Mod Con Boiler upgrade, heat loss calc

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BMWpowere36m3

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I'm renovating my home, '56 single-story ranch ~ 1600 sqft, gutted and currently installing drywall. My plumbing supply guy turned me onto a Lochivar Knight Mod Con and Crown DWH... he figured I would need a WHN085 (69 MBH IBR). The house originally had an oil-fired boiler, domestic coil, baseboard and crappy insulation
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I've ran heat loss calculations using the SlantFin tool (IBR method).

Assumptions:
  • Interior Temp 70 F
  • Design Day Temp 7 F (CT)
  • Unheated basement (mostly below grade)
  • Ventilated Attic (R49)
  • Infiltration, 0.75 to 1 exchanges per hour
Results are 39,100 - 42,700 BTU's/hr (depending on ACH)

I've been told given the size of my house and recent work (vinyl siding, tyvek, insulation, windows/doors, some crack sealing and caulking) I should be fine with the WHN055 (44 MBH IBR) or even a CDN040 (37 MBH IBR).

Thoughts?
 

Dana

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That's correct- either of those boilers would work just fine.

The IBR output number is irrelevant unless the boiler is in an uninsulated shed/garage outside of the thermal envelope of the house. The D.O.E. output number is what counts if the boiler is in the basement. Even if the basement isn't specifically heated, the standby & distribution losses of the boiler still accrue to cover the heat load by lowering the heat loss of the first floor space to the basement.

And...

The SlantFin tool is an old-school IBR type calculation method, guaranteed to be at least 15% above reality for most homes. A 1950s ranch house with R13 cavity insulation, R49 roof, and at least clear-glass tight storms would typically have heat load in the mid 30s @ +7F, not 39-43K. If you air-seal and insulated the foundation walls to at least R10 it would probably come in near (or even under) 30,000 BTU/hr, since the heat loss out of the basement is likely to be 15-20% of the whole-house load (even if the basement drops to 50F during cold snaps), due to the very low R value of uninsulated above grade foundation & band joist.

If you have low-E storm windows or low-E replacement windows and installed R5 or better foam under that vinyl siding (a code min wall for your climate zone under IRC 2012) you're probably looking at a design heat load in the mid-20s, maybe as high as 30KBTU/hr if the foundation is not yet insulated.

It may be better to use a high mass condensing unit like an HTP Versa rather than a Lochinvar. The ratio of radiation output at low water temps to the boiler's min-fire output may limit how low you can run the system temps, and with it the amount of condensing efficiency you get out of the Lochinvar. Rather than focus on the high-fire output, look at the min-fire figures- lower is always better for low-mass mod-cons, particularly if you have low mass fin-tube rather than cast iron radiation. The thermal mass of the HTPs make it self-buffering to keep it from short cycling at any water temp, no matter how limited the radiation output is at that temp.

So, how many feet of baseboard do you have (per zone, if broken into zones) and how many zones?
 

BMWpowere36m3

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That's correct- either of those boilers would work just fine.

The IBR output number is irrelevant unless the boiler is in an uninsulated shed/garage outside of the thermal envelope of the house. The D.O.E. output number is what counts if the boiler is in the basement. Even if the basement isn't specifically heated, the standby & distribution losses of the boiler still accrue to cover the heat load by lowering the heat loss of the first floor space to the basement.

And...

The SlantFin tool is an old-school IBR type calculation method, guaranteed to be at least 15% above reality for most homes. A 1950s ranch house with R13 cavity insulation, R49 roof, and at least clear-glass tight storms would typically have heat load in the mid 30s @ +7F, not 39-43K. If you air-seal and insulated the foundation walls to at least R10 it would probably come in near (or even under) 30,000 BTU/hr, since the heat loss out of the basement is likely to be 15-20% of the whole-house load (even if the basement drops to 50F during cold snaps), due to the very low R value of uninsulated above grade foundation & band joist.

If you have low-E storm windows or low-E replacement windows and installed R5 or better foam under that vinyl siding (a code min wall for your climate zone under IRC 2012) you're probably looking at a design heat load in the mid-20s, maybe as high as 30KBTU/hr if the foundation is not yet insulated.

It may be better to use a high mass condensing unit like an HTP Versa rather than a Lochinvar. The ratio of radiation output at low water temps to the boiler's min-fire output may limit how low you can run the system temps, and with it the amount of condensing efficiency you get out of the Lochinvar. Rather than focus on the high-fire output, look at the min-fire figures- lower is always better for low-mass mod-cons, particularly if you have low mass fin-tube rather than cast iron radiation. The thermal mass of the HTPs make it self-buffering to keep it from short cycling at any water temp, no matter how limited the radiation output is at that temp.

