Calculating Boiler Standby Heat Loss

9% is a lot more than I would have expected. That is very interesting.

For a back of the envelope calculation -- quite reasonable, and quite correct. Just round it off to around 20,000 BTU/hour. As @EBEBRATT-Ed noted, some of that is wandering off up the stack -- but a good deal of it (I'm not quite sure where Mr. Cates gets his 9% figure, but that's of no importance) is "lost" to the space in which the boiler is placed.

Kindly note the quotation marks around "lost". if we were using this boiler for power, that would be a problem; we'd like to recover as much of that heat as we could, and would surely be using heat recovery on the return water from the exhaust stream, or on the boiler feedwater (which would, properly implemented, bring us to the heat input from the higher heating value -- a gain of about 7% right there). However, presumably this boiler is in a building. One of the more common errors in working with systems is to consider only part of the system, and not the whole system. That heat is not lost -- it is transferred to the space in which the boiler is located. If it were not so transferred, to maintain the same heat loss from the whole system -- the house -- if you managed to perfectly insulate the boiler you'd have to burn more fuel to feed the compensating radiators.

The reason we insulate pipes in both steam and hot water and ducts in air is not to conserve the heat, but to ensure that it gets to where we want it to be and, in steam, to reduce condensation in the mains which can cause other problems.

I am sure that there are those who would love to see boilers better insulated. But they need to realise that the heat radiated from the boiler is not lost from the system standpoint. Granted, one does want a reasonable balance between heat radiated from the boiler and heat directed elsewhere, but it is a balance, and like most balances the optimum point is variable (the two obvious extremes are that the heating device is located directly in the middle of the occupied space (think wood stove) where insulation is about the last thing one wants, and the heating device located fifty feet from the occupied space in the backyard, where the heat really is lost). Bottom line -- it is unlikely that there is anything to be gained in most situations from additional insulation for the boiler.

I think I would double check your results by using boiler and water weight.

> @gmcinnes said:
> @RayWohlfarth Last time it was measured by a tech with an analyzer, a month or so ago, it was 83%.

The tech probably read 83% off the analyser. That's an inaccurate number based on many interpolated data by the software.
Actual efficiency is Thermal efficiency.
BTU = GPM x 500 x DELTA T.

Like Ray said, it may be quite a bit lower.
I'll bet it's more like 60-70%.

E.g.. 100k input 80% boiler.

The pump moves 13gpm. Your temperature rise is 8. 13 x 8 x500 = 52000 but output. 52%
However boilers and furnaces are under input usually. (At 3.5") So 100kbtu at 3.5"wc is actually about 86kbtu input. So it's not as bad as it looks . Closer to 60%.

Let's assume the 9% is correct. What do we think is the best way to optimize or mitigate for this loss factor?

It's not worth debating with you, @SeymourCates .

These threads can get kinda boring though if everyone agrees to disagree.

@gmcinnes

What makes up the rest of the heating system?

I like your financial math, @gmcinnes . Makes sense to me. And, given the setting for your boiler, it probably makes sense to see about reducing standby losses, within reason...

Your boiler has a rated output of 413 k/btu. What kind of application is it? Roughly what is the size and length is the distribution piping?
A grossly oversized boiler incurs more inefficiencies due to the increased start/stops.

It looks like your system water is more like 35 gallons plus the boiler.
What is your on/off differential? How long is a typical cycle?
I wonder if there is an option to downfire that model.
A boiler like that will run very inefficiently at the beginning and end of a cycle. If it cycles to frequently, it will bring down the overall efficiency significantly.
There is a great illustration of this in Modern Hydronic Heating. Ill see if I can find it.

Your methodology is correct. Your boiler is probably 80% efficient, but you should check the name plate to verify that. A fair amount of heat is lost through the stack even with the damper in place. Probably about 60% of your losses Go up the stack. The rest go to the surrounding area. There are devices that actually close off the flu when the boiler is not operational. They're motorized dampers. They will cut your losses dramatically. I'm a certified energy manager and mechanical engineer with 40 years experience in the residential and commercial energy efficiency programs at a local utility. Your utility might offer you incentives to install the damper I speak about or more efficient boiler systems like a condensing unit. People think they're very efficient but you have to remember they have standby losses too and are only at the rated efficiency at full load. Personally at my house I have three forms of heat source. Oil, propane, and a Bosch heat pump which has a COP of three down to about 15° f . Depending on the price of the fuel, I control which source I'm using utilizing a residential energy management system and some custom relaying I've developed. When oil is $4.50 a gallon I'm running that heat pump down to 15° when it just won't keep up anymore. Now with oil about $ 2.80 a gallon I switch over to oil from the heat pump at 30°. It provides the best economical heating of the home.

First off, I totally agree with @DCContrarian : any heat "lost" from the boiler itself into the structure is not a loss. When performing the kind of analysis that is in play here, it is essential to consider the entire system as a whole, and not just bits and pieces of it — though the latter is much easier, and much more common.

Stack losses are a different matter. Unhappily, they are much more difficult to account for. This is particularly true of installations with a barometric damper on the breeching, as it becomes necessary for any kind of accuracy to determine the additional air flow rate through the stack vs. normal infiltration into the structure. One also needs to account for the heat transfer from the stack into the structure if it is an interior stack, since that is not a loss.

That said, it is reasonably clear that a tight stack damper in the breaching, but located after the barometric, will save some energy. How much? Very hard to say. Balanced against that one has to consider the reliability question of such dampers, and balance that against the energy loss. It has been my experience that automatic stack dampers are the least reliable part of a heating system, and that their reliability is low enough that I simply will not use them. There is another factor to consider here, too: if there is a barometric on the breaching, it will kill any draught through the boiler itself (particularly if the boiler is a positive pressure combustion design), and thus virtually eliminate the loss through the stack of the residual heat in the firebox.