without insulating the foundation to IRC 2015 levels (R10 continuous insulation, in your US climate zone 4 location) would be a mistake, even in the boiler room. Or maybe that should be "especially in the boiler room", since the standby losses of the boiler are truly lost with only an R1-ish foundation walls to contain that heat from the outdoors, and R2-3-ish partition walls of the closet separating it from the fully conditioned space.

While a 2x4/R13 studwall would meet code from a thermal performance point of view, it's a serious mold risk. A combination of rigid foam between an insulated studwall & foundation, or continuous foam strapped to the foundation with 1x4 furring through screwed with masonry screws onto which the wallboard is mounted works. If using reclaimed roofing polyiso (2" minimum) it's usually cheaper than 2x4/R13 solution. There are multiple threads on this site going into the details & options, but in a place with summers as humid and winters as cool as yours it's important to get it right. I can go into those details here, if you like.
 

Nathan Davis

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Take heed to jadnashua's suggestion to consider retiring the boiler, especially if it is ridiculously oversized (which is typical, not rare.) What vintage &/or model boiler are we talking about here? If it was first put in service during the Clinton administration or early it's not insane to consider replacing it with something newer, more efficient, and more right sized for the actual loads. A wall hung condensing boiler taking it's combustion air from the outdoors can usually be mounted directly on an insulated wall or concrete wall.

To get a handle on your oversizing factor and what size you would be more appropriate, run fuel-use based load calculations on some wintertime gas bills. Use only winter billing periods, to minimize the error introduced by other gas appliances in the house. In my neighborhood 3x oversizing is the median (as if people were expecting a cold snap that got down to -180F or colder), which already takes a real hit in as-used AFUE (especially in an uninsulated basement), but I've seen oversizing factors twice that bad.

Finishing the basement without insulating the foundation to IRC 2015 levels (R10 continuous insulation, in your US climate zone 4 location) would be a mistake, even in the boiler room. Or maybe that should be "especially in the boiler room", since the standby losses of the boiler are truly lost with only an R1-ish foundation walls to contain that heat from the outdoors, and R2-3-ish partition walls of the closet separating it from the fully conditioned space.

While a 2x4/R13 studwall would meet code from a thermal performance point of view, it's a serious mold risk. A combination of rigid foam between an insulated studwall & foundation, or continuous foam strapped to the foundation with 1x4 furring through screwed with masonry screws onto which the wallboard is mounted works. If using reclaimed roofing polyiso (2" minimum) it's usually cheaper than 2x4/R13 solution. There are multiple threads on this site going into the details & options, but in a place with summers as humid and winters as cool as yours it's important to get it right. I can go into those details here, if you like.

I'm unsure if it is worth replacing the boiler yet. I replaced it in 2008 so it still has lots of life remaining. It is an 83% efficiency 200,000 btu unit that heats about 4000 sq ft. I don't find it oversized because it can barely keep up when it gets down to 0 F here in Missouri. It is a single zone unit. I'd prefer to have 4 condensing units, one for each floor and one for the basement, but the replacement cost would be quite high. I think the payback would probably take well over 10 years, although the house would certainly seem more comfortable with a 4 zone system. There is no heating in the basement, so I am wondering if I should add a small condensing unit that serves only the basement.
 

Jadnashua

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It sounds like the first step would be to add some insulation and tighten up the house! That would also help quite a bit with the cooling load, which can be quite long of a season in parts of your state.

Many utilities will do a blower door test for a nominal, and maybe no cost. Some will also subsidize what is needed to tighten up the house as well. Drafts make any house uncomfortable, and that sized boiler should be able to heat that area easily if tight and reasonable insulation.
 

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The boiler is almost certianly oversized for the space heating load by at least a factor of 2. (Run the fuel use numbers to prove/disprove that thesis.) It's probably oversized for the total amount of radiator too, but is saved from short cycling by the thermal mass of the high volume radiators, and the fact that it operates as a single zone. The limitation on barely keeping up is almost certainly the amount of radiator, not the amount of burner.- you could install a 500,000 BTU/hr boiler on the same system and still barely keep up at 0F. If the system gets broken up into four zones it's very likely to exhibit short-cycling on zone calls.