So, how many feet of baseboard do you have (per zone, if broken into zones) and how many zones?

Good info!

Walls (2x4): R15 cavities, vapor barrier (inside), Tyvek (outside), plywood sheathing and vinyl siding (no foam board underneath).

Windows and doors: all replaced (Anderson/Pella low-e) with a NFRC U-Factor averaging 0.3.

Foundation: uninsulated poured concrete, with about 2-3' above ground (remaining below). Don't plan on insulating basement, but I already sealed the band joisted with expanding foam. It has 4 single pane windows (obviously leaky). With temps upstairs of 55*, 20* outside, I've measured 45* in the basement. (House currently in drywall stage... no interior doors, no finish flooring, no mechanicals).

Infiltration.... that is a big unknown. People say anywhere from 0.5 to 3 ACH, which has a huge impact on heatloss.

The house layout has changed and the old CI boiler and baseboard were tossed. I'm considering either long lengths of HO baseboard (like S/F Pak 80) or radiant panels (Buderus, Veha, Myson, Vasco) in an attempt to keep supply and return temps down.

Here is a layout of the house:
Visio-9 Great Oak Home.jpg

My goal is to maintain a comfortable temp throughout the house (68-72F), having the bathrooms run slightly higher and be able to lower the temps in the bedrooms at night (purely for comfort).[/QUOTE]
 

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A few comments:

R15 2x4 16' o.c. walls (25% framing fraction) comes in at about R10 whole-wall after factoring in the thermal bridging of the framing and adding the R-value of the plywood and wallboard. R13 comes in at R9.5, which isn't a big performance difference. The higher air-retardency of R15 is worth it though. An R13 +5 IRC 2012 code min wall comes in between R14-R15, which is a pretty big uptick over cavity-fill-only solutions, and would allow you to dispense with the interior vapor barrier, making the assembly more moisture resilient.

Ideally it's best to blower-door test the house after the windows and exterior doors are in, and before the wall insulation goes up, which allows you to find and fix all of the leaks in the wall sheathing. If you don't have a lot of the drywall up yet it may still be worth it. If you can get the leakage at 50 pascals under 1500 cfm you can pretty much ignore the infiltration numbers.

Not insulating the foundation is a mistake. Do the math:

A poured concrete foundation has a U-factor of about 1 BTU/hr per square foot per degree F. Given that the difference in basement temp and upperfloor temp is 10F when it's 20F outdoors, with a 70F first floor it might run around 57F in the basement when it's 7F outside for a delta-T of 50F. It looks like you have about 220' of perimeter, so even with a 2' average exposure you're looking at 44o square feet. 440' x 50F x U1= 22,000 BTU/hr. Hell, even if it's only 47F for a 40F delta-T, that's still 17-18KBTU/hr- that's half your calculate heat load (!). If you add R1o continuous insulation that drops the U factor to under 0.1, and the heat load for that portion of wall to under 3000 BTU/hr, even at much higher "coasting" temperature north of 60F, maybe even 67F, a delta of 60F: 440' x 60F x U0.1= 2640 BTU/hr.

Even if the concrete is a special mix magically U0.5 (not likely), and the foundation heat loss is "only" 10,000 BTU/hr insulating to R10 knocks your whole house load to under 30K. The heat load of the below grade slab & wall still count, but nothing like the above grade concrete. Insulating the slab may not be worth it at buck-a-therm gas prices, but the walls definitely are, and your floors upstairs will be warmer during cold snaps.

Even 2" of cheap EPS (R8) strapped to the foundation with 1x furring TapConned to the foundation is huge. If it's a budget issue, reclaimed roofing foam is 1/4-1/3 the cost of virgin stock. I did my foundation with 3" reclaimed roofing polyiso from a local reseller. It cut fuel use by more than 15%, and that's from a much higher heat load for the rest of the place than you're talking. Percentage-wise it's probably bigger for your house than mine.Search your local craigslist materials section for the keywords rigid insulation- you'll probably find a few of the regional sellers. At virgin-stock pricing R1o foam would run $1 per square foot or more. Reclaimed R10 would run 25-35 cents per square foot, plus whatever it takes for the 1x4 furring, TapCons and 1/2" wallboard (required by code as a thermal barrier against fire for the foam.)

Going with low-temp panel radiators is MUCH more comfortable than low-temp convectors. Panel radiators would be worth the upcharge in comfort terms in any rooms where you might be hanging out while awake.

