When is it time to *replace the boiler*

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Chefwong

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This is not the hot water tank forum, where generally you replace it when it's sprung a leak ;-)

I know I've been on borrowed time with our HW tank. If I had to guess, the boiler in this house has not been touched for the last 60 years since it was built. If I had to guess. It works fine....the circulating pump has been replaced. The shell of this beast has a spot with bondo and another spot that looks to be getting weak.

But she runs, heats the house , etc.

Walking blind here. Got what seems to be a good heating mechanic coming by next week to check out the site and estimate a replacement. Any tell-tale signs I should be looking for in a good heating mechanic.
 
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Dana

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After 60 years of service a cast iron boiler's steady state efficiency has dropped by 10 percent or more due to erosion/corrosion on the turbulence-inducing features on both the water & fire sides of the heat exchanger plates. If it's a gas-burner it probably started out at about 80% steady state efficiency in 1955, and is probably doing 70% (steady state) now.

If it's output is 3x your peak heating load or higher (a good bet, if the insulation, windows & air sealing of the house have been upgraded since 1955) it's as-used AFUE is a good 5-10% lower than it's steady state thermal efficiency. So you're probably looking at ~65% for as-used AFUE, possibly lower.

An oil burner that old would be doing even worse, due to the higher corrosion of oil exhaust vs. gas.

Short of a Manual-J, you can still reasonably size the boiler using fuel use against heating degree-days data and the boiler's nameplate efficiency (which would establish an upper bound.) If you have two actual (not estimated) recent gas meter readings with the exact dates, or oil use between two exact fill-up dates this winter, with your ZIP code we can calculate the implied heat load at the 99% outside design temp.

The AFUE testing assumes 1.7x oversizing, which is the upper limit of what you would want to install there. At 1.4x oversizing you would be 100% covered (with margin!) at the 100 year historical low temperatures, and even 1.25x oversizing would be more than adequate for managing recent years' polar vortex events. Given the age of your existing boiler and it's presumed lower-than-nameplate efficiency, you will still have margin if the output is sized exactly to the fuel-use calculated numbers.

So, what is your fuel use between date-X and date-y at some interval this past winter? (If you were in Cancun for 2 weeks with the thermostat set to 55F during that period, use a different period.)
 

Chefwong

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Heating Mechanic came and gone. He told me to call when when the boiler breaks down, but there was no need to replace it, as anything new would be comparable than my current unit.

He advised if I was spend about 2X of what the direct replacement would bem, he could replace it with a HE unit, but to recoup the cost vs. benefits, would be not worth the expense.
 

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Direct replacement? Why would anybody even suggest that?

That is nearly ALWAYS a mistake, preserving the errors of the past! The thing was probably way oversized in 1955, and by now the house has probably seen air-sealing, insulation, and window upgrades, making it even MORE oversized.

Did the guy run a combustion analysis to figure out it's raw combustion efficiency?

Did he run any type of heat load calculation?

What is the oversizing factor on the boiler?

A code-minimum efficiency boiler that is right-sized to the heating load would use substantially less fuel than a limping 3-4x oversized beast.

When fuel prices are low it may be true that keeping the thing until it breaks is the most cost-effective path, even if it's only running 60% efficiency, if all you are concerned about is net-present-value of future energy cost savings. But with a boiler that old it's good to have the replacement path figured out ahead of time, rather than having to make a snap decision during the coldest week in January.

Seriously, it takes very little effort to run a heat load calculation based on fuel-use and heating degree-days. It's not perfect, but it's a very clear stake in the ground, and will tell you the approximate oversizing factor of the old boiler. If it's way oversized (more than 1.7x), when the fateful day arrives when it's finally toast you can seize the opportunity moment to right-size it rather than replacing it with something similar. Rather than paying 2x for a high efficiency units, you'd pay something alike 0.9x what a direct replacement would be. You would have increased comfort and still save a double-digit percentage on annual fuel use.

So again, pick a recent gas billing period, tell us the amount of fuel used, the exact meter-reading dates, and your ZIP code (for more accurate weather data.)
 

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I am remodeling my basement in a historic (1923) home and I want to get a new boiler and have it installed in a new location. The existing boiler is a 28 year old cast iron Burnham and is located in the middle of the room feeding big old cast iron pipes. The pipes in the basement are going to get redone but I intend to reuse all the original cast iron radiators throughout, which have radiator covers. I got a bid from a guy who proposes to install a Triangle Tube Prestige Trimax Solo 110. I have no idea if that is the right size boiler. Can you please help me verify?

My zip code is 53704. I had already insulated the majority of the basement walls and I plan to insulate the attic with spray foam under the roof (where there is currently minimal fiberglass). I don't intend to insulate any of the walls. Thanks in advance for your help!