Even though it drops below 0F at times, the 99th percentile temperature bin in Springfield MO is +9F. The load should be calculated at +9F, with no more than a 1.4x oversizing factor on the boiler, for non-moduationg boiler. At even 1.2x oversizing for the load at +9F it doesn't begin to lose ground until it's -7F or so outside. At 1.4x oversizing it's good to -15F. It's only cooler than +9F for 87 hours out of an average year, and never for 87 consecutive hours.

If insulating and air sealing the basement the total load will be going down too, since even an unheated basement that drops to +50F indoors during cold snaps is still losing a LOT of heat to the outdoors 24/7 during the winter, heat that is being supplied by the boiler & distribution plumbing. Sealing & insulating the basement will sometimes have negative load relative to the amount of standby and distribution loss from the boiler & plumbing, unless the plumbing gets insulated too.

The heat load of the basement isn't big enough to warrant it's own boiler, even if you DON'T insulate it. But DO insulate! Spending the money on reducing the heat load of the basemen by more than 3/4 costs less than installing a mod-con boiler. A hydronic baseboard zone running off the water heater (isolated with a plate-type heat exchanger) is one fairly cheap solution. A 3/4 ton mini-split heat pump might be another, depending on how many rooms it's getting divided up into. Run room-by-room heat load calculation if you want to optimize it.

If the windows in this are old-school single panes with clear glass storm windows there may be reasonable options for upgrading the window performance with air sealing and replacing the storm windows with low-E storms, which will bring a single-pane wood sash window up to current code-minimum at a fraction of the cost of a replacement window. That can lower the peak load, making it more likely that the rooms stay at temperature during colder tempratures, and with less of a cascading draft of cold air coming off the window it's more comfortable at any indoor temperature.
 

Nathan Davis

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The boiler is almost certianly oversized for the space heating load by at least a factor of 2. (Run the fuel use numbers to prove/disprove that thesis.) It's probably oversized for the total amount of radiator too, but is saved from short cycling by the thermal mass of the high volume radiators, and the fact that it operates as a single zone. The limitation on barely keeping up is almost certainly the amount of radiator, not the amount of burner.- you could install a 500,000 BTU/hr boiler on the same system and still barely keep up at 0F. If the system gets broken up into four zones it's very likely to exhibit short-cycling on zone calls.

Even though it drops below 0F at times, the 99th percentile temperature bin in Springfield MO is +9F. The load should be calculated at +9F, with no more than a 1.4x oversizing factor on the boiler, for non-moduationg boiler. At even 1.2x oversizing for the load at +9F it doesn't begin to lose ground until it's -7F or so outside. At 1.4x oversizing it's good to -15F. It's only cooler than +9F for 87 hours out of an average year, and never for 87 consecutive hours.

If insulating and air sealing the basement the total load will be going down too, since even an unheated basement that drops to +50F indoors during cold snaps is still losing a LOT of heat to the outdoors 24/7 during the winter, heat that is being supplied by the boiler & distribution plumbing. Sealing & insulating the basement will sometimes have negative load relative to the amount of standby and distribution loss from the boiler & plumbing, unless the plumbing gets insulated too.

The heat load of the basement isn't big enough to warrant it's own boiler, even if you DON'T insulate it. But DO insulate! Spending the money on reducing the heat load of the basemen by more than 3/4 costs less than installing a mod-con boiler. A hydronic baseboard zone running off the water heater (isolated with a plate-type heat exchanger) is one fairly cheap solution. A 3/4 ton mini-split heat pump might be another, depending on how many rooms it's getting divided up into. Run room-by-room heat load calculation if you want to optimize it.

If the windows in this are old-school single panes with clear glass storm windows there may be reasonable options for upgrading the window performance with air sealing and replacing the storm windows with low-E storms, which will bring a single-pane wood sash window up to current code-minimum at a fraction of the cost of a replacement window. That can lower the peak load, making it more likely that the rooms stay at temperature during colder tempratures, and with less of a cascading draft of cold air coming off the window it's more comfortable at any indoor temperature.

These are all great suggestions.

All 44 of my huge double-hung wood windows are original 1928 vintage. A previous owner installed storm windows, and 4 years ago I installed a 70% reflective film on the inside of each storm window. It helped a lot for the summer as well as the winter seasons. I've been contemplating replacing the iron window weights with spring coil retractors, and then filling the weight cavities with foam. But I'm unsure what else to do unless I replace all the windows. The windows are all beautiful wood units, so wood replacement windows would be expensive (44 double hung windows), and the payback cycle may be fairly long.