If you're going to micro-zone it on a room-by-room basis you'd need a buffer tank if using a mod-con. But the smallest HTP Versa is probably a better choice. If you run it as a single zone you'll do fine with the smallest Lochinvars.
 

BMWpowere36m3

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A few comments:

R15 2x4 16' o.c. walls (25% framing fraction) comes in at about R10 whole-wall after factoring in the thermal bridging of the framing and adding the R-value of the plywood and wallboard. R13 comes in at R9.5, which isn't a big performance difference. The higher air-retardency of R15 is worth it though. An R13 +5 IRC 2012 code min wall comes in between R14-R15, which is a pretty big uptick over cavity-fill-only solutions, and would allow you to dispense with the interior vapor barrier, making the assembly more moisture resilient.

Ideally it's best to blower-door test the house after the windows and exterior doors are in, and before the wall insulation goes up, which allows you to find and fix all of the leaks in the wall sheathing. If you don't have a lot of the drywall up yet it may still be worth it. If you can get the leakage at 50 pascals under 1500 cfm you can pretty much ignore the infiltration numbers.

Not insulating the foundation is a mistake. Do the math:

A poured concrete foundation has a U-factor of about 1 BTU/hr per square foot per degree F. Given that the difference in basement temp and upperfloor temp is 10F when it's 20F outdoors, with a 70F first floor it might run around 57F in the basement when it's 7F outside for a delta-T of 50F. It looks like you have about 220' of perimeter, so even with a 2' average exposure you're looking at 44o square feet. 440' x 50F x U1= 22,000 BTU/hr. Hell, even if it's only 47F for a 40F delta-T, that's still 17-18KBTU/hr- that's half your calculate heat load (!). If you add R1o continuous insulation that drops the U factor to under 0.1, and the heat load for that portion of wall to under 3000 BTU/hr, even at much higher "coasting" temperature north of 60F, maybe even 67F, a delta of 60F: 440' x 60F x U0.1= 2640 BTU/hr.

Even if the concrete is a special mix magically U0.5 (not likely), and the foundation heat loss is "only" 10,000 BTU/hr insulating to R10 knocks your whole house load to under 30K. The heat load of the below grade slab & wall still count, but nothing like the above grade concrete. Insulating the slab may not be worth it at buck-a-therm gas prices, but the walls definitely are, and your floors upstairs will be warmer during cold snaps.

Even 2" of cheap EPS (R8) strapped to the foundation with 1x furring TapConned to the foundation is huge. If it's a budget issue, reclaimed roofing foam is 1/4-1/3 the cost of virgin stock. I did my foundation with 3" reclaimed roofing polyiso from a local reseller. It cut fuel use by more than 15%, and that's from a much higher heat load for the rest of the place than you're talking. Percentage-wise it's probably bigger for your house than mine.Search your local craigslist materials section for the keywords rigid insulation- you'll probably find a few of the regional sellers. At virgin-stock pricing R1o foam would run $1 per square foot or more. Reclaimed R10 would run 25-35 cents per square foot, plus whatever it takes for the 1x4 furring, TapCons and 1/2" wallboard (required by code as a thermal barrier against fire for the foam.)

Going with low-temp panel radiators is MUCH more comfortable than low-temp convectors. Panel radiators would be worth the upcharge in comfort terms in any rooms where you might be hanging out while awake.

If you're going to micro-zone it on a room-by-room basis you'd need a buffer tank if using a mod-con. But the smallest HTP Versa is probably a better choice. If you run it as a single zone you'll do fine with the smallest Lochinvars.

I used this calculator (for walls and ceilings) and the number agrees with the values in the S/F tool… so I thought they were right. It seems to indicate with "typical" 2x4 construction with R13 insulation equals ~ R13.3 as a "composite".

Since I had R15, I just plugged that in at got ~ R14.3 for the wall. THe U-factor of 0.07 is in agreement with the S/F tool. In any event, its all said and done as far as installation. Siding is hung, walls/ceilings insulated and drywall hung/taped.

As you alluded to, the biggest heat losses in the house (calculated) are the floors and infiltration. The tool uses a U-Factor of 0.15 for finished floors, uninsulated, above an unheated basement. I think the floor heatloss was on the order of 15k BTUs/hr.

That's good info regarding the basement… something to think about in the future, but not currently planned. Right now I need to get the house done and living in it.

I'm trying to determine what size boiler will suite my house and what type of emitters I want to use. My apprehensive to the boiler size is purely based on my parent's house (basically same layout, sq ft, uninsulated basement, renovated 18 yrs ago, new windows, siding, insulation, etc…). They had a Burnham 136 MBH boiler that was put in when they moved in 18 yrs ago and baseboard throughout the house.