Last four readings were:
Reading Date 3/6/2015 Therms: 353 Heating Degree Days: 1,465
Reading Date: 2/6/2015 Therms: 309 Heating Degree Days: 1,307
Reading Date: 1/8/2015 Therms: 302 Heating Degree Days: 1,313
 

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Your boiler had (or had) a steady state combustion efficiency of 80%. It burned through 964 therms, 20% of which went up the flue, for a net of 0.8 x 964= 771 therms (77.1 MMBTU) of heat going into the heating system.

That was over 4079 HDD, or a ratio of 77.1/4079= 0.0189 MMBTU/HDD, or 18,900 BTU/ HDD.

In a 24 hour day, that's 788 BTU per degree-hour.

The 99% outside design temp for Madison is -6F, the presumptive balance point, is +65F, so that's a total of 71F heating degrees.

That implies a design heat load of 71F x 788 = 55,948 BTU/hr.

The Solo-60 can deliver that- it's good for about 57K in condensing mode, only 53K if it's running above the condensing temps, which isn't sufficient margin, but it's awfully close.

In a 1923 home you did the right thing by insulating & air sealing the basement, which probably cut the heat load by at least 10,000 BTU/hr. Air sealing and insulating the attic will also peel off a few thousand BTU/hr too, but maybe not as much- it depends a lot on just how air- leaky it is, and how lousy the fiberglass insulation installation was. It's probably worth doing blower-door & infra-red imaging directed air sealing on the place before picking a boiler. The Solo 110 will clearly get you there, but if we can better predict the reduction that you'll get with the sealed & insulated roof deck, we can see if that buys you enough margin. Ideally you'd be able to get the heat load down to the 45K range (or at least under 50K) to be able to run with the Solo-60, and you probably CAN. With a heat load of 45K @ -6F (633BTU/hr per heating degree) you'd be good down to -19F before it begins to lose ground, even running the Solo-60 at non-condensing temps.

How much roof air, how much attic-floor area, and how deep is the "minimal fiberglass"?

How much above-grade foundation is still uninsulated?
 

mattmeier

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There isn't any uninsulated above-grade foundation exposed. The grade comes right up to the top of the foundation believe it or not. So far I spray foamed about 70% of the sill plates and rim joists on top of the foundation. Inside the foundation itself has 2*4 framing with bat insulation but is not yet drywalled. I took it this far a couple years ago so its largely reflected in the energy usage I provided.

The under side of the roof in the attic has old (20 - 30 years?) R-19 paper faced bat insulation in poor condition. There are a couple roof vents but no soffit or roof vents. Based on ice dams I belive air infiltration is a problem through the fiberglass so I plan to remove the fiberglass and spray foam the under side of the roof. The roof is a steep pitch and I have a partially finished attic but most of the roof is accessible above the finished space and behind knee walls. The house is about 950 S.F. on the first floor, 800 SF on the second floor and roughly 500 SF of finished space in the attic for a total of 2250 S.F. above grade. The finished basement is/will be about 950 S.F., same as the first floor.

Correction from prior post: The boiler was made in 1983 so 32 years old. The plate says DOE HTG CAP BTU/HR: 136,000. IBR BTU/HR: 118,300. Input BTU/HR: 164,000
 

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Dana

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There isn't any uninsulated above-grade foundation exposed. The grade comes right up to the top of the foundation believe it or not. So far I spray foamed about 70% of the sill plates and rim joists on top of the foundation. Inside the foundation itself has 2*4 framing with bat insulation but is not yet drywalled. I took it this far a couple years ago so its largely reflected in the energy usage I provided.

The under side of the roof in the attic has old (20 - 30 years?) R-19 paper faced bat insulation in poor condition. There are a couple roof vents but no soffit or roof vents. Based on ice dams I belive air infiltration is a problem through the fiberglass so I plan to remove the fiberglass and spray foam the under side of the roof. The roof is a steep pitch and I have a partially finished attic but most of the roof is accessible above the finished space and behind knee walls. The house is about 950 S.F. on the first floor, 800 SF on the second floor and roughly 500 SF of finished space in the attic for a total of 2250 S.F. above grade. The finished basement is/will be about 950 S.F., same as the first floor.

Correction from prior post: The boiler was made in 1983 so 32 years old. The plate says DOE HTG CAP BTU/HR: 136,000. IBR BTU/HR: 118,300. Input BTU/HR: 164,000


The boiler's steady state efficiency boiler is 136,000/164,000 = 83%, not 80% as estimated previously. That raises the implied heat load to 55,948 x 83/80= 58,046 BTU/hr

Crappy low density R19s are nearly worthless as-installed, so you're probably looking at a U-factor no better than U0.1, which means that at an interior design temp of 70F and an outdoor temp of -6F, every square foot of roof area has a heat loss of about U0.1 x 7.6 BTU/hr, not counting the infiltration losses. You're probably looking at about 1100-1200' of roof area, so that's about 8000-9000 BTU/hr + infiltration losses, call it 12K.