Probably the biggest loss is through the exterior walls. The exterior walls are real plaster over hollow clay tile with a brick veneer exterior. So there is very little insulation in the exterior walls (probably about an r3 or 4), and it would be very difficult to add insulation to the walls.

I have added 12" insulation to the attic for a total of 15" of insulation. I can still add foam board to the basement walls, and it would be fairly easy to add more hot water radiators. However, 35 windows already have a hot water radiator under them, and there are another 5 radiators elsewhere, and I already replaced the hot water heater with a tankless condensing unit. I'm unsure what else to do unless I replace the single boiler with several condensing units--it's a three story house. Presently I spend about $2500/year for NG heating. So even if I can increase the efficiency another 15% (to save about $400/year), the payback period for three condensing units would be quite long.

However, I really like the idea of installing baseboard radiators in the basement. Presently, it is totally unheated, and they would be relatively easy to install around the outside walls. So I'm presently thinking of adding foam board and baseboard radiators in the basement.

What is the average life expectancy of a single boiler unit like mine? I think the brand is Utica?
 

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The hollow multi-core terra cotta blocks are higher-R than they might seem, far less thermally conductive than red brick, and less thermally conductive than concrete or cinder blocks. How thick are they?

Is there an air gap between the blocks and brick veneer?

Is there wood lath behind the plaster, or is it directly applied to the terra cotta block?

Assuming a 6" terra cotta block you're looking at about R1.5 for the block, R0.5 for the brick veneer, R1 for the interior & exterior air films (combined), and R0.5-R1 for the plaster (depending on lath vs. direct-applied, for an R-value of ~ R4. If there's a cavity between the brick veneer & structual block it'l be closer to R5.

If the storm windows are tight for (usually) less than $100/window you can re-glaze them with hard-coat low-E glass and bring the total window performance up to about code min, (about R3) even before counting the window film performance, but it's probably not the highest priority. Blower-door directed air sealing comes first.

The tankless water heater can easily heat the basement with a plate-heat exchanger isolated loop, without impacting hot water use if designed right. Fin-tube baseboard delivers about 150-200 BTU/hr per running foot at an average water temp of 110-115F (set the tankless to 120F). So if the calculated heat load of the basement after insulating is say, 4000 BTU/hr, it'll take only about 20-30' of baseboard to heat the place. But the minimum-fire output of most tankless units will be higher than that. To keep from short-cycling the tankless into low efficiency it'll either need higher thermal mass radiation like high-volume radiators, or sufficient baseboard to emit the full min-fire output into the zone. It's not rocket science to design one of these, but it does take at least some napkin-math. A heat exchanger, a couple of pumps, and a zone relay is a lot cheaper than a boiler, and it will be more efficient than adding load onto the oversized cast iron.

It's probably worth installing a retrofit heat purge economizer control onto the boiler, even if it's not short-cycling due to the high thermal mass of your radiators.
 

Nathan Davis

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The boiler is almost certianly oversized for the space heating load by at least a factor of 2. (Run the fuel use numbers to prove/disprove that thesis.) It's probably oversized for the total amount of radiator too, but is saved from short cycling by the thermal mass of the high volume radiators, and the fact that it operates as a single zone. The limitation on barely keeping up is almost certainly the amount of radiator, not the amount of burner.- you could install a 500,000 BTU/hr boiler on the same system and still barely keep up at 0F. If the system gets broken up into four zones it's very likely to exhibit short-cycling on zone calls.

Even though it drops below 0F at times, the 99th percentile temperature bin in Springfield MO is +9F. The load should be calculated at +9F, with no more than a 1.4x oversizing factor on the boiler, for non-moduationg boiler. At even 1.2x oversizing for the load at +9F it doesn't begin to lose ground until it's -7F or so outside. At 1.4x oversizing it's good to -15F. It's only cooler than +9F for 87 hours out of an average year, and never for 87 consecutive hours.

If insulating and air sealing the basement the total load will be going down too, since even an unheated basement that drops to +50F indoors during cold snaps is still losing a LOT of heat to the outdoors 24/7 during the winter, heat that is being supplied by the boiler & distribution plumbing. Sealing & insulating the basement will sometimes have negative load relative to the amount of standby and distribution loss from the boiler & plumbing, unless the plumbing gets insulated too.

The heat load of the basement isn't big enough to warrant it's own boiler, even if you DON'T insulate it. But DO insulate! Spending the money on reducing the heat load of the basemen by more than 3/4 costs less than installing a mod-con boiler. A hydronic baseboard zone running off the water heater (isolated with a plate-type heat exchanger) is one fairly cheap solution. A 3/4 ton mini-split heat pump might be another, depending on how many rooms it's getting divided up into. Run room-by-room heat load calculation if you want to optimize it.

If the windows in this are old-school single panes with clear glass storm windows there may be reasonable options for upgrading the window performance with air sealing and replacing the storm windows with low-E storms, which will bring a single-pane wood sash window up to current code-minimum at a fraction of the cost of a replacement window. That can lower the peak load, making it more likely that the rooms stay at temperature during colder tempratures, and with less of a cascading draft of cold air coming off the window it's more comfortable at any indoor temperature.

I am finding your recommended heat calculations fascinating. Interestingly, when I replaced the old boiler in 2008, I replaced a 340,000 btu unit with a 200,000 btu unit, simply because I couldn't believe that such a large boiler was really necessary. And that is what you are also saying. In trying to run the calculations, they discuss using a 60 degree day versus a 65 degree day for either a 2x4 wall or a 2x6 wall. In my case, the exterior walls have NO wood. The exterior walls are plaster over hollow clay tile with a layer of brick veneer on the exterior. I'm unsure of the r value of this wall, but I would guess that it's probably only a R 4 or 5, at best, and maybe even lower! So I'm also unsure if I should run calculations using a 65 degree day, as used for a 2x4 insulated wall. Any ideas?
 

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The hollow multi-core terra cotta blocks are higher-R than they might seem, far less thermally conductive than red brick, and less thermally conductive than concrete or cinder blocks. How thick are they?

Is there an air gap between the blocks and brick veneer?

Is there wood lath behind the plaster, or is it directly applied to the terra cotta block?

Assuming a 6" terra cotta block you're looking at about R1.5 for the block, R0.5 for the brick veneer, R1 for the interior & exterior air films (combined), and R0.5-R1 for the plaster (depending on lath vs. direct-applied, for an R-value of ~ R4. If there's a cavity between the brick veneer & structual block it'l be closer to R5.

If the storm windows are tight for (usually) less than $100/window you can re-glaze them with hard-coat low-E glass and bring the total window performance up to about code min, (about R3) even before counting the window film performance, but it's probably not the highest priority. Blower-door directed air sealing comes first.

The tankless water heater can easily heat the basement with a plate-heat exchanger isolated loop, without impacting hot water use if designed right. Fin-tube baseboard delivers about 150-200 BTU/hr per running foot at an average water temp of 110-115F (set the tankless to 120F). So if the calculated heat load of the basement after insulating is say, 4000 BTU/hr, it'll take only about 20-30' of baseboard to heat the place. But the minimum-fire output of most tankless units will be higher than that. To keep from short-cycling the tankless into low efficiency it'll either need higher thermal mass radiation like high-volume radiators, or sufficient baseboard to emit the full min-fire output into the zone. It's not rocket science to design one of these, but it does take at least some napkin-math. A heat exchanger, a couple of pumps, and a zone relay is a lot cheaper than a boiler, and it will be more efficient than adding load onto the oversized cast iron.

It's probably worth installing a retrofit heat purge economizer control onto the boiler, even if it's not short-cycling due to the high thermal mass of your radiators.

You guessed correctly about the exterior wall. It is plaster directly applied over 6" hollow clay tile with about an inch of airspace between the tile and brick veneer. So I may have an r-4 or 5 exterior wall, at best. I am intrigued about the possibility of using my tankless water heater to heat the basement--never thought of that one. I can get baseboard hot water heat exchangers on Amazon.com for fairly reasonable prices. Like you say, a couple of pumps, and a zone relay probably won't cost too much, and may be more efficient than tapping into the existing how water boiler supply and return pipes.

I am also intrigued with your idea about installing a heat purge economizer. I've never heard of one, but it sounds worthwhile to price out.

I would still like to change the iron window weights for tape balance springs and foam filled weight cavities, but I cannot find tape balances at a reasonable price. I found one company that sells them at $46 per pair for an 18 lb sash, so new balances for all 44 windows would cost over $2000. And that assumes that I modify all the double hung windows into single hung, and even then I'd still have to buy the foam. The total cost would be about $66 per window, or almost $3000 for all 44 windows. Even if I consider my labor free, the payback through savings could take a very long time.
 

