Heat Loss Calculation

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John Molyneux

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In an earlier thread you guys helped me get my head around how to arm myself with enough good data to argue against what seems to be a common inclination for heating contractors to want to install oversized equipment. I just wanted to follow up for the benefit of others to reinforce that using actual energy use versus seasonal heating degree days seems to be a pretty good way of getting to a heat loss number that is at least accurate enough for a decision about whether you need a 50-, 80-, or 100,000 btu/hr boiler.

Our 1400 sq/ft New England cape is currently heated with a 125,000 (DOE) btu/hr hot water boiler feeding cast iron radiators. Existing radiator capacity is approx. 80,000 btu/hr at 180F AWT. The boiler's obviously way too big based on how much it cycles.

Dana's initial estimate based on my actual fuel use per heating-degree-day this past winter was 25,000 btu/hr. We adjusted this up to 32,000 when he found out I had kept the house in the low- to mid-60's during the measurement period. I subsequently re-ran the calc with a range of assumptions and got very comfortable that the btu/hdd formula put me in the 30,000 btu/hr range. Then I ran the SlantFin software (which I'm led to believe tends to overstate), using some very conservative assumptions, and came up with 36,000, just to give myself an upper bookend.

I just got the results of an independent energy audit and their professional opinion is .... 29,000 btu/hr. (They think they can drive that down to 20,000 btu/hr with a variety of air sealing and insulation, much of which won't cost me anything thanks to the existing state and utility rebate programs here in Maine.)

So at my implied heating loss of 450 btu/degree-hour, I only needed 27,000 btu/hr on our coldest winter day this year, and only needed around 20,000 btu/hr on average over the period December-March. Armed with that info, I can now have an informed discussion with my heating contractor why he was recommending a Solo 110 that can only modulate down to 30,000 btu/hr rather than the Solo 60 that modulates from 16,000 up to 60,000!

So thanks to all.
 
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Dana

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A fuel use/HDD calc is far from a WAG, it's a measurement, using the heating equipment as the measuring instrument!

If keeping the house at 68-72F (the original presumption), using base 65F is fine.

But since you kept it in the low 60s, choosing base 55F as the HDD base temperature to figure out a BTU/hr per degree-F and projecting that forward for some higher interior temp it puts a hard upper bound on it, since it's intentionally overstating reality.

Fuel use against HDD becomes much less accurate when using shoulder season fuel use periods, since the lower heat loads of spring/fall mean the hot water use and passive solar gains become larger fractions of the total heat inputs/outputs, injecting more uncertainty & error.

I'm not surprised that the Manual-J came in at about 29K, which pretty much splits the difference between what the original & obvious under-shoot number was when using base 65F, and the obvious (and intentional) overshoot established using 55F as the temperature base. The Slant-Fin tool is a straight-ahead I=B=R methods tool, which doesn't account for internal heat sources (not even jacket losses on the boiler that get built into the AFUE numbers). So it's not surprising that it came in 24% higher than the Manual-J.

Beating the design load down to 20K from 29K with retrofit weatherization & insulation is a credible number for a 1400' 2x4 framed house, and could even be cost effective without subsidy depending on what they found, but with subsidies it's pretty much a no brainer.

There are other boilers that modulate lower than the Solo 60, but it's a matter of finding local installers & support for those boilers. If your heating contractor who is comfortable with Triangle Tube is otherwise capable (and just doesn't do heat loads), the Solo 60 isn't a terrible mis-match, since it's min-fire output isn't too far away from your average winter load. It'll be a bit oversized for the average mid-winter load of the "after" picture, but IIRC you have more than sufficient radiation & thermal mass- it'll won't short-cycle unless you micro-zone the place to death.

I'm hoping the heat load education isn't lost on your heating contractor. He has probably been similarly oversizing other projects "just to be sure", but real heat loads are nowhere near what the traditional xx BTU/hr-ft^2 rule-of-thumb guys have been using. It's not that rules of thumb are bad, it's just that have been using really FAT thumbs. Real heat loads on most reasonably tight 2x4 construction come in at about 15 BTU/hr-ft^2 @ 0F, give or take, which is about where you'll be in the "after" picture of the planned weatherization. I've yet to encounter a rule-of-thumb HVAC contractor in New England using any thing less than 25 BTU/hr-ft^2, which would have oversized it 25% from your "before" picture. Many are using 35 BTU/hr-ft^2, which STILL would have put you in a Solo-60, not a -110. I wonder what this contractor was thinking- not even a 1400' uninsulated tar-papered shack would require the full output of the Solo-110, and would probably still be within Solo-60 range.