If you spray use foam on the underside of the roof deck it MUST be closed cell only, applied in lifts of no more than 2" at a time. At 2" closed cell foam is a Class-II vapor retarder at about 0.5-0.7 perms, which is sufficiently vapor tight to prevent high moisture build-up in the roof deck in winter, but sufficiently vapor open that the roof deck can still dry. If you go with 4" foam it becomes even more vapor tight, which isn't always a good thing, since the roofing above & foam below becomes effectively a moisture trap. Since the foam is being thermally bridged by the rafters it's effectiveness is being undercut too, so as a general rule it's better to stop the foam at 2", and fill out the remainder of the rafter depth with high density fiber such as rock wool, or high density fiberglass. If the rafters are milled 2x6 (5.5" deep) R15 batts would fill that out nicely if you trim it carefully to fit with no compressions, waves or voids. (A batt knife is a cheap & useful tool here, but an 8" bread knife works about as well. The "whole assembly" thermal performance difference between a 2" foam + 3.5" rock wool stackup and 5.5" of closed cell foam is less than R2 after factoring in the thermal bridging of the rafters. If the rafters are deeper than that, go with whatever it takes for a full fill of fiber.

Then, to avoid condensation issues at the foam/fiber boundary it's worth installing a smart vapor retarder like Certainteed MemBrain. You can get it from Menards- if they don't stock it at the local store, you can buy it from their online store for about $100 for an 800 square foot sheet/roll, shipped to your home. That's about 2x as expensive as 6 mil polyethylene vapor barrier, but unlike polyethylene it won't create a moisture trap. During the winter when the air is dry it's vapor retardency drops to less than 1 perm (a Class-II vapor retarder), but if any significant moisture accumulates or leaks in raising the relative humidity of the entrained air in the cavity it causes the vapor permeance to rise quickly to over 5 perms or higher (Class-III vapor retardency) which gives it a very reasonable drying rate. The combined effect of slow moisture accumulation and fast drying keeps you from ending up with wet fiber insulation in the dead of winter, even if the fiber-R is many times that of the foam-R.

The thermal bridging of the rafters will continue to be a factor in ice damming. One way to deal with that is to rip 2" wide strips of 1.5-2" rigid foam and glue it to the rafter edges, then insulate between the foam strips with cheap fiber such as split R15 batts. Full sheets of foam covering the insulation between the rafters aren't a good idea here, since the vapor retardency of the foam impedes drying, creating a potential moisture trap. In this stackup, put the MemBrain between the foam strips and the rafters- if installed on the conditioned space side of the foam thermal breaks it's far more likely to tear, bringing the potential for air transported moisture getting in.

Assuming you have 2" of ccSPF, 5.5" of rock wool, and 2" of rigid polyiso as a thermal break over the rafter edges you would have a center-cavity R of about R36-R38, but with the R9 rafter edge breaks you end up with a "whole-assembly" R of about R34-R35 due to the thermal bridging of the rafters, which is a U-factor of about U0.28-U0.03, which is a 70% reduction in the conducted losses out of the roof. That means the 8K-9K + infiltration losses are now reduced to ~2.5K and a dramatically reduced infiltration loss, peeling at lest 5500 BTU/hr off the heat load, but probably not more than 10K.

Which means the Solo-60 could still be a bit marginal since the heat load is still probably over 50K, but it also means the Solo-110 is 2x oversized, with a min-fire output of ~28-29K, more than half the design load, which means it won't modulate much except during the coldest weeks of winter. A better fit would be a mod-con with 65-70KBTU/hr of DOE output, something like the Burnham ALP-080 or similar, which has a min-fire output of about 15-16K, which is less than 1/3 of your design heat load, and less than your average winter load. That means it can be tweaked to where it modulates MOST of the time, and only cycles on/off a lot during the shoulder seasons, and will have a much lower propensity to short-cycle at very low water temps (lower temps= higher condensing efficiency.) Another good fit would be the Lochinvar KBN081 which also modulates down to ~15K out, but like the ALP-080 can still deliver ~70K at non-condensing temps if needed.
 

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Similar question. I'm in the process of replacing a 25-year-old WM GV Gold boiler with 140,000 btu heat input and trying to decide between a Solo60 or Solo110 (the choices available via my preferred contractor). The current boiler is obviously oversized and cycles like crazy. We have cast iron radiators upstairs on one zone and a second zone comprised of a small run of fin tube in half the basement which is finished.

