Thermal post-purge of cast iron boiler
I recently installed Resideo L6006C strap-on aquastats on the supply pipes coming out of our Weil-McLain WGO-5 oil boilers. These are cold-start boilers that start with water temps around 65 degrees, run for 30-40 minutes, then shut down with supply water temps of 130-140 degrees.
The strap-on aquastats are set to continue running the circulators after boiler shutdown until the supply water temp drops to 90 degrees. This thermal post-purge recovers some (hopefully most) of the BTU's remaining in the boiler water and cast iron after shutdown, and circulates those BTU's out to the pipes and radiators in the house.
I had made some previous guesstimates at how many BTU's might remain in the boiler water and cast iron after shutdown, but now with the post-purge aquastats installed, I was able to measure return and supply water temps on a minute-by-minute basis after a boiler run this morning, and calculate how many BTU's were extracted from the boiler during the post purge.
Here is a graph showing the delta T (supply water temp minus return water temp) over a 20-minute period following boiler shutdown:
While runnning, the boiler produces a delta T of about 20 degrees, at a flow rate of about 12 GPM. Knowing the flow rate and the delta T at one-minute intervals during the post-purge lets us calculate the BTU's carried out of the boiler by the circulating water during the post purge.
I'll skip the math and get to the result: the purge extracts about 5,200 additional BTU's from the boiler. So what percentage is that of our total BTU's?
During the boiler run time of 29 minutes, the known oil input rate of 1.18 GPH gives us a total BTU input of 79,800.
But since the boiler combustion efficiency is only about 75% (after considering dry gas and vapor latent heat loss), only about 60,000 of those BTU's got absorbed by the water and the boiler as useful heat. The rest of those BTU's went up the flue.
So using 60,000 useful BTU's as the denominator of our recovery percentage:
(5,200/60,000) x 100 = 8.6%
Which means that the post purge improved our useful heat output by as much as 8.6%.
In reality, even without post-purge, some of those BTU's would have been recovered anyway from the boiler via gravity circulation. We don't know how many BTU's, but let's assume half. That means our post-purge improved our recovery by half, or 4.3%.
So it looks like the post purge could be giving us anywhere from about 4-8% more useful heat from a 29-minute boiler run.
Comments
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At some risk, may I point out that those BTU which you extract will need to be replaced when the boiler fires up from cold… have you figured that into the equation?
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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@Jamie Hall Yes, the boilers typically idle 3-4 hours or more between burns, so they are almost always dead cold at startup anyway, as are the radiators, pipes, and water.
The post purge merely speeds the natural cooling process by purging as much of the residual heat as possible into the building, thereby minimizing residual heat loss up the flue.
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What what? 5 section you said? Got an eraser?
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@HVACNUT Because our boilers are so massively oversized relative to building heat loss, I asked our boiler tech to downfire as much as Weil McLain will allow, which is 80% of rating. So he installed 1.0 GPH nozzles running at 140 psi, which gives the 1.18 GPH actual input rate I used in the calcs above.
1.18 GPH/1.45 GPH rated input = 0.81, or 81% of rated input, so we're at the lower limit of what Weil McLain allows for the WGO-5.
If we were running at rated input, we'd be getting about 350 MBH total gross output (from 2 WGO-5's) in a house that has a heat loss of about 100,000 BTU/hr at zero degrees design temp. So these boilers have over 3x the needed capacity, which is part of our efficiency problem. I'm just trying to make marginal improvements with post purge, etc, until these boilers croak and we can get something better matched to our needs, or maybe repipe one of the boilers primary/secondary to heat the whole building, as suggested by @EdTheHeaterMan .
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@jesmed1 - I added a similar setup (although I used a PLC because my setup has 3 zones) to solve a similar problem, and I'm very happy with it so far! If your system is way oversized (mine is at least 4x vs the max heat loss!) and the boiler isