Help choosing a new boiler/hot water heater
Hi all,
So my old gas-fired Teledyne Laars boiler went to boiler heaven a couple of days ago, and I urgently need to replace it as the weather is cold here in Southwestern Ontario, Canada.
I was able to get two companies out quickly, to quote replacements for me. However, since buying a new heating and hot water system is a long-term and very expensive commitment (and I do not really have the money for this), I want to make sure I do my due diligence and learn as much as possible so I can make the right decision. Even though I have no heat, and it's like 5 degrees C (41 F) outside right now, I don't want to rush into it.
First of all, my Google-fu has revealed I should do a "heat loss calculation." I asked one of the contractors who quoted me about this, and he said I do not need to do this because the boiler he quoted me will "automatically modulate" so I don't really need to have an exactly-sized boiler installed. I think I still want to do it, (should I?) but what kind of company do I search for that would do this report?
Secondly, I learned that "short-cycling" is bad. I don't know why it is, but it is. So, I want to make sure i do not buy an oversized boiler.
My house is a 1961 brick bungalow (one-storey) with full concrete block basement. The house is 45' x 25' so 1125 square feet per level. The home has been insulated with R-50 in the attic, not sure about the main floor but it's pink fiberglass, and the basement is 75% R7.5 (1.5" styrofoam) and 25% R12 (blanket wrap). It has 77 feet of copper fin baseboard radiators on the main floor, and nothing in the basement for heat. (I would like to add an option for a future basement zone in case I ever finish the basement).
Edit: design temp is 5F
I also have a separate hot water tank, which is old and both contractors suggested I replace it at the same time with a "combi boiler". I do not know the domestic hot water load of my house. It currently has 1 full bathroom, dishwasher, kitchen and laundry sinks, and a washing machine. I am thinking of using the space that the current huge boiler and hot water tanks are in, and making that a second bathroom down the line. The combi boiler has the advantage that it would provide that space. The disadvantage as I understand it, is you have to make a trade-off between oversizing for heat and undersizing for domestic hot water.
Current quotes were for IBC SFC-125-1 and a Navien NCB-240/110H.
Questions:
- Should I do the heat loss calculation, or is it true that with the modulating boilers this is no longer needed? If so, what type of company does it, so I can find one?
- Is a combi boiler good for my situation or should I get one with a separate indirect hot water tank? This will add a good 33% more to the price, and eliminate the possibility of a second bathroom, so I would want to have a good understanding of the pros and cons.
- Are the boilers I have been quoted, reliable, long-lived, and good for my situation?
- I had a Lochinvar brand boiler at a previous house because it was a good brand. Should I try to find someone in my area who sells them or is IBC/Navien okay?
Comments
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Does the home heat well now?
If so the 77’ of fin tube tells you how many btu/ hr you need to supply
77x 500 btu/ft of fin tube is 38,000
So a 50,000 boiler will be adequate for what you have and some additional capacity for the basement
A 60- 80,000 boiler that modulates would be close
Lochinvar or IBC in that size range. Would be fine
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
Well, a heat loss calculation would be nice, and normally I'd say that it should be done. There are other ways to get at least close, however. One of the simplest is to look at that baseboard. You have 77 feet of it, and as a general rule the most you can get out of baseboard is around 500 BTUh per foot — so even being generous, it's unlikely that you need more than around 40,000 BTUh from the boiler. Another ballpark is to look at the area of the house — which is, I'll admit, a pretty poor guide! — but again, that suggests a maximum heating load of about 40,000 BTUh.
Now the fun begins. The folks who mentioned that for a combi boiler there is a tradeoff between "right size" for heat and "right size" for domestic hot water are exactly right. Your domestic need, if supplied by a combi (or other on-demand system) is likely to be up around 200,000 BTUh, accounting for the future bathroom, but with the plumbing you mention both boilers you mention should be adequate.
Both are actually oversize for your likely heating load, but both "modulate" — that is, they can (and do, if set up properly) reduce the firing rate to more closely match the load, and both should be able to modulate down to match the conservative numbers I quoted above.
They will likely have to stop and start (cycle) on warmer days — but if set up properly that won't be a problem.