The house is a 1940 typical New England center chimney cape in Maine (04105). We have blown-in cellulose in the walls and ceiling and full replacement windows and will be doing some additional air sealing. In addition to heat the boiler will supply a Smart30 indirect HW tank. We are currently on propane (converting to natgas). Recent fuel deliveries for this winter: March 23, 101 gallons; Feb 26, 110 gallons; Feb 4, 84 gallons; Jan 14, 99 gallons; Dec 31, 140 gallons; Dec 5, 138 gallons; Oct 23, 107 gallons. (Includes cooking and hot water via the already-installed Smart30.)

Is that enough info for you to provide an opinion on the 60 vs. 110? The 110 is the contractor's preference. If I want to go with the 60 I'll have to push him very hard and give him a good justification. In the alternative, I have an energy audit scheduled and will probably get a decent heat loss calculation before I have to decide, but your opinion would certainly be helpful.

Thanks in advance.
 

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Similar question. I'm in the process of replacing a 25-year-old WM GV Gold boiler with 140,000 btu heat input and trying to decide between a Solo60 or Solo110 (the choices available via my preferred contractor). The current boiler is obviously oversized and cycles like crazy. We have cast iron radiators upstairs on one zone and a second zone comprised of a small run of fin tube in half the basement which is finished.

The house is a 1940 typical New England center chimney cape in Maine (04105). We have blown-in cellulose in the walls and ceiling and full replacement windows and will be doing some additional air sealing. In addition to heat the boiler will supply a Smart30 indirect HW tank. We are currently on propane (converting to natgas). Recent fuel deliveries for this winter: March 23, 101 gallons; Feb 26, 110 gallons; Feb 4, 84 gallons; Jan 14, 99 gallons; Dec 31, 140 gallons; Dec 5, 138 gallons; Oct 23, 107 gallons. (Includes cooking and hot water via the already-installed Smart30.)

Is that enough info for you to provide an opinion on the 60 vs. 110? The 110 is the contractor's preference. If I want to go with the 60 I'll have to push him very hard and give him a good justification. In the alternative, I have an energy audit scheduled and will probably get a decent heat loss calculation before I have to decide, but your opinion would certainly be helpful.

Thanks in advance.

Without even doing the math I can tell you the -110 is ridiculously oversized for the load, but lets do the math anyway.

Between 5 December 5 and 23 March you used 140 + 99 + 84 + 110 + 101 = 534 gallons of propane. A gallon of propane has 91,500 BTU of source fuel content, so that's a total of 48,861,000 BTU.

Using the Portland ME airport's weather station as source for the base-65 heating degree day data from degreedays.net, from 6 December through 23 March there were 4361 heating degree days.

That's a ratio of 11,204 BTU / HDD, or (/24 =) 467 BTU per degree-hour.

Portland's 99% outside design temp is +2F, which is 63F heating degrees below the presumptive base 65F heating/cooling balance point temperature.

63 heating degrees x 467 BTU/degree-hour = 29,421 BTU/hr (source fuel) Assuming 85% efficiency for the GV Gold (probably optimistic- I didn't bother to look it up) that implies a heat load of about 0.85 x 29,421= ~25,000 BTU/hr

That is 25% of the output of the Solo-110, and is even below it's minimum-firing rate output, so it would be INSANE to install the -110 there.

Things that could skew this calculation are issues such as being away on vacation for weeks/months during that period with the thermostat turned down to 50F, using a wood stove or pellet stove as auxilliary heating to save on propane, keeping the house at 60F and just bundling up to stay warm, etc. But even with some of those factors it's unlikely to change the boiler choice.

The Solo-60 puts out over 50,000 BTU/hr even at high water temps well above the condensing zone, or about 2x the heat load implied by the fuel use. That means you'd be good down to about -60F for an outdoor temp, a temp probably not seen in coastal ME since the last ice age.

A typical 0F heat load ratio for an insulated 2x4 framed house with wood siding, & insulated windows (or tight clear storms over wood-sash double hungs) is about 15BTU/hr per square foot of conditioned space. With a high window area/floor area ratio it can be over 20BTU/ft-hr, but it's almost never below 12 BTU/ft-hr. So for crude rule-of-thumb sanity checking the implied heat load indicates a house between ~1200'- 2000' of floor area (also pretty typical for a 1940s vintage cape.)

How much (and what type) radiation, divided into how many zones? (If multi-zone, spell out the radiation per zone.)
 

