Sizing Replacement Boiler

Steve Minnich_3

Hi Jim,

I e-mailed you a short while ago. Let me know if that works for you.

Condensing boilers are not for every application.

Steve

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Jim H.

Best Boiler Size

I am replacing a grossly oversized and inefficient boiler in a three story building in Chicago that originally heated all floors. Now the boiler heats only the upper two floors. I calculate the heat loss at around 90,000 BTU/Hr. The remaining connected radiation is around 99,000 BTU/Hr. and seems to heat well enough.

For a heat loss of 90,000 BTU/Hr., the boiler program recommends between 90,000 and 109,000 BTU/Hr. IBR. The quotes I've got have recommended boilers between 127,000 BTU/Hr. and 163,000 BTU Hr. IBR. If I run the calculation at a water temperature of 160 (instead of 180) the boiler recommendation is then even smaller, between 56,000 and 76,000 IBR.

For efficiency and comfort, what is the best boiler size?

Brad White_132

The smallest possible boiler

Jim-

The smallest possible boiler to meet the heat loss is the only one you want. If we take your 90,000 heat loss as accurate, you want a boiler with that output. In a fast-responding ModCon, you could also drive an indirect hot water heater within the same model with no additional capacity.

I have to ask, is that 90 MBH heat load based on an insulated structure? Can it be further reduced? This is the time to seize the opportunity. One other contributor to this site, (sorry to forget whom for credit is due) said that insulation is fuel bought at today's prices. Anyway, you get the point.

Hot water boilers are ALWAYS sized to as accurate a heat loss as you can calculate. The quotes you are getting in the 127,000 and 163,000 range have more capacity than your radiators could possibly emit- and for what possible purpose? That would put you back where you were, oversized after replacement. Regardless if those are input or especially if output ratings, they are too large. The quotes are still useful though- they help you eliminate contractors from the competition. To be charitable, they just may not know any better so we would first try to bring them along and tell them much what I am saying here in this entire response.

To repeat: Size to the heat loss and certainly no more than the radiators can emit.

Of course, the manufacturers have only so many increments available. For this reason and if you have natural gas, a Modulating Condensing ("ModCon") boiler is a great way to go. The modulation alone absorbs the sizing tolerances (especially if tightly sized to the maximum boiler output). The condensing part, in my opinion, is a welcome bonus which gives you another 10 to 15 percent. Such a deal.

The I=B=R sizing has a 15% factor in there for piping and pick-up, the assumption being that you will start from a cold, high-mass radiation system every day. With constant circulation and proper control management this is moot. I size by net output.

I am not clear on what you mean by running the boiler at a lower water temperature, how that allows you a smaller boiler. It still has to meet the heat loss.

May I assume that your 99 MBH of radiator is at 180 degree supply water (170 average)? That is not a huge margin but is enough to allow condensing and reset over the course of a year for enough hours to make it worthwhile. If your house could be improved by insulation and storm or replacement windows, you radiation surplus increases. This allows you to run at even lower water temperatures. Don't get jiggy now. Breathe... breathe.... all is good.

Jim H.

Mod Con Replacement Boiler - Chicago Contractor?

Brad,

Thanks very much for your response, it confirms that I am on the right track.

For the heat loss calc, I assumed no insulation. I appreciate your comments on insulating and I will where I can.

Do you know someone in the Chicago Area can who would install a mod con boiler with provision for an indirect hot water heater? I contacted the one listing in "Finding a Professional" on the Wall with no response as yet.

The old timer I have will not install a mod-con boiler because "he has had too many problems with them" and will only warranty his work if he installs a 127,000 output unit where the heat loss is less than 90,000.

So I need someone local who is on the same page as far as technology.

Thanks,

Jim H.

Brad White_9

Jim

I think there are several but do not want to volunteer any. Be patient for this is the busy season. Branch out too and know your own schedule.

The contractor or any contractor who says he has too many problems is obviously not the one for you; there may be reasons of course but it sounds dismissive without explanation. But to install a larger unit to compensate for...what now?... That makes no sense to me whatsoever.

Sort of like saying I have trouble with Menolo Blahnik high fashion shoes so I wear waist high waders instead.