Calculating Boiler Standby Heat Loss

gmcinnes

I figured I'd have a go at calculating the standby heat loss of my boiler. I'd appreciate if you guys could take a look and see if I've missed anything or done anything silly:

1. First I shut off the boiler and circulation pumps so I'm only measuring the heat loss from the boiler at idle.
2. Note that the boiler drops from 187F to 154F over two hours. That's not enough information though 'cause I don't know how much thermal mass that drop has taken place over.
3. However I do know that the nozzle is rated at 2.75GPH and there are 138,500 BTUs in a gallon of oil.
4. So I turn the boiler back on and measure it takes 6.9 minutes of firing to raise the temp from 154F to 190F. Therefore, given a nozzle at 2.75GPH, 43906 BTUs were added, and that means 1219 BTUs per degree.
5. As we said, we know the boiler dropped 33F over 2 hours. Using the BTUs per degree calculated above, it lost 40247 BTUs, which means the BTU loss / hour must be 20123 BTUs / hr.

Obviously the above figure is a very back of the envelope calculation, but is there anything crazy about what I've done?

EBEBRATT-Ed

Seems to make sense. But just two facts. Some of this heat lost is up the stack and gone for good.

Some heat loss through the jacket is lost to the building so it is not really lost, just isn't in the water or the boiler any longer

SeymourCates

Your calculated value of 43906 is based upon input. Actual net at 80% efficiency would be 35,040.

This represents 9.2% of input which is typical of CI boilers. Sadly, other than Buderas, none of the industry stalwarts want to do what is necessary to limit this extreme loss.

gmcinnes

Ah. Of course. I forgot the stack losses. Thanks guys.

@SeymourCates
9.2% is pretty high, for the want of a tiny marginal cost increase in insulation. Seems like pretty low hanging fruit! I'm adding some more myself, and I'll see if I can measure a difference.

@EBEBRATT-Ed
Yes, the heat is lost to the house. The speed with which it makes it up to and out the attic is another problem I'm working on

Eastman

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

RayWohlfarth

I think you are assuming the boiler is 80% which it may not be if there is scale on the water side or soot on the fireside. You cannot accurately measure it without a combustion analyzer or if you can calculate the btu's leaving the boiler.

gmcinnes

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

gmcinnes

@Eastman Perhaps not so surprising if you were standing next to this boiler Best radiator in the house!

Jamie Hall

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.

hot_rod

Could the calculation be done by knowing the weight of the boiler, and the volume of water weight?

The Euro boilers tend to have better insulation packages, denser and thicker insulation. Possibly a code requirement, indirect tanks also.

If the boiler is in a cold basement, or room with open combustion air, keeping as much heat in the block would reduce cycling and energy consumption I imagine. Many conventional boilers have hi and low combustion air vents nearby, so the loss would go to the great outdoors.

If the boiler is within the insulated heated envelop, the loss would be counted as heat to the space.

Zman

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

STEVEusaPA

What's the purpose of this exercise? Trying to figure out a heat loss for the home, or just how much heat your boiler is losing per hour.
If it's the former, than a standard heat loss would be more accurate.
If it's the latter, what do you plan to do with the info?
Just curious.

SeymourCates

Any boiler that resides in the basement or a crawl space absolutely loses all of this energy to unconditioned space. None of it is "recovered" to heat the rooms where the occupants desire the energy.

It is a fallacy to believe the energy "rises" and actually contributes to any heating of the real conditioned space.

Zman

SeymourCates said:

Any boiler that resides in the basement or a crawl space absolutely loses all of this energy to unconditioned space. None of it is "recovered" to heat the rooms where the occupants desire the energy.

It is a fallacy to believe the energy "rises" and actually contributes to any heating of the real conditioned space.

I think it depends on the design. Most basements and crawlspaces I see are entirely within the conditioned spaced, a handful are debatable, none are entirely outside.

icy78

> @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%.

Jamie Hall

SeymourCates said:

Any boiler that resides in the basement or a crawl space absolutely loses all of this energy to unconditioned space. None of it is "recovered" to heat the rooms where the occupants desire the energy.

It is a fallacy to believe the energy "rises" and actually contributes to any heating of the real conditioned space.

Phooey. My cat doesn't agree with you.

Eastman

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

SeymourCates

@Jamie Hall

You might ask the cat if he as ever installed a mod-con and observed a 25% fuel savings as compared to a steam boiler or a CI HW boiler.

I'm sure the cat will say that he has.

Then, ask him where the efficiency improvement came from if the combustion efficiency difference is only 10% as we both know that it is.

That mod-con doesn't have that giant heat disposal unit that is present in the caveman equipment. Believing that all that heat goes to the rooms with the thermostat(s) is ludicrous. A good percentage of it is disposed in the cooler basement or crawl space and does nothing to heat the occupied space. The cat will understand that energy will continue to flow from the occupied space to the cooler basement or crawl space despite the presence of the boiler.

Even with the equipment on the first floor of the building, where you might think some energy would transfer to the living area, you'll likely find the combustion airflow for the boiler will easily dispose a good portion of the energy transferred to the surrounding air. You won't get much benefit from it.

Jamie Hall

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

STEVEusaPA

The return of @Hatterasguy ? About 7 of us think it's you again. We figured you show your hand when you got challenged.
I think he (you) did try to come back with different/new accounts.

Eastman

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

Gordy

Any losses from the boiler to a space in which it’s temperature is less than the setpoint of the intended space the boiler is to heat is indeed a loss in efficiency through standby losses. For the boiler.