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The classic heat purging economizers are the Becket Heat Manager (manufactured by Intellicon) and the Intellicon 3250 HW+, which are pretty easy for DIY retrofit on many boilers, but there are others. Newer boilers often come with smarter controls included. The Beckett Aqua Smart, and Hydrostat 3250 Plus are sometimes OEMed into new boilers, but can also be retrofitted.

Short of replacing sash weights with coil spring units, retrofit pulley seals can dramatically reduce the air leakage at the sash weight pockets for low money if the window trim is already tightly caulked:

11123.jpg


(I ended up installing a bunch of these in my 1923 vintage bungalow.)

Build a spreadsheet using a U-factor of U0.20 (=R5) for the above-grade exterior wall area, and U0.35 for the window area, run the room by room heat load numbers I=B=R style, assuming a 60F temperature difference (69F indoors, 9F outdoors) and see what you come up with for conducted heat loss. Assuming you have 6-9" of fluff in the attic, use U0.04 for the ceiling losses of top floor rooms. Add a foot of height to the wall area to account for the joists between rooms. Use U0.5 for solid wood doors, U0.8 for paneled doors and single-pane wood sashed windows (no storms).

The basic formula is:

Area x U-factor x 60F= heat loss

eg: If a top floor room is say, 13 x 15', and has 9' ceilings, and two 12 square foot windows (measure the full area inside the casing trim, not just the glass), and just one 15' of exterior wall, the conducted heat loss is:

Ceiling:

195 square feet x U0.04 x 60F= 468 BTU/hr

Windows:

24' x U0.35 x 60F= 504 BTU/hr

Walls: The gross area (including joist depth) is (9'+1') x 15' = 150', less 24' of window is 126' of wall area.

126' x U0.20 x 60F= 1512 BTU/hr

Add it all up and you're at 2484 BTU/hr, a ratio of about 13 BTU per square foot of conditioned space for that room, before air infiltration factors, and before subtracting off the heat of mammalian bodies (230BTU/hr per sleeping human, etc.)

It's useful to track it room by room then add up the whole house numbers. With the room by room numbers you can also look at the load per square foot EDR of the radiators, which would tell you how well balanced the heating system design is, and to get a handle on the water temperatures necessary to meet the 99% load (and colder.) A typical cast iron boiler set up for 190F output and a 20F in-to-out delta-T for a 170F return puts out ~170BTU per EDR' from the radiators. If you crank the boiler up to 215F you can get a bit more out of the rads, but not a lot more. Don't expect more than 200 BTU /ft-EDR out of them with any hot water boiler, and not more than 160 BTU/ft-EDR with most condensing boilers, if considering that option. Knowing the approximate load/radiation ratio can be a critical point in that decision tree.

For the basement load, just count the square feet of foundation above grade. If insulated with 2" of roofing polyiso with half-inch wallboard on the interior you're looking at about U0.065- U0.075, if it's the terra-cotta + brick veneer construction. So if you have 200' of perimeter with 2' of exposure, no windows, the above grade losses prior to insulation would be

400' x U0.2o x 60F= 4800 BTU/hr, and probably another 2500-4000 BTU/hr of below grade loss (including the slab), potentially ~7000-9000 BTU/hr.

After insulation it would be about:

400' x U0.075 x 60F= 1800 BTU/hr of above grade losses, and probably less than 2500 BTU/hr for the rest, something like 4000 BTU/hr total, possibly only 3000 BTU/hr depending on the R-value of the soil below the slab.

But run the conducted loss numbers, see where they fall before including any infiltration/ventilation loss estimates.
 

Nathan Davis

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The classic heat purging economizers are the Becket Heat Manager (manufactured by Intellicon) and the Intellicon 3250 HW+, which are pretty easy for DIY retrofit on many boilers, but there are others. Newer boilers often come with smarter controls included. The Beckett Aqua Smart, and Hydrostat 3250 Plus are sometimes OEMed into new boilers, but can also be retrofitted.

Short of replacing sash weights with coil spring units, retrofit pulley seals can dramatically reduce the air leakage at the sash weight pockets for low money if the window trim is already tightly caulked:

11123.jpg



(I ended up installing a bunch of these in my 1923 vintage bungalow.)