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Wow, seems like you really know your stuff and I certainly appreciate your time. I would like to clarify your suggested insulation stack. First, I assume any vents would get sealed during the initial spray foam application? The MemBrane would be installed over the rockwool and fastened directly to the edges of the rafters. Then add 2" polyiso over the top of the membrane to the rafter edges and then add additional rockwool between the polyiso strips on the outside of the membrane?

Also, I saw one of your other threads about micro zones. Right now the 1st and second floor are all one zone - mostly cast iron radiators. The uninsulated cast iron pipes seem to heat the basement just fine right now but those would get removed. Installer proposes up to 45' of baseboard in the basement. Ideally I would have one zone per floor including the basement but it sounds from your other thread that you would be concerned with the baseboards causing the short cycles if it were the only zone calling for heat. Do you recommend two or three panel radiators in my case in lieu of baseboards?
 

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Math is not my strong suit, so thanks for that. Here's some more info...

We were at home during the entire winter period but typically keep the thermostat set in the low- to mid-60s -- not bundled up per se but that's where we're comfortable.

There is a combined total of 9 cast iron radiators on the first and second floor with a total of about 160 individual sections. Main radiators are 22 inches tall with smaller ones in the 2 bathrooms. All the radiators are on a single zone. We generally have the upstairs radiators turned down. In other words, even without doing any math it's pretty obvious we have more than enough radiation to keep the house comfortable.

We have a second zone heating a finished basement room with 15 feet or so of fin tube baseboard. It is used relatively infrequently.

Your floor space estimate is right on the money -- the house has 1400 square feet of living space according to my mortgage appraisal.
 

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Wow, seems like you really know your stuff and I certainly appreciate your time. I would like to clarify your suggested insulation stack. First, I assume any vents would get sealed during the initial spray foam application? The MemBrane would be installed over the rockwool and fastened directly to the edges of the rafters. Then add 2" polyiso over the top of the membrane to the rafter edges and then add additional rockwool between the polyiso strips on the outside of the membrane?

Also, I saw one of your other threads about micro zones. Right now the 1st and second floor are all one zone - mostly cast iron radiators. The uninsulated cast iron pipes seem to heat the basement just fine right now but those would get removed. Installer proposes up to 45' of baseboard in the basement. Ideally I would have one zone per floor including the basement but it sounds from your other thread that you would be concerned with the baseboards causing the short cycles if it were the only zone calling for heat. Do you recommend two or three panel radiators in my case in lieu of baseboards?

You have the roof insulation stackup right:

*Shingles

*Roofing felt
^^^^^^^^^ about 0.1 per vapor retardency above the roof deck- no drying to the exterior, expecially when covered with snow or raining!
*Roof deck

*2" ccSPF (~0.6 perm vapor retardency) | 2" of rafter depth

*xx inches of rock wool | xx inches of rafter depth

*Certainteed MemBrain stapled/lapped and sealed (variable vapor permenace, less than 1-perm during dry winter air, but about 10 perms at 75% RH.)

*yy inches of rock wool (or fiberglass) | yy inches of rigid polyiso (go for at least 1.5" of depth, which is R9- the same R-value of 7.5" of hemlock rafter)

*1/2" wallboard (unpainted, or latex only if painted)- long screwed or ring-shank nailed through to the rafters 12"-16" o.c.

The foam thermal break on the rafters needs to be at least 1/2" wider than the rafter edge to get the majority of the benefit, so 2-2.5" wide would be about right for a milled rafter with a 1.5" nominal width. It means a bit of width trimming to get the split batts to fit between the foam strips without compression, but that's pretty quick with an insulation knife or bread knife. It's fine to use split R13 fiberglass for the bottom part if it's easier for you to split them easier, but you'll find splitting R15 rock wool batts evenly isn't really that tough. Batts designed for 2x4 cavities are 3.5" deep, so if you use 1.5"polyiso for the thermal break the split doesn't have to be absolutely perfect- it'll be a compression fit.

If you use 2" polyiso (R12-R13) for the thermal break it's better to go with low-density R19s, since 1/2 x 3.5" isn't a full depth fill, but a compressed 1/2 x 5.5" split batt would still fill the space completely.

With 45' of baseboard in the basement zone and the Solo-110, at ~28-29,000 BTU/hr of output at minimum fire that's 625-650 BTU/hr per foot, which balances at an output temp of about 190F, well above the condensing temperature zone (130F AWT or lower). If you tried to run the -110 at condensing temps it would short-cycle like crazy on that little radiation- you might as well go with a cast-iron boiler if you need water that hot to keep it from short cycling on zone calls.

With the ALP-080 or Sol0-60's 15-16,000 BTU/hr output you're talking 350BTU/hr per foot, which balances at about 140F AWT. You can probably work with that, but you'd still be better off with 60-70' of fin tube baseboard, even with the smaller boiler if you hope to be able to get condensing efficiency out of it without short-cycling.