One could say it’s nice to walk into the area the boiler resides, and say well it’s nice, and warm in here. Now one could also say the heat lost from the boilers standby is heating the space adjacent to the boiler room indirectly. But then that space should have already been at setpoint from the heating of the boiler. So the adjacent space would either be over heated, or lacked the ability to reach desired set point.

gmcinnes

Looks like I opened a can of worms here!

For what it's worth, my boiler is massively oversized, and in a leaky rubble basement. It's also located under a formal dining room that's a "Christmas and Thanksgiving" room. i.e. it's rarely used.

Although I understand that the standby heat is not "lost", in my particular case, since it's is radiated / convected to part of the house that is rarely used, it's not very useful to me.

Unfortunately, we recently moved here, the boiler relatively new, and switching out to a right-sized high-efficiency unit is financially prohibitive just now.

So, until such time as I can afford an upgraded system, I'm trying to do what I can to reduce oil usage. DHW is from an indirect, so I'd also like to keep standby losses on this 413MBH monster lower in the summer too.

It sounds like, given the ballpark numbers calculated here, it would be worth adding extra insulation. If I reduce standby heat losses by even a few percent it will pay itself back in a few months.

gmcinnes

@STEVEusaPA I'm just trying to figure out the standby losses on the boiler specifically.

Circumstances prevent upgrading the boiler at the moment, but it's at 413MBH boiler in a 3000 sq. ft. home, and it also runs an indirect DHW. Keeping that massive hunk of iron hot all year might bankrupt me before I get the opportunity to upgrade it So I was trying to figure out how to get my fuel bill down.

gmcinnes

@hot rod @Zman Boiler weight + water weight would be a great way to double check, but I'm just a lowly homeowner and I have no idea how to figure those things out.

Zman

What model boiler?

gmcinnes

The other goal in trying to figure out standby loss is my total "wasted" oil. ie stack loss + standby loss, to figure out whether it's worth replacing this boiler early. Here's what I decided:

1. If my combustion efficiency is ~=83% as measured by the tech, and my standby losses are ~=9% as measured in this back of the envelope calculation, then my "useful" oil is ~=72%
2. The absolute most oil I could save, in an imaginary world, where everything could be 100% efficient is 28%.
3. Realistically, I'd probably get, at best, ~=90% combustion efficiency and cut standby losses in half to, say ~=4%. Meaning my "useful" oil is ~=86%. For a difference of ~=14%.
4. My annual fuel bill is ~= 12K. So the ballpark savings might be ~ $1700 annually.
5. I've had 4 quotes for an upgrade, and they've ranged from 15K to 22K, giving a payback time of ~9 - 13 years.
6. There's probably about 10 years left in this current CI boiler.
7. A newer High Efficiency boiler probably has a life expectancy of 15 years.


On balance, given the payback times, it's probably not worth switching boilers. But I'm going to investigate adding insulation to the current boiler, and adding Tekmar controls to get boiler purging etc.

gmcinnes

@Zman H.B. Smith 19-A. 4 Sections, one of them has a (disused) sidearm connector.

Eastman

@gmcinnes

What makes up the rest of the heating system?

gmcinnes

@Eastman Mostly cast iron radiators. One 50 gal indirect DHW. Two unit heaters in the garage. Total EDR excluding DHW is 287407

Jamie Hall

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...

Leonard

You said " leaky rubble basement" sounds like field stone foundation you can look out thru the gaps. Little cement to patchup the "vent" holes maybe?? At least keep the wind out....

In my NH house the un insulated hydronic pipes heat the cellar to ~ 65 and that keeps the uninsulated floor of above living space comfortable.

Zman

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.

Zman

It looks like your boiler holds 49.2 gallons of water. If you figure the piping holds another 20, you would have 69.2 gallons * 8.36 lbs/gallon, that would be 579 pounds. Water has a specific heat of 1, so the water in the boiler would lose (or gain) 579 btus per degree.

The boiler looks like it weighs about 1711 pounds. Cast iron has a specific heat of .11. The boiler itself will lose (or gain) about 181 per degrees.

In this example you said the boiler is losing 16.5 degrees per hour. 16.5 * (579 + 181) = 12,540 Btu/hr give or take....

gmcinnes

@Zman The application is DHW and heat in a 3000 sq. ft. house. Think it might be just a little too big? Still, you gotta play the hand you're dealt.

The distribution pipe is mostly 1 1/2" Black Iron, with some 2" thrown in. I haven't measured the length, but it's probably ballpark 300'.

Thanks for going to the trouble to get these numbers for me.

The reason for trying to figure out the stack / standby losses was to figure out the economics of replacing this monster early.

Based on my calculations it looks like it's not quite worth replacing until it fails. Based on your calculations it's probably even less worth replacing because more of the fuel is "useful" (in the sense that more of its BTUs are making it to where I want it to heat and less is going to standby losses).

In the meantime, there's not much I can do about the stack losses, but I thought I might be able to drop the standby losses with addition of some insulation. The cost is so low, it's worth a try.

Jamie Hall

Sounds like you have a plan there, @gmcinnes .

Zman

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.

gmcinnes

@Zman I have a Honeywell L4006A aquastat. I can't see anywhere to adjust the differential so I think it's 2° F or 5° F fixed according to the data sheet. Don't know for sure though.

Surprisingly the boiler doesn't seem to run too often, or short cycle, but I can't put my finger on exactly why. It is one bright spot in the setup though