Build a spreadsheet using a U-factor of U0.20 (=R5) for the above-grade exterior wall area, and U0.35 for the window area, run the room by room heat load numbers I=B=R style, assuming a 60F temperature difference (69F indoors, 9F outdoors) and see what you come up with for conducted heat loss. Assuming you have 6-9" of fluff in the attic, use U0.04 for the ceiling losses of top floor rooms. Add a foot of height to the wall area to account for the joists between rooms. Use U0.5 for solid wood doors, U0.8 for paneled doors and single-pane wood sashed windows (no storms).

The basic formula is:

Area x U-factor x 60F= heat loss

eg: If a top floor room is say, 13 x 15', and has 9' ceilings, and two 12 square foot windows (measure the full area inside the casing trim, not just the glass), and just one 15' of exterior wall, the conducted heat loss is:

Ceiling:

195 square feet x U0.04 x 60F= 468 BTU/hr

Windows:

24' x U0.35 x 60F= 504 BTU/hr

Walls: The gross area (including joist depth) is (9'+1') x 15' = 150', less 24' of window is 126' of wall area.

126' x U0.20 x 60F= 1512 BTU/hr

Add it all up and you're at 2484 BTU/hr, a ratio of about 13 BTU per square foot of conditioned space for that room, before air infiltration factors, and before subtracting off the heat of mammalian bodies (230BTU/hr per sleeping human, etc.)

It's useful to track it room by room then add up the whole house numbers. With the room by room numbers you can also look at the load per square foot EDR of the radiators, which would tell you how well balanced the heating system design is, and to get a handle on the water temperatures necessary to meet the 99% load (and colder.) A typical cast iron boiler set up for 190F output and a 20F in-to-out delta-T for a 170F return puts out ~170BTU per EDR' from the radiators. If you crank the boiler up to 215F you can get a bit more out of the rads, but not a lot more. Don't expect more than 200 BTU /ft-EDR out of them with any hot water boiler, and not more than 160 BTU/ft-EDR with most condensing boilers, if considering that option. Knowing the approximate load/radiation ratio can be a critical point in that decision tree.

For the basement load, just count the square feet of foundation above grade. If insulated with 2" of roofing polyiso with half-inch wallboard on the interior you're looking at about U0.065- U0.075, if it's the terra-cotta + brick veneer construction. So if you have 200' of perimeter with 2' of exposure, no windows, the above grade losses prior to insulation would be

400' x U0.2o x 60F= 4800 BTU/hr, and probably another 2500-4000 BTU/hr of below grade loss (including the slab), potentially ~7000-9000 BTU/hr.

After insulation it would be about:

400' x U0.075 x 60F= 1800 BTU/hr of above grade losses, and probably less than 2500 BTU/hr for the rest, something like 4000 BTU/hr total, possibly only 3000 BTU/hr depending on the R-value of the soil below the slab.

But run the conducted loss numbers, see where they fall before including any infiltration/ventilation loss estimates.

HA!! I've got a humdinger of a homework assignment! This will take me a while, but it looks like fun.
 

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HA!! I've got a humdinger of a homework assignment! This will take me a while, but it looks like fun.

OK--I finally got finished inputting all the numbers for each room, windows and doors. This has developed into a fairly large spreadsheet since it the house is 5800 sq ft.

From the furnace I found the following:
Furnace input rating = 199,999 Btu
DOE rating = 165,000 Btu
Water rating= 143,000 Btu

After running all the calculations for each room and each radiator--the total projected heat loss for the house = 178,000 Btu, based on a 60F day (see the attachment below for details). In addition to the hot water boiler, the master bath has 15 Amp electric radiant heat under the floor. The Master Bath heating loss is 22,000 Btu. It used to feel cold until I added the radiant heat, but it feels fine now.

I guess all this means that the house may be slightly under heated, but it still seems to feel fine except when it gets below 0F.

My wife complained this winter that the kitchen feels cool when she is not cooking to heat it up. So last week I ran 130' of pex radiant heat under the kitchen floor, and tied it into the main boiler line. The numbers show that the heat loss in the kitchen is 18000 Btu, and the heat loss through the basement should be 4400 Btu. Now I'm wondering if I should use a small condensing boiler (about 20,000 Btu) to serve the kitchen pex and basement radiators? Any ideas?

As you said, insulating the basement seems like a no-brainer.