Now here is much the most important point: boilers are not plug and play. Not even close. Therefore the most important part of the project is not the brand on the boiler (full disclosure; I rather like the IBC better) but the ability of the installer to get it piped and connected to your system correctly, get the necessary sensors, including outdoor reset, properly installed — and the willingness of the installer to stand behind their work and follow up as needed. Only you can readily judge that for the two quotes you have gotten.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
Thank you for your reply.
"Does the home heat well now?" I mean, not now, because the boiler decided to start shooting off CO and the home had to be evacuated and the gas is shut off now.
But if you mean before, then yes, the now-defunct boiler heated well, but it was 83% efficiency and 100,000 BTU input and 83,000 BTU output, which seems high. I don't even know how often it kicked on/off because it was quiet and I didn't notice. But it could supply whatever temperature I called for, no problem. (well, until problem - lol)
Edit: I want to leave the possibility of heating the basement in the future, so that would add more linear feet to the radiators.
The IBC boiler (SFC-125) says it has a range from 15,800 to 89,000 BTU/h. The Navien NCB 240/110 has a range 13,000 - 110,000 BTU/h - however I have read some poor reviews of Navien, are they lower-end?
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Also, is the combi-boiler recommended/works well for domestic water? I have 0 experience with on-demand hot water.
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Thank you! That's great insight! Very helpful.
The Navien boiler has a space heating range of 13,000 - 110,000 BTU/h and a domestic water range of 13,000 - 199,900 BTU/h
The IBC boiler has a space heating range of 15,800 to 89,000 BTU/h and a domestic water range of 16,500 BTU/h to 125,000 BTU/h
So it seems like the IBC boiler might struggle a bit more with the domestic hot water, and it also doesn't modulate as low as the Navien. But I've seen poor reviews on the Naviens. So… I'm still not sure.
i will investigate the Lochinvar Noble model tomorrow.
You made a good point about the installers/technicians. Unfortunately, I was not able to be here when the repair technician came to look at my existing system (he was from one of the quoting companies) nor when the salesmen came. (it was unavoidable). However, I will call each of them to discuss my questions over the phone and try to get a sense of their knowledge level.
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I have had a few combi boilers in my various homes and shops A 120,000 is about as small as you want to go. You should get 2 gpm or so of continuous hot water with a 120,000 in your area.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
Okay! I guess I should measure how many GPM my showerhead and various appliances use :) Thanks!
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2 gpm should provide plenty for a shower, and maybe a dishwasher at the same time
It would not handle a shower a load of wash at the ssmr time. Unless you run a cold cycle😉
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
The Lochinvar pages aren't helpful. They give a BTU/h range for the DHW but not for the space heating.
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Okay. I did some heat loss calculations according to this tutorial and it seems like both boilers are oversized (which I expected)
I used my actual fuel consumption from both January 2023 and January 2024 (this is my highest consumption month). Of course, it includes the water heater, so the space heating is an overestimate. I did for a 65F and 60F balance point, giving me four numbers (60 and 65F, for 2023 and 2024).
The low number is 16,736.4 BTU/h and the high is 35,349 BTU/h. This jives with @hot_rod estimate of 38,000 BTU max.
The ASHRAE sizing guide says to take that number (I used the highest) and multiply by 1.4 to get the recommended size. So mine would be 49,488 BTU/h for space heating.
The IBC boiler has a modulating range of 15,800 BTU/h to 89,000 BTU/h so it's within that, but I'm a little concerned about short-cycling the rest of the time (basically, September-December and March-May). Is this a valid concern?
I still have to figure out my GPM measurement for my hot water…
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Hi, There are very good 1.5 gallon per minute showerheads and aerators. Replacing those fixtures could allow you to not install such an oversized boiler…. or keeping a separate tank for your hot water would isolate the loads and let you install a boiler sized just for space heating. Choices! 🤔
Yours, Larry
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Is there a way to figure out what the lower range on a modulating boiler should be for my scenario, so I can avoid getting a boiler that short-cycles half the time? Is this a valid concern?