Panel radiators are certainly more comfortable than baseboard and have more thermal mass to work with, but are substantially more expensive. Unless you're planning to hang out in the basement a lot, it's cheaper to just run more baseboard. If going with panel radiators you'll need the cumulative 180F-output rating on the order of 30,000 BTU/hr to not short-cycle the smaller boiler, twice that for the Solo-110. That would be on about a $1000 cost-adder over the cost of 60'-70' of baseboard ($11-12/ft typical) for the smaller boiler case.
 

Dana

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Math is not my strong suit, so thanks for that. Here's some more info...

We were at home during the entire winter period but typically keep the thermostat set in the low- to mid-60s -- not bundled up per se but that's where we're comfortable.

There is a combined total of 9 cast iron radiators on the first and second floor with a total of about 160 individual sections. Main radiators are 22 inches tall with smaller ones in the 2 bathrooms. All the radiators are on a single zone. We generally have the upstairs radiators turned down. In other words, even without doing any math it's pretty obvious we have more than enough radiation to keep the house comfortable.

We have a second zone heating a finished basement room with 15 feet or so of fin tube baseboard. It is used relatively infrequently.

Your floor space estimate is right on the money -- the house has 1400 square feet of living space according to my mortgage appraisal.

The thermal mass of the cast iron and water volume of the rads definitely help the short-cycling issues. Nothing is going to keep it from short-cycling on the basement zone though. Even at 180F AWT (190F water out, 170F back) 15' of baseboard is emitting barely half the min-fire output of the Solo-60, and at 130F AWT (beginning of condensing, about 88% combustion efficiency assuming 125F return water) it's only emitting about 1/4 of the min-fire output. As long as the main zone is calling for heat when you're running the basement zone it'll be fine though, and if you rarely run the basement zone on it's own it's a "who cares?" sort of deal.

Yours is DEFINITELY a Solo-60 sized house- don't even THINK about dropping a Solo-110 in there, if those are your choices. But you probably have sufficient thermal mass & radiation on the main zone to run at temps well into the condensing zone.

Care to estimate the EDR per section of your radiator type(s)? (See Page 7 of this also.) There are 22" tall rads 10" deep, and 22" rads that are only 3" deep, so the type kinda matters.

A somewhat typical case might be a 5" deep Arco Sunrad or Burnham Radiant (popular under-window or recessed kneewall radiators of the era) at about 2.25' per section, so for 160 sections of that is 360'. At 130F AWT radiators emit about 70BTU per EDR sq. ft. , so that's 70BTU/hr x 360' 25,000 BTU/hr, which would be GREAT for the Solo-60, since it could be in condensing mode even during design conditions, and DEEP into condensing at the average winter temps!

It's marginal but still OK (due to the high thermal mass) for the ridiculously oversized Solo-110. With that radiation you'd be in the mid-90s for efficiency with either, but the Solo-110 would be cycling on/off, never modulating, with more wear & tear on the boiler due to the greater numbers of ignition cycles. Seriously- there is no point to a modulating boiler that never modulates, and that's what you would have with the 110.

Example pics of some 20" x 5" rads:

Sunrad:
DSC01310.JPG



Radiant:
4sectsunrad.JPG


If the foundation isn't insulated (even in the unheated part), it's worth doing that at some point. It'll raise the average floor temp slightly on the first floor, and it may make running the basement zone unnecessary.
 

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The thermal mass of the cast iron and water volume of the rads definitely help the short-cycling issues. Nothing is going to keep it from short-cycling on the basement zone though. Even at 180F AWT (190F water out, 170F back) 15' of baseboard is emitting barely half the min-fire output of the Solo-60, and at 130F AWT (beginning of condensing, about 88% combustion efficiency assuming 125F return water) it's only emitting about 1/4 of the min-fire output. As long as the main zone is calling for heat when you're running the basement zone it'll be fine though, and if you rarely run the basement zone on it's own it's a "who cares?" sort of deal.

Yours is DEFINITELY a Solo-60 sized house- don't even THINK about dropping a Solo-110 in there, if those are your choices. But you probably have sufficient thermal mass & radiation on the main zone to run at temps well into the condensing zone.

Care to estimate the EDR per section of your radiator type(s)? (See Page 7 of this also.) There are 22" tall rads 10" deep, and 22" rads that are only 3" deep, so the type kinda matters.

