Heat load calculation for replacement boiler - double-check my work?
I'm looking at replacing the HTP-EL150 that has had a bunch of problems (see my other posts for those adventures). The first thing I did was to run some heat load calculations. Instead of the more complicated and error-prone Manual-J calculation, I decided to use nearly three years worth of our LP purchase history combined with heating degree-day spreadsheets and the 99% outside design temperature in our area.
The results show that the 150k BTU boiler that we currently have is extremely oversized for what we're actually consuming in order to heat the home. My calculations indicate that a 50k BTU unit would more than cover our needs.
I'm leaning towards the Triangle Tube floor model INSFS110S (BTU Max Output of 110k).
Since that's such a large difference, I'm looking for additional pairs of eyes to poke holes in this calculation which I outline below:
- For a given period (2/25/22 thru 8/18/22) determine the number of gallons of LP consumed: 218.8g
- Determine therms contained within 218.8g of LP: 218.8 * 0.916 = 200.42 therms
- Determine therms delivered to the home: 200.42 therms * .91 = 183.05 therms (.91 is the assumed efficiency % of the HTP-EL150).
- Get the BTU value of therms delivered to the home: 183.05 * 100000 = 18305000 BTU
- Determine total BTU per degree-day for the time period: 18305000 / sum of the 60F degree-day values measured from the local airport over the time period in step 1: 9310.36
- Determine the BTU per degree-day hour: 9310.36 / 24 = 387.93
- Determine the implied heat load for the time period for 60F degree-days. Our 99% outside design temperature here is -4F. (60 - -4) * 387.93 = 24,827 BTU/hr.
I performed this calculation over 8 periods and ended up with an average BTU/hr rating of 29418 using the 60F degree-day data and 25919 using the 65F degree-day data. When you multiply these values by the ASHRAE 1.4x sizing factor you end up with 41185 for 60F data and 36287 for 65F data.
These numbers suggest that replacing the current 150k output boiler with another 150k output boiler would be extreme overkill. Even replacing with a 110k boiler (appears to be the smallest floor model Triangle Tube offers) indicates a very oversized installation!
I welcome thoughts on these calculations and thoughts on the Triangle Tube brand and model (110k) mentioned above!
Comments
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A few comments.
Checking the heat load from Feb to August is not covering the winter months which could result in inaccuracies.
Do you heat DHW from the boiler?
The way your calculations are you would allow for not thermostat setbacks in other words you would set the thermostat and forget it.
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Did you subtract out your other propane use? Won't change the answers much but get you more accurate numbers.
Quick look at the calcs, it looks fine. Most people miss the fact that if you actually needed a 150k boiler, you would not be able to afford the heating bill.
The 1.4 sizing factor is actually the MAX recommened oversizing but that is for furnace and heat pumps. For a boiler especially with decent turndown, you can oversize as long as you are not micro zoned. The turndown on the Triangle tube is not that great, the 5.5:1 on the 110k means min modulations of about 20k which is pretty much heat load at design temp. I would try to find something with a 10:1 turndown.
Heck, if that propane use included DWH/dryer/stove, so your heat loss is even less, you can tap your water tank with a plate HX or Taco X block and easily heat your place.
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@EBEBRATT-Ed Agree, Feb-Aug isn't the best range, but there are seven other ranges that cover the period of three years and the numbers average out OK. Yep, DHW is heated by the boiler as well and stored in an indirect tank. We typically have the house around 62-63F during the winter, which is probably a lot lower than "normal".
@Kaos I didn't subtract out the use for the DHW which would be the only other consumer of the propane. Figured it wouldn't make that huge of a difference either. I took another look at the specs for the Triangle Tube INSFS110S and it shows a turndown of 8:1, so a bit better than what you listed and I think that is fine for our usage (hydronic system provides the home heat).
I'm putting together a schematic of the whole system right now so that it's easy to converse about what's actually present… stay tuned.
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The TT 110 has a minimum output of 13750 BTUH. Your MAXIMUM requirement, as stated is 29418 BTUH. The minimum on the TT is nearly ONE HALF of your maximum. This boiler will cycle constantly between October and December and again between March and May. I discourage you from procuring this unit.
Kaos has the preferred solution of using HWH with a plate HX IF your radiation is sufficient with 145F SWT.
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Exactly right. The entire purpose of a mod-con is to set it up so that it can run 24x7 for the entire heating season. That's where you get the most efficiency. If you purchase it wrong, or set it up wrong, you might as well have procured a CI boiler that is much simpler to operate and maintain. You wasted all your funds and efforts.
I have an installed HTP UFT with a minimum firing rate of 7200 net and it will run from October to April with no shutdown in a 1900 ft. colonial in NY. This required very careful programming to achieve AND it might suffer from the inability to make the 'stat setting on the first floor on a very cold windy night in January. Of course, the homeowner is dissatisfied in such situations……………….but the boiler will last for 20 years!