A 20 BTU rule of thumb for older construction, or 15 BTU for new construction would (almost) never leave the client cold. But any HVAC contractor should just bite the bullet on the price of a Manual-J software package and learn how to use it (without applying thumbs on the inputs, such as ridiculously low outside design temps, or overstating window U-factors, etc.). By the time they've done a half-dozen Manual-J heat load calculations using accurate inputs their assumptions about the size of real heat loads will be re-adjusted forever. Even without running the numbers they would then be able to look at a 1400' house and know right away that it won't need anything like a Solo-110, and they might start looking for even smaller modulating equipment than the Solo-60, since many smaller houses really don't need a Solo-60 either.
 

John Molyneux

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A fuel use/HDD calc is far from a WAG, it's a measurement, using the heating equipment as the measuring instrument!

If keeping the house at 68-72F (the original presumption), using base 65F is fine.

But since you kept it in the low 60s, choosing base 55F as the HDD base temperature to figure out a BTU/hr per degree-F and projecting that forward for some higher interior temp it puts a hard upper bound on it, since it's intentionally overstating reality.

Fuel use against HDD becomes much less accurate when using shoulder season fuel use periods, since the lower heat loads of spring/fall mean the hot water use and passive solar gains become larger fractions of the total heat inputs/outputs, injecting more uncertainty & error.

I'm not surprised that the Manual-J came in at about 29K, which pretty much splits the difference between what the original & obvious under-shoot number was when using base 65F, and the obvious (and intentional) overshoot established using 55F as the temperature base. The Slant-Fin tool is a straight-ahead I=B=R methods tool, which doesn't account for internal heat sources (not even jacket losses on the boiler that get built into the AFUE numbers). So it's not surprising that it came in 24% higher than the Manual-J.

Beating the design load down to 20K from 29K with retrofit weatherization & insulation is a credible number for a 1400' 2x4 framed house, and could even be cost effective without subsidy depending on what they found, but with subsidies it's pretty much a no brainer.

There are other boilers that modulate lower than the Solo 60, but it's a matter of finding local installers & support for those boilers. If your heating contractor who is comfortable with Triangle Tube is otherwise capable (and just doesn't do heat loads), the Solo 60 isn't a terrible mis-match, since it's min-fire output isn't too far away from your average winter load. It'll be a bit oversized for the average mid-winter load of the "after" picture, but IIRC you have more than sufficient radiation & thermal mass- it'll won't short-cycle unless you micro-zone the place to death.

I'm hoping the heat load education isn't lost on your heating contractor. He has probably been similarly oversizing other projects "just to be sure", but real heat loads are nowhere near what the traditional xx BTU/hr-ft^2 rule-of-thumb guys have been using. It's not that rules of thumb are bad, it's just that have been using really FAT thumbs. Real heat loads on most reasonably tight 2x4 construction come in at about 15 BTU/hr-ft^2 @ 0F, give or take, which is about where you'll be in the "after" picture of the planned weatherization. I've yet to encounter a rule-of-thumb HVAC contractor in New England using any thing less than 25 BTU/hr-ft^2, which would have oversized it 25% from your "before" picture. Many are using 35 BTU/hr-ft^2, which STILL would have put you in a Solo-60, not a -110. I wonder what this contractor was thinking- not even a 1400' uninsulated tar-papered shack would require the full output of the Solo-110, and would probably still be within Solo-60 range.

A 20 BTU rule of thumb for older construction, or 15 BTU for new construction would (almost) never leave the client cold. But any HVAC contractor should just bite the bullet on the price of a Manual-J software package and learn how to use it (without applying thumbs on the inputs, such as ridiculously low outside design temps, or overstating window U-factors, etc.). By the time they've done a half-dozen Manual-J heat load calculations using accurate inputs their assumptions about the size of real heat loads will be re-adjusted forever. Even without running the numbers they would then be able to look at a 1400' house and know right away that it won't need anything like a Solo-110, and they might start looking for even smaller modulating equipment than the Solo-60, since many smaller houses really don't need a Solo-60 either.

Unfortunately, I'm not sure I really educated this particular contractor. He kept pushing back on pick up load (what if I lose power for a couple days?) and DHW recovery. You guys have thoroughly addressed those issues in other threads. I got him to the point where he'd at least agree to install what I wanted but he wasn't about to agree that it was the right thing to do. Consequently I'm expanding my horizons in terms of contractors and boiler manufacturers.

Any thoughts on good choices for boilers smaller than a TT-60?
 

BadgerBoilerMN

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Bravo!

We install Lochinvar's CDN040 and the Bosch Greenstar 57 for small loads and many condensing tank water heaters for combi DHW and space heating loads below 50mbuth.

Stick with it!
 
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John Molyneux

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Bravo!

We install Lochinvar's CDN040 and the Bosch Greenstar 57 for small loads and many condensing tank water heaters for combi DHW and space heating loads below 50mbuth.