Coincidentally, my A/C compressor is now 13 years old, and the evaporator unit is now 23 years old. So I am also contemplating replacing everything preventatively before it breaks during the middle of summer. At 5 tons, it has been marginal, but I've heard that is more desirable than being oversized. Yet, it can't keep up when temperatures go above 95F. How do I convert these heating numbers into something that indicates the appropriate size of an A/C system?
 

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Dana

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I'm having hard time figuring out the spreadsheet- something seems seriously askew!

For instance the first row shows 36 linear feet of 10' tall U0.2 exterior wall with a 45 foot window and 50 square feet of door.

That's a net wall area of (36 x 10)- 45' (window) -50' (door) = 267 square feet, which at a 60F temperature difference would yield wall losses of:

267' x U0.2 x 60F= 3204 BTU/hr

But at the end of row it lists a heat loss of 2445 BTU/hr of window & door losses (which seems right), and 15,900 BTU/hr of wall loss.

Where is the other (15,900 - 3204 =) 12,676 BTU/hr of wall loss coming from for that room?

As-shown in the spreadsheet, just your first floor has more than twice the heat load of my 2x4 framed 2400' sub-code antique with 1600' of insulated basement whole house! It can't be right.

Only the exterior walls have heat a loss. The partition walls and floors are at the same temperature on both sides of the wall or floor assembly , with no heat actually leaving house.

Also, if it's just a clear-glass window + storm and no window film, use U0.5 for the windows. If it's a wood sashed single pane with no storm, use U1.

There is no correlation between heating loads and cooling loads. A large fraction of the cooling load is due to solar gains through windows (which don't get subtracted from heating loads since the coldest temps are at night.) In your location/climate another double-digit percentage of the cooling load is latent load- humidity, which is also plays no part in a heating load calculation.

With the boiler inside of conditioned space, with no insulated wall, ceiling or floor between it and fully heated space it's the DOE output that matters. The net water rating would only matter if the boiler was located outside the thermal boundary of the house, when the standby losses and some of the distribution losses are truly lost, and don't end up heating any part of the inside of the house.

Also (maybe it's a regional dialect issue?), in most of the US "furnace" refers exclusively to hot air heating systems- what you have is a boiler.
 

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I'm having hard time figuring out the spreadsheet- something seems seriously askew!

For instance the first row shows 36 linear feet of 10' tall U0.2 exterior wall with a 45 foot window and 50 square feet of door.

That's a net wall area of (36 x 10)- 45' (window) -50' (door) = 267 square feet, which at a 60F temperature difference would yield wall losses of:

267' x U0.2 x 60F= 3204 BTU/hr

But at the end of row it lists a heat loss of 2445 BTU/hr of window & door losses (which seems right), and 15,900 BTU/hr of wall loss.

Where is the other (15,900 - 3204 =) 12,676 BTU/hr of wall loss coming from for that room?

As-shown in the spreadsheet, just your first floor has more than twice the heat load of my 2x4 framed 2400' sub-code antique with 1600' of insulated basement whole house! It can't be right.

Only the exterior walls have heat a loss. The partition walls and floors are at the same temperature on both sides of the wall or floor assembly , with no heat actually leaving house.

Also, if it's just a clear-glass window + storm and no window film, use U0.5 for the windows. If it's a wood sashed single pane with no storm, use U1.

There is no correlation between heating loads and cooling loads. A large fraction of the cooling load is due to solar gains through windows (which don't get subtracted from heating loads since the coldest temps are at night.) In your location/climate another double-digit percentage of the cooling load is latent load- humidity, which is also plays no part in a heating load calculation.

With the boiler inside of conditioned space, with no insulated wall, ceiling or floor between it and fully heated space it's the DOE output that matters. The net water rating would only matter if the boiler was located outside the thermal boundary of the house, when the standby losses and some of the distribution losses are truly lost, and don't end up heating any part of the inside of the house.

Also (maybe it's a regional dialect issue?), in most of the US "furnace" refers exclusively to hot air heating systems- what you have is a boiler.

The first line indicates 36' of a 10' high wall at U .2, 45 sq ft of window area, and 50 sq ft of entry door. Actually there are 2 entry doors (double) with two 1'x7' side lights for a total unit size of 8' wide x 7' high. So it would actually total almost 56 sq ft. However, I included the spreadsheet as a way to let someone else double check my numbers, and I'm very glad I did. When I made the calculation for heat loss in the wall, I accidentally omitted the .2 for the U factor. Well, that reduces the final figure by 1/5th ! I'm glad you found that. Now I can start to correct the figures!
 