My plumbing supply guy (did his own calcs) says a WHN085 should be fine with a DWH and others are saying 40-50 MBH boilers (like CDN040 0r WHN055). Obviously a large range 40-140 MBU, however I understand the "oversizing" of boilers was typical "in the day".

What got me on this "journey" was reading more about hydronics and figuring out the install for my house. If I didn't know any better I would have gotten the WHN085 and Crown DWH and zoned each bedroom, bathroom and kitchen+living room separately with 4 zones, 4 circs, 4 thermostats and baseboard throughout.
 

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The R value of the framing is a slightly optimisitic in that Colorado Energy calc as are the contributions of air films, but it's "good 'nuff."

Whatever you calculated for floor losses drops by something like 75% if you insulate the foundation walls to R10 or better (R15 continuous insulation is IRC 2012 code min for US climate zone 5, which includes CT). So if the calc came in at ~15,000 BTU/hr, it'll drop to something like 3000-4000 BTU/hr. If you insulate the slab too it drops even further, but that's a separate discussion. (Probably not worth doing unless you can tolerate the hit in ceiling height.

Even if you don't insulate any other part of the basement on the first go-around, if mounting the boiler on an exterior wall, put at least R10 between the boiler & plumbing manifolds when you install it, since it would be a super PITA to deal with that later. Putting 1.5-2" of foam (any type) against the wall and building a studwall with unfaced batt insulation to the interior side of that with 3/4" OSB or plywood facing the interior gives you something solid to mount the boiler, pumps & plumbing onto. Put an inch of EPS or XPS under the bottom plate of the studwall as a capillary & thermal break to keep the bottom plate from accumulating moisture through the slab (or summertime air.) TapCon the bottom plate to to the slab through the foam.

Upsizing the boiler for domestic hot water is insane if you're heating water with an indirect running as "priority" zone. The output of a mod-con is higher than most 40-50 gallon standalone tanks. You might have issues with the smaller boiler if you're heating an outdoor hot tub, but unless there are special circumstances like that, size the indirect for the biggest tub you have to fill, and size the boiler for the space heating load.

When I started upgrading my place about a decade ago ( a 2400' circa 1923 1.5 story bungalow in Worcester MA, 99% outside design temp +5F) I ended up scrapping a Burnham P206 (DOE output of about 130K, IBR of about 120K) that was only about 12-15 years old. The heat load characteristics of some of the rooms were SO different from the main areas that I decided to micro zone, and centered the heating system around a ~50 gallon buffer tank (that is also a "reverse indirect", with a potable coil inside the tank), and designed the radiation to deliver the heat at a single temp 125F AWT. When all zones are calling for heat the entering water temps at the boiler are around 115, with the boiler output of ~145F, and a buffer tank set point of 130F.

Manual-J calcs put my design load in the mid-30s, IBR methods put it at ~40K, so it's very similar calculated loads to yours. Note the pre-existing boiler was more than 3x oversized(!) and was more than 2x oversized for the load even before air sealing & insulating. I'm radiation limited to something between 40-45K of output, at the water temps I'm running, and the place sails through -5F & cooler weather just fine. The exterior surface area, insulation values, and window U-factors of your house are all better than mine EXCEPT for the foundation insulation. You really don't need more than 40K of boiler even before insulating the foundation, and would have significant temperature margin below +7F outdoor temps with a 40K boiler after insulating the foundation, shedding another 10K or more of peak load.

My radiation is a kludgey combination of radiant floors, re-used re-painted antique Arco SunRad recessed radiators (architecturally appropriate for a 1920s house), a section of cast iron baseboard (for the upstairs bathroom), with the "Hail Mary" backup being the pre-existing hydro-air air handler that had been served by the oversized P206. The air handler is capable of heating the whole house on it's own even with 120F water, since it too was 3x oversized, but the distribution & temperature balance between rooms is pretty lousy. When it's under 10F outside (or if using deep setbacks) the hydro-air kicks on, but most of the time the radiation is carrying the load. With outdoor reset I'd be able to heat it totally with the radiation 100% of the time, but I wouldn't run the buffer setput cooler than 125F under any circumstances, for domestic hot water performance reasons. Setting up the boiler for a fixed output of 140-150F the entering water is always in the condensing zone (with a reasonable delta-T) except for the last minute or so of burns when NOTHING is calling for heat, and it can't short-cycle on zone calls even though the smallest zone can only emit ~2500BTU/hr, (well under the minimum modulation level of any mod-con.)