A somewhat typical case might be a 5" deep Arco Sunrad or Burnham Radiant (popular under-window or recessed kneewall radiators of the era) at about 2.25' per section, so for 160 sections of that is 360'. At 130F AWT radiators emit about 70BTU per EDR sq. ft. , so that's 70BTU/hr x 360' 25,000 BTU/hr, which would be GREAT for the Solo-60, since it could be in condensing mode even during design conditions, and DEEP into condensing at the average winter temps!

It's marginal but still OK (due to the high thermal mass) for the ridiculously oversized Solo-110. With that radiation you'd be in the mid-90s for efficiency with either, but the Solo-110 would be cycling on/off, never modulating, with more wear & tear on the boiler due to the greater numbers of ignition cycles. Seriously- there is no point to a modulating boiler that never modulates, and that's what you would have with the 110.

Example pics of some 20" x 5" rads:

Sunrad:
DSC01310.JPG



Radiant:
4sectsunrad.JPG


If the foundation isn't insulated (even in the unheated part), it's worth doing that at some point. It'll raise the average floor temp slightly on the first floor, and it may make running the basement zone unnecessary.
 

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The type of radiator is the one of the left in the attached image and has 5 tubes (I think they're tubes, not columns, right?). They are painted but it looks like they're stamped "US" on the end plugs. According to the Columbia chart the EDR looks like it would be 2.67. However, the radiators look similar to the "Slenderized" ones that Burnham says have an EDR 0f 3.00 (for 23-inch radiator as measured from the floor).

Per the Columbia Heating formula my largest radiator comes in at about 9,500 (2.67 X 21 sections X 170). That would give me more than 38,000 Btu/hr just on the first floor, and slightly less than that on the second floor, so about 75,000 to 80,000 Btu/hr of total radiator capacity depending on which EDR I use. Does that make sense?

DOE heating capacity of existing boiler is 125 MBH. I can easily tell by how much it cycles that it's way oversized. Are there any circumstances where you'd actually want a boiler that has a significantly higher output than what the radiators can even handle? (Assuming an efficient indirect DHW tank.)

Do I need to be overly concerned about pickup demand? My contractor mentioned something about being able to get the house warm on a cold day after a power outage, etc. Seems like planning for such a rare event is overkill.

And to make sure I understand correctly, the heating loss calculation you did represents average winter heat loss (which would mean design day load is higher)? But that it included fuel use related to cooking and DHW load (meaning that actual heat load is lower)?
 

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Dana

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Radiator output at condensing temps is not 170BTU/ft^2-hr. That is the output at 180F average water temps. Looking at the nomograph on p.2 of the Columbia sizing document, the output at 130F AWT (the highest end of the condensing zone, where it's doing about 90% efficiency) is about 70 BTU/ft^2-hr.

So, if it has 75,000 BTU/hr of radiator output at 180F AWT, it has about 75,000 x 70/170= 30,900 BTU/hr of radiator output capacity at 130F AWT (not too far off my WAG example case.) That's still higher than your heat load.

About that heat load...

If the thermostat is set to 60-63 F you can't use base-65F heating degree days. If nobody was living there and there were NO plug-loads running in the houses (no refrigerators, DVRs, lights, etc.) it might be appropriate to use base-60F, but lets worst-case it using 55F.

Using base-55F out to 3381 HDD between 6-December and 23 March, over which you used 48,861,000 BTU of propane (source). Assuming 85% efficiency that's 41,531,850 BTU that went into the heating system, the rest went up the flue.

That's a ratio of 41,531,850 / 3381= 12, 284 BTU/HDD, or (/24= ) 512 BTU per degree-hour.

At 55F balance point and an outside design temp of +2F that's 53 heating degrees, for an implied heat load of 53 x 512 BTU/hr 28,160 BTU per/hour when the interior temp is 62F, which would be 60F heating degrees.

Without modeling it more accurately, using the same 512BTU/hr, if you raise the interior temp to 70F that adds 8 heating-degrees, or 8F x 512= 4096 BTU/hr to the heat load, bringing the total heat load up to 28,160 + 4096= 32, 256 BTU/hr @ 70F indoors, +2F outdoors.

That's 29% higher than the previous 25,000 BTU/hr fuel-use calculated estimate, but actually a worst-case number- worse than reality, since with a 60-62F interior temp your heating/cooling balance point is probably higher than 55F. It's close enough to know that the Solo-110 is more than 3x oversized, and would (almost) never modulate, and would not be capable of tracking the heat load with nearly continuous burns the way the Solo-60 would. This is still WELL within the output of the Solo-60, though if you kept the house at 70F you might be slightly above the condensing zone on design day with your radiation (or would have to tweak the outdoor reset curves pretty carefully to keep it in the condensing zone.)