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Lochinvar WHB085N: 8,500 BTU low fire (turndown ratio of 10:1). It seems like you will get the best operation from that unit in the fringe months.
Lochinvar WHB055N goes as low as 8,300 BTU input but only has 55000 input for you DHW recovery.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Calcs look fine, not surprised by the result. What’s the radiation?
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Not from the indirect. You can use regular power vented water heater or one of the high efficiency ones (ie Polaris) and tap it for space heat. Most regular water heaters have a 45k to 55k burner, you can get ones with larger burners specifically meant for combi heat, these get low to mid 80% efficiency when used for space heat..
Since you already have a complete setup with an indirect, it doesn't much sense to go this route. Find a boiler with better turndown and swap out what you have. This would be pretty much a 1 to 1 swap with minimal changes.
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@np_mdbr, so I intended to do a heat loss calc today, which I've never done before, and was all ready for a frustrating internet search looking at different methods, followed by confusion and uncertainty — and then your post popped up with a ready made recipe. Thank you.
I too got a number a lot lower than my current boiler's rating, and I have some questions, but if I asked them here I feel like I'd be hijacking your thread, which is about your situation not mine, so I'm just starting a new thread. It will happen to look a bit like yours, in fact I might paste your recipe but with my numbers. If I do I will attribute it properly, please don't sue me for plagiarism!
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@seized123 Glad this post was helpful for you! We're all here to learn from the experts, so I appreciate that you're benefiting as much as I am!
@LRCCBJ as a hypothetical, if I went with a unit rated at 199 BTUH max with an 8:1 turndown (yep, realize 199 is WAY above my max target BTUH) wouldn't this also avoid incessant cycling as the unit would be very near my target BTUH of 29.4 when it was down around 8:1 (24.8 BTUH) or 8:2 (49.75 BTUH)?
@Kaos Thanks for that suggestion. The boiler that I have right now is around 5:1 turndown, not very impressive. I like the suggested model from @EdTheHeaterMan but it looks like it's a hard-to-get item in my area and may not be a familiar for the local techs. It also doesn't have some of the features that the TT has such as the wifi connection, the Caleffi 548 separator and the option of hooking up from either the left or right side…
@Hot_water_fan I'm assuming you're referring to the method of heat transfer ("radiation")? It's an in-floor hydronic system with a large concrete slab in the basement and wood floors above.
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as a hypothetical, if I went with a unit rated at 199 BTUH max with an 8:1 turndown (yep, realize 199 is WAY above my max target BTUH) wouldn't this also avoid incessant cycling as the unit would be very near my target BTUH of 29.4 when it was down around 8:1 (24.8 BTUH) or 8:2 (49.75 BTUH)?
Yes, if you went with the 199, it can modulate down to the target of 24.8.
Now, think about how many times per calendar year will it be cold enough outdoors to warrant 24.8 heatloss for the building?
Answer: Maybe 1 day………..one really cold day………..what we refer to as the "design day".
Every other day, the unit will cycle. And it will cycle endlessly in the warmer seasons.
This would be the worst possible solution you could select.
But, folks do it all the time so they can have endless hot water from two showers simultaneously!!!!
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If you have high mass like radiant slab as part of your emitters, turndown doesn't matter as much. Even at low fire, the run-time will be long enough that the boiler will not short cycle.
I would still try to get something with better turndown though. There are other units out there that can do very low output (ie HTP ELU-85WBN). I would ask around what your local suppliers carry and try to find something there.
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Want an easy Load Calc spread sheet? try this one. https://centralboiler.com/pdf/FORM-1504WH.pdf
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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@EdTheHeaterMan I did the form, will give details on my heat load thread — spoiler below, however:
@np_mdbr did you try Ed's form? I did after a bunch of measurements. It gave me a few unexpected insights as to where my largest heat losses might be. But the end result was radically different from the fuel bill method. I'd be very interested in your results if you try the square foot method.
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@seized123 I haven't tried the form from @EdTheHeaterMan yet, but it looks in-depth and very thorough! I suppose the fuel bill method really depends on whether you're comfortable with the levels you've experienced over X years as a homeowner or not.
My main driver for going the fuel bill route was that we've been relatively comfortable over these past three years (minus all of the boiler issues that have cropped up), so I'm relatively confident that the numbers match that which would continue to provide us comfort.
Also, there's a lot more room for error when you're taking measurements, etc. but of course it depends on how meticulous one is!
It is interesting that your numbers are so different between the two methods… worth more discussion for sure.
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I think you’ll find the fuel loss method is more accurate, faster, cheaper, and easier. The place for a manual J is in a pro’s hands, especially if it’s a new build. The big issue is the infiltration. You won’t know that without a blower door test. The fuel method is based on reality, which is a perk!
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