Stick with it!
I already have a TT Smart indirect so I'm thinking I'll just match it up with a boiler.

Since you recommended it, I assume there's no reason to be concerned about the unique-sounding alloy HX in the Greenstar?

Any reason to choose Lochinvar Cadet vs. Knight other than cost?
 

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We have had the Bosch boilers in the field for about 5 years now. I think the new alloy will be a winner. We installed quite a few Buderus GB142's and take care of many more. Their issues with HX are legend, but mostly self-inflicted when water quality was regularly ignored. We use FERNOX to commission and service all boiler systems, commercial and residential.

If you have an ABC Bosch certified tech install the Greenstar you will have a 5 year parts and labor warranty.
 

Dana

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Unfortunately, I'm not sure I really educated this particular contractor. He kept pushing back on pick up load (what if I lose power for a couple days?) and DHW recovery. You guys have thoroughly addressed those issues in other threads. I got him to the point where he'd at least agree to install what I wanted but he wasn't about to agree that it was the right thing to do. Consequently I'm expanding my horizons in terms of contractors and boiler manufacturers.

Any thoughts on good choices for boilers smaller than a TT-60?

Seriously? With the Solo-60 and a 29K load you have 25KBTU/hr+ of excess capacity. Even if it stayed at the 99% outside design temp you'd be fully up to temp in 2-3 hours. With the Solo-11o you'd be up to temp in under 90 minutes, but so what? If it's warmer than the 99th percentile outside design temp it'll be up to temp even sooner.

And, just for yuks, it's useful to pretend that Maine is in a first-world country, where power outages that last for a couple of days are relatively rare, and power outages that last for a couple of days that coincided with outdoor temps at the 99th percentile temperature bin have not occured since the power grid was established in Maine. I suppose theoretically it COULD happen, but oversizing by 3x to avoid an hour or so of lesser comfort during the recovery ramp (and paying for it in lower efficiency and higher maintenance over the lifecycle of the equipment) just in case that happens seems idiotic.

Historically in coastal ME the rare power outages of that duration are either from hurricane events at outdoor temps well above freezing, or after major ice storms that occur at temps fairly close to 32F, and occasionally during big nor'easter snow storms, which are also on-shore wind flows of higher temp. If it occurred during a severe cold snap at or below the 99% outside design temp, odds are better than 50% that the heating system water would have frozen, and the boiler might be toast anyway (unless you drain the system in anticipation). During the ice storm of December 2008 (that affected Portland ME as well) I was out of power for five days. The overnight lows hit +10F a couple of nights, and I took measures to ensure that the upstairs bathroom plumbing didn't freeze up, but do I remember (or even care) how long it took to bring the house up to temp after power was restored? What I remember most clearly is mopping up the minor flooding in one corner of the basement due to the sump pump being idled during a period of high water table, and getting the power company to deal with the flaky neutral connection between the meter and the power pole. Several dozen houses in that were out of power even longer than me (a different utility company, later fined by the state regulators) ended up with frozen heating systems. But those events are extremely rare.
 

John Molyneux

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Bravo!

We install Lochinvar's CDN040 and the Bosch Greenstar 57 for small loads and many condensing tank water heaters for combi DHW and space heating loads below 50mbuth.

Stick with it!
Thanks Badger. Found a Bosch-certified shop that thought I should consider the new Greenstar floor-standing model. Any thoughts on floor standing vs. wall-mounted?
 

BadgerBoilerMN

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One of my favorite things about condensing boilers is that most of them can be hung from a wall. This usually makes venting easier and frees up valuable floor space. On occasion a floor model will make replacement of and old cast iron boiler easier, so do what feels good.

By the way, what town are you in?
 

John Molyneux

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One of my favorite things about condensing boilers is that most of them can be hung from a wall. This usually makes venting easier and frees up valuable floor space. On occasion a floor model will make replacement of and old cast iron boiler easier, so do what feels good.

By the way, what town are you in?
Falmouth, Maine.
 

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Sweet!

The SSC has a built-in pump.

You have to have some help piping and pumping any condensing unit. Local distributor and contractor support is critical.
Any opinions on NTI? Doesn't modulate quite as far down (only to 17,000) but I have a contractor coming who likes them a lot.
 

Dana

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If the plan is to go ahead and retrofit-insulate the place down to the anticipated ~20K load, stick with the Utica.

The NTI would be too big, since its min-fire output would be well above your average winter load, and barely below your new-improved design heat load and would almost never modulate. It would be OK for a 30K design load, but not for your post-improvements load.
 

BadgerBoilerMN

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Dana has it. It is all about the base...load. Stay focused and get the heat source "output" right. Then a choice, if there is one.

I would rather have the "worst" condensing boiler properly sized, than best over-sized boiler.
 
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