It's also close enough to reality to make me guess that your foundation is probably not insulated(?) and /or the house is leaking substantial amounts of air, since at 1400' of house that's a heat load / conditioned space ratio of more than 20 BTU/hr-ft^2. Taking off the Jr. Heating Designer hat and donning the building-efficiency nerd chapeau, it seems highly likely that there is still some cost-effective insulation & air sealing remediation possible on this house.
 

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Radiator output at condensing temps is not 170BTU/ft^2-hr. That is the output at 180F average water temps. Looking at the nomograph on p.2 of the Columbia sizing document, the output at 130F AWT (the highest end of the condensing zone, where it's doing about 90% efficiency) is about 70 BTU/ft^2-hr.

So, if it has 75,000 BTU/hr of radiator output at 180F AWT, it has about 75,000 x 70/170= 30,900 BTU/hr of radiator output capacity at 130F AWT (not too far off my WAG example case.) That's still higher than your heat load.

About that heat load...

If the thermostat is set to 60-63 F you can't use base-65F heating degree days. If nobody was living there and there were NO plug-loads running in the houses (no refrigerators, DVRs, lights, etc.) it might be appropriate to use base-60F, but lets worst-case it using 55F.

Using base-55F out to 3381 HDD between 6-December and 23 March, over which you used 48,861,000 BTU of propane (source). Assuming 85% efficiency that's 41,531,850 BTU that went into the heating system, the rest went up the flue.

That's a ratio of 41,531,850 / 3381= 12, 284 BTU/HDD, or (/24= ) 512 BTU per degree-hour.

At 55F balance point and an outside design temp of +2F that's 53 heating degrees, for an implied heat load of 53 x 512 BTU/hr 28,160 BTU per/hour when the interior temp is 62F, which would be 60F heating degrees.

Without modeling it more accurately, using the same 512BTU/hr, if you raise the interior temp to 70F that adds 8 heating-degrees, or 8F x 512= 4096 BTU/hr to the heat load, bringing the total heat load up to 28,160 + 4096= 32, 256 BTU/hr @ 70F indoors, +2F outdoors.

That's 29% higher than the previous 25,000 BTU/hr fuel-use calculated estimate, but actually a worst-case number- worse than reality, since with a 60-62F interior temp your heating/cooling balance point is probably higher than 55F. It's close enough to know that the Solo-110 is more than 3x oversized, and would (almost) never modulate, and would not be capable of tracking the heat load with nearly continuous burns the way the Solo-60 would. This is still WELL within the output of the Solo-60, though if you kept the house at 70F you might be slightly above the condensing zone on design day with your radiation (or would have to tweak the outdoor reset curves pretty carefully to keep it in the condensing zone.)

It's also close enough to reality to make me guess that your foundation is probably not insulated(?) and /or the house is leaking substantial amounts of air, since at 1400' of house that's a heat load / conditioned space ratio of more than 20 BTU/hr-ft^2. Taking off the Jr. Heating Designer hat and donning the building-efficiency nerd chapeau, it seems highly likely that there is still some cost-effective insulation & air sealing remediation possible on this house.

Wow Dana -- You are an excellent teacher. Thanks so much for taking the time on this. Your guess is again correct. We have blown in cellulose in the attic and walls and replacement windows, but the foundation/basement is not insulated. We don't have air infiltration to the point it's drafty but there are some obvious candidates for additional insulation and air sealing. We're having an energy audit next week and we have some pretty good rebates available.

I'm still unclear about one thing: my total fuel use of 534 gallons propane between Dec 5 and March 23 includes cooking and domestic hot water (via an indirect tank). Cooking is probably insignificant in the scheme of things but is hot water significant-enough to affect the calculation?
 

Dana

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Hot water use may be as much as 15-25% of the annual fuel use, but it's not going to affect the wintertime numbers enough to matter. The other uses for that fuel DO upsize the implied load number by a few percent for a winter period. But then again, so does the assumption that a 25 year old ~4x oversized cast iron boiler actually delivers 85% efficiency. AFUE testing presumes 1.7x oversizing, which has a duty cycle high enough that the boiler's average efficiency is close to it's raw combustion efficiency. At 4x oversizing it's operating at less than half the presumptive duty cycle, and that low duty cycle comes with higher standby losses.

But the amount of oversizing of the implied load from those assumptions isn't going to affect the boiler choice much, if at all, and only if the more carefully done fuel-use numbers fall too close to the maximum-fire output capacity of the boiler, which is exactly maxmeier's situation. In his case the Solo-60 might leave him cold on design-day even after his planned round of weatherization, but bumping up to an ~80,000 BTU-in/70K-out boiler gives him a bit of margin even for the pre-weatherization load, but without the potential loss in modulation that he would suffer by going with something as oversized as the Solo-110.
 
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