96% AFUE Boiler and Closed Radiators

jim426March 11, 2012

Interesting "problem". Had a super hi-tech 96% AFUE boiler installed (Burnham Alpine 105K BTU). Boiler was sized for the house, even though there are many rooms we don't normally use in the Winter. Sizing was the only reasonable thing to do - would hate to sell the house and tell someone the furnace is undersized if all rooms are heated.

With the old traditional boiler, we shut off the radiators in the rooms we didn't use (including our bedroom as we like it cold). This saved money. The guy who installed it as well as the Burham rep said this isn't a good idea now - because it causes the return water to be warmer than design (makes sense, not enough of the boiler output water is being transferred to the air), and the 96% boiler is most efficient when run in "condensing mode" which can only happen if there is sufficient differential between the input and output temperatures.

But there is math here that doesn't seem to add up. The efficiency of the furnace at its worse is still better than any traditional boiler, let's say it's 80%. It seems to me if you are only heating half the house, you still win because you must be using a lot less gas at 80% than with all rooms on at 96% (and it's actually better than that - due to the fancy digital readout on the unit I can see it stays in max efficiency (condensing) mode for quite a while when heat is first called for since the temp differential is high, but then it switches to non-condensing mode until thermostat is satisfied.

The installer and the Burham rep seem quite convinced I should leave all radiators on all the time. They seem very knowledgable - what am I missing here?

It will take a very long time to compute the actual fuel cost savings, made even more complicated by the fact that this was also an oil to gas conversion, and we've had a warm Winter. Need to satisfy my gut feeling in the short term.

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What is the boiler jacket made of?

Boilers that do not reach normal operating temp can have corrosion problems, and running one 'hot' is going to take a toll on circulation pumps.

    Bookmark   March 13, 2012 at 3:23PM
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Boiler jacket is stainless steel.

The issue is that with some of the radiators closed, it reaches operating temp too quickly and spends less time in condensing mode. "Too hot" is probably not an issue as the computer reduces the gas to keep that (and stack temperature - this is a PVC pipe exhaust) under control.

My question is more related to gas usage.

    Bookmark   March 14, 2012 at 12:29PM
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I am not a condensing boiler expert, but this is what I know from what I read.

Condensing boilers are supposed to be about 10 percentage points higher in efficicency than conventional boilers. So I think your 96% AFUE boiler should operate no lower than 86% when it is not condensing. When the display shows the it is not a maximum efficiency, I think the fall off in efficency is gradual. The efficiency is still very good on an absolute scale.

Have you tried turing on all the radiators to see how it affects the operation of the boiler? You may find out the water temperature is still too high even in this mode.

What about lowering the temperature of the boiler output? My understanding is the water temperature needs to be below 131 degrees F in order to get the maximum condensation.

I think the additional energy savings of not having the boiler operating in the optimum efficiency mode is small. Enjoy your new boiler and don't agonize over this.

    Bookmark   March 14, 2012 at 3:14PM
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