Had the HTP Versa been available at the time it probably would have been a better (and cheaper) choice, and easier to design in, but technology marches on, eh?
 

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So I got the blower door test completed today...

1670 CFM @ -50 pascals, ACH50 of 4.8-9.6 depending if I include the basement (1300 sqft each).

At the moment the house has no mechanical ventilation, no finished flooring (only subfloor with gaps between sheets), drywall hung and taped, no electrical trim (switches/recep), no recessed light trims and no glass door for chimney (only damper). When the blower was running I could feel quite a bit of air coming in from the recessed lights, around the fireplace masonry (façade torn down) and any subfloor gaps/penetrations. Whereas the windows, doors, electrical boxes, drywall seams were good.

I’m surprised its this low given that a lot of trim work and flooring still need to go in. Per Energy Star guide, the local correction factor is ~19, so 0.25-0.51 ACH!
 

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Code-max leakage under IRC 2012 is 3 ACH/50, counting all space that is inside the pressure boundary of the house. Caulking between top plates and caulking the wallboard to the framing, and the electrical boxes to the wallboard go a long way to eating into wall leakage. There may still be some big leaks on plumbing stack & electrical penetrations into the attic.

The fireplace is probably a good fraction of the leakage as-is. Gasketed flue-top dampers seal better than the steel flaps in the fireboxes, but the masonry often leaks badly too from micro-cracks, etc. after 50+ years of service.
 

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Code-max leakage under IRC 2012 is 3 ACH/50, counting all space that is inside the pressure boundary of the house. Caulking between top plates and caulking the wallboard to the framing, and the electrical boxes to the wallboard go a long way to eating into wall leakage. There may still be some big leaks on plumbing stack & electrical penetrations into the attic.

The fireplace is probably a good fraction of the leakage as-is. Gasketed flue-top dampers seal better than the steel flaps in the fireboxes, but the masonry often leaks badly too from micro-cracks, etc. after 50+ years of service.

A couple of notes:

During the test, the contractor mentioned that 1300 CFM (@ -50 pascals) is the minimum before needing mechanical ventilation for my square footage. I'm almost positive he only used the first floor volume and did not include the basement (even though it was wide open to the first floor). My understanding is that 0.35 ACH is that minimum, which translates to ~ 6.65 ACH50 (for my area). So depending on whether I count the basement or not, I fall right around that number.

Foundation sill plate/header/subfloor joints were foamed (around the exterior perimeter). Exterior sheathing was caulked from the inside where it met the top and bottom plates. All plumbing and electrical penetrations were fire-caulked (even old unused penetrations). I'm sure there are other areas that could benefit as well, including: current single pane basement windows/frames, bottoms of drywall sheets (meets bottom plate), any recessed electrical boxes/lights and the chimney.

I'm pretty happy with the results, to me this is the worst case scenario… as more trim work and flooring go in it'll only get better. Maybe not super tight, but I don't want to mess with mechanical ventilation other than in the bathrooms and kitchen.
 

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The concerns about mechanical ventilation are way overwrought, an artifact of the ASHRAE 62.2 ventilation requirements (which has many well-argued detractors as being way too high). To hit ASHRAE 62.2 ventilation with "natural" air exchanges requires a house as leaky as stated, but it's a ventilation rate that will lead to drier-than-healthy air in winter, and is not necessary for good indoor air quality (unless people smoke, use hair sprays daily, and use aerosol disinfectants obsessively, etc.) Most people are smart enough to crack a window or turn on the exhaust fans if the air is getting stinky or stale.

Even if you went with mechanical ventilation, a single exhaust fan operating on a duty-cycle (or dehumiditstat) control is not expensive to install or operate and could meet the ASHRAE 62.2 requirement (if you actually LIKE it to be that dry indoors in winter!). In a tight/very-tight wood sheathed house it can be important to monitor the indoor humidity during cold weather, and ventilate if it creeps over 35%. Getting religion about running kitchen exhaust vents while cooking, and putting an vacancy sensor switch with a 15-30 minute time-out on the bathroom exhaust fan (manual-on, automatically turns off at a programmed preset 15-30 minutes after the last time it sensed an occupant) is usually good enough to keep odors & humidity down, and the indoor air quality high. The $30 Leviton IPP15 or IPV15 series are pretty good. Lutron's MS-OPS2 series is pretty good too, often available at box stores for under $25. (The MS-OPS2 can be switched for either occupancy or vacancy sensing modes.)

As a mechanical contractor building to code, those ventilation systems would be required in new houses built that tight, but there is no reason to actually operate them at the ASHRAE rate. Most people can figure out what works for them.
 
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