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The first line indicates 36' of a 10' high wall at U .2, 45 sq ft of window area, and 50 sq ft of entry door. Actually there are 2 entry doors (double) with two 1'x7' side lights for a total unit size of 8' wide x 7' high. So it would actually total almost 56 sq ft. However, I included the spreadsheet as a way to let someone else double check my numbers, and I'm very glad I did. When I made the calculation for heat loss in the wall, I accidentally omitted the .2 for the U factor. Well, that reduces the final figure by 1/5th ! I'm glad you found that. Now I can start to correct the figures!

OK--I'm really glad you found that calculation error. An extra head and extra 2 eyes makes a huge difference! Anyway, the corrected table is attached. It shows that the correct total heat loss is only 60000 Btu. So yes, it actually seems very over-sized, and I should have no problem to add the pex radiant loop to my kitchen (at 6100 Btu for wall and window loss) as well as the 4400 Btu for the basement.

This has me wondering if I should swap my existing boiler for one at half the size.

I'm still laughing at the mistake.
 

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Dana

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There have been no adjustments for air infiltration losses, which may be considerable (or not), nor have we subtracted of internal heat sources such as warm bodies, DVRs, refrigerators, any other 24/7 electrical plug loads.

You probably have a least 5000 BTU/hr of infiltration loss, but also 2000 BTU/hr of plug loads and live bodies. Air leakage of

100 cfm x 60 minutes/hr x 0.018 BTU/cubic foot per degree F x 60F = 6480 BTU/hr

Your real air leakage could be half that (not likely) , or it could be 3 times that (more likely, but can be improved upon.)

To verify the 60K heat load number, run a fuel use based heat load calculation on some wintertime gas bills. The domestic hot water use error will be counterbalanced by the solar gains (pretty much, more or less) in winter, but not during the rest of the year, so stick to wintertime usage. That would put a firm stake in the ground for the as-is-where-is heat load prior to air sealing and insulation improvements.

If replacing a 165K output boiler with an 80K boiler, there are some fairly inexpensive (as cheap or cheaper than cast iron) condensing 80K top 120K (input) wall hung boilers with 10:1 turn down ratios that might work. I'd have to pore over the load per square foot EDR on the rads a bit closer and a fuel use sanity check to make that call, but at least you have a starting point to work from with that spreadsheet.
 

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There have been no adjustments for air infiltration losses, which may be considerable (or not), nor have we subtracted of internal heat sources such as warm bodies, DVRs, refrigerators, any other 24/7 electrical plug loads.

You probably have a least 5000 BTU/hr of infiltration loss, but also 2000 BTU/hr of plug loads and live bodies. Air leakage of

100 cfm x 60 minutes/hr x 0.018 BTU/cubic foot per degree F x 60F = 6480 BTU/hr

Your real air leakage could be half that (not likely) , or it could be 3 times that (more likely, but can be improved upon.)

To verify the 60K heat load number, run a fuel use based heat load calculation on some wintertime gas bills. The domestic hot water use error will be counterbalanced by the solar gains (pretty much, more or less) in winter, but not during the rest of the year, so stick to wintertime usage. That would put a firm stake in the ground for the as-is-where-is heat load prior to air sealing and insulation improvements.

If replacing a 165K output boiler with an 80K boiler, there are some fairly inexpensive (as cheap or cheaper than cast iron) condensing 80K top 120K (input) wall hung boilers with 10:1 turn down ratios that might work. I'd have to pore over the load per square foot EDR on the rads a bit closer and a fuel use sanity check to make that call, but at least you have a starting point to work from with that spreadsheet.

I am certainly going to ask my wife to retrieve the gas bills from last year, and this one, too.

Your comment about air infiltration is interesting:
First of all, the $3000 window remodeling to install tape balances and foam in the weight cavities would likely address one of the more significant air infiltration sources.
Second, I may have inadvertently introduced a major air infiltration problem when I installed a radon mitigation system last year. I have 3 sump pumps and a large crawl space under the kitchen. So the radon pump for all 4 sources pulls 200 cfm, and that air has to be replaced from somewhere. Although it uses a 200 cfm fan, I estimate that it probably actually expells about 100 cfm. It's ironic that your example uses 100 cfm. But I don't know any way to correct this, other than to turn it off. Since radon causes cancer, I'm unsure of a good alternative.
 
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