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You are a bit confused with the term "short cycle". If we have a conventional boiler that has an output of, say, 90K BTUH and a small zone that has the capability to deliver 12K calls the boiler, the boiler must short cycle because it only needs to deliver 12K and it has an output of 90K. It will probably run for one minute………..stop for 5 minutes………..run for one minute………..repeat.
Now, if you utilize a mod-con that can get down to 12K and the zone requires only 7K (at warmer temps), the boiler is going to run for probably 5-10 minutes and then shutdown. It will restart (depending on the amount of water in the system) in about 10 minutes and the cycle will repeat until the zone is satisfied (probably well under 30 minutes……….depending on how the outdoor reset is calibrated). This is "cycling" but we do not consider it "short cycling".
Of course, the lower the minimum output that you can obtain, the less cycling will occur. And, note that the cycling will only occur in the shoulder seasons where the outdoor temp is above……….say 45-50F. Below these temperatures, the building requirements are 12K or more and the boiler will run constantly (from November to April).
What you don't want to do is to buy a large mod-con that can modulate down to 18K. Now you definitely will cycle for more of the season because 18K is a significant load and most zoned systems will not deliver that amount except in the coldest outdoor temps.
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Thanks, this is helpful. You're right. I'm definitely learning as I go here, and in a hurry lol. I did more math today than I've done in the past 10 years probably :) I figured that if the boiler was constantly having to shut off and on because its capacity was too high, that was "short-cycling"
Is there any special calculation I can do to determine what the lower bound of the modulation should be? You suggested at least 12K, and that 18K minimum would be too high. How did you come about those numbers if I may ask?
Additionally, currently there is only 1 zone, which is the main floor. The full basement is not heated. Let's say I decide to finish the basement 7-10 years from now, and add heating down there. Should I account for this when sizing the boiler, (since I guess the linear feet of copper fin will now be doubled), or will a boiler sized for the main floor still work when the second run is added?
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Is there any special calculation I can do to determine what the lower bound of the modulation should be? You suggested at least 12K, and that 18K minimum would be too high. How did you come about those numbers if I may ask?
There is no magical lower limit that is suitable for every building. I actually view it as a percentage of the heatloss at design (the maximum expected requirement for the building on the coldest day of the year. If your house with a single zone requires 45K BTUH on the coldest day, I would like to see a mod-con that can achieve 20% of that value………….say 9K………….for it's minimum firing rate. If you have a large mod-con and it only achieves 18K, you are looking at an output that is 40% of the heatloss at design. So, that boiler is going to cycle significantly more than the smaller one that has a minimum firing rate of 9K.
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Additionally, currently there is only 1 zone, which is the main floor. The full basement is not heated. Let's say I decide to finish the basement 7-10 years from now, and add heating down there. Should I account for this when sizing the boiler, (since I guess the linear feet of copper fin will now be doubled), or will a boiler sized for the main floor still work when the second run is added?
If you were to size the boiler perfectly to the heatloss on the design day for the existing first floor, there would be no additional margin to utilize for the basement.
However, basements require a fraction of the energy that a first floor requires because most of them are underground and the "outdoor" temperature is near 50F (below ground).
You probably will procure a boiler with a max output of 60-70K and this will be more than sufficient to heat both the first floor and the basement. Just be sure to insulate the basement walls above ground level. Massive loss of energy through the blocks (or the poured cement) to the ambient temperature outdoors.
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Thank you so much!
Yeah, the smallest boiler I have even been able to find that exists, is 80,000 BTU/h. If I can find one that can do 10:1 turndown then that should work great.
I am guessing, that a combi boiler is not going to work for me, since they don't make them that small (80,000 BTU) that I can find, and even if they did, I wouldn't be able to warm up more than like 1 GPM of domestic water with it 😁
I really wanted a combi boiler for space saving reasons, but not if I have to compromise on every other metric.
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Don't buy a Navien
Don't buy a combi
Install a 60,00-80,000 btu boiler and an indirect hW tank. Lochinvar and IBC are ok.
Short cycling is hard on the equipment but in that range with modulation you should be ok.
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This is great advice. Install a boiler and indirect water heater. Less costs of service and repairs and you will be happier with the results in the end.
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Thanks, this is helpful!
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