Could my envelope really be this good? And how bad is it to oversize a modcon?

Bernie_the_Brewer · 3:20AM

I am in the middle of speccing a new modcon boiler to replace a ~70-year-old, 220 kBTU/h cast iron hot-water boiler in my large, 1925 duplex (~4500 sqft). I did a lot over the last couple of years to improve the building envelope. Attic insulation, air sealing, blow-in wall insulation, weatherstripping, etc. This has helped a lot. But I still have lots of drafty windows, sizable stack infiltration, lots of single-pane windows, etc.

Nonetheless, for the last two heating seasons, I have been monitoring the gas usage on a daily basis, and calculating the gas used per degree day. I claim that the design heat load (at -5F) is less than ~70 kBTU/h. The monthly gas bills say the same.

The “problem” is that this works out to only about 16 BTU/h/sqft. This seems low. Therefore, the contractors I am interviewing want to put in much larger boiler than my calculations and gas bill would warrant. (Of course, none of them wants to do a Manual J.)

I am all on-board with the accepted notion at HH of NOT oversizing a boiler. However, one question I have how bad is it to oversize a modcon in particular? I claim I really should get a boiler of approximately 80 kBTU/h. Contractors are begrudgingly considering ~130 kBTU/h (based on 29 BTU/sqft).

But how bad is that, really? If their recommended modcon has a minimum output (with a 10:1 turndown ratio) of ~13 kBTU/h, would that work okay? Two salient considerations: (A) Even at 120F AWT, my smallest zone can absorb ~13 kBTU/hr (and 37k at 170F); and (B) According to my calculations, the outside temperature would have to be higher than 54F to have the heat load drop below 13K, causing the boiler to short-cycle.

So, would it be correct to say that a modcon is much more forgiving for oversizing? Should I fight with these guys to put in a smaller boiler, (or find someone who will), or am I going to be saved by the turndown ratio and I should just allow them to put in a bigger unit than I think it needs?

Thanks for your thoughts.

EdTheHeaterMan · 3:40AM

You need to trust your numbers. Contractors really have no business specifying heating equipment without doing some form of heat-loss calculation first.

That said, the modulating-condensing boiler (mod-con) has changed the boiler world quite a bit because it greatly reduces the traditional oversizing problems we used to see with fixed-input ON/OFF boilers.

Since this is your own building — and you probably are not working on other systems every day — there is always a chance your numbers could be a little off. One place that often causes trouble is the infiltration assumption. If the air-changes-per-hour (ACH) value is set too high or too low, it can skew the load calculation noticeably.

My advice is simple: find a contractor who is willing to perform a proper load calculation and walk you through the numbers.

That’s the contractor you want to buy the system from.

PS: thanks for reminding me… I need to get my envelope ready for church tomorrow

Good Night

Steamhead · 3:59AM

@EdTheHeaterMan said "My advice is simple: find a contractor who is willing to perform a proper load calculation and walk you through the numbers.

That’s the contractor you want to buy the system from."

THIS!

hot_rod · 12:30PM

the biggest unknown is always the infiltration. To pin-that down a blower door test is a good option

Do the test on a cold day and use an IR camera to pinpoint the leakage

Another check it to measure and calculate the heat emitters. If the building has been heating adequately then you have enough emitter. So check the boiler sizing against that number.

Ideally you would size the boiler to where the load will be after you finish all the building upgrades. As you upgrade the building the SWT drops for even more mod con advantage

I agree the oversizing it not a big issue with a mod con. Most all models can be limited to a fixed output with 10-1 a typical turndown

Modulation and ODR for the win

EdTheHeaterMan · 1:56PM

OR just go with the lowest bid and hope for the best. That is how the government was able to put a man on the moon!🤗

Kaos · 2:47PM

People tend to overestimate heat loss for houses. 30but/sqft does exit but it is the outlier not the norm. Heat loss on on a multiplex tends to be lower as there are a lot of internal gains from people and equipment.

Fuel use based heat loss calculation is the most accurate for any existing building where there are too many unknowns about insulation and infiltration. Trust your numbers and size off that.

I'm not surprised about your numbers, typical residential boiler is 3x oversized around me.

The hard part will be convincing the installer to trust your number. Even a good man J can be quite a bit off for older houses. The best you can hope for is to install the smallest modcon they are comfortable with, even with a small zone it is rarely the case that only the smallest zone is calling for heat, so cycling won't be all that much different between an 80k and 130k unit.

Besides a proper install, the most important part is good ODR. Make sure the installer puts in the outdoor sensor and show you how to adjust the curve as the default ones tend to be very conservative.

Bernie_the_Brewer · 3:37PM

Thanks, all. I appreciate the feedback.

Although I have had a whole-house energy audit done, they won't do a blower-door test due to the presence of asbestos in the house. (Fortunately for me, there is almost no exposed asbestos anymore, but I know the pipes in the walls are asbestos covered.)

And yes, unsurprisingly, there is PLENTY of emitter capacity! I have catalogued all of the emitters, and I claim that I can meet design-day load at 120F AWT.

I have not yet received final design proposals from any of the contractors, so I will wait and see if I was able to convince anyone. (To be fair, one of the three contractors I contacted immediately asked for my gas bills and intended to measure all of the emitters before I gave him all of that info. The other two just pulled out rules of thumb. We'll see.)

Thanks.

EBEBRATT-Ed · 3:52PM

If you are going off the actual gas usage there is no risk in downsizing the boiler. Actual usage is actual usage. Infiltration means nothing as it is already figured in your fuel usage. Find a contractor that will install the correct size boiler.

Could my envelope really be this good? And how bad is it to oversize a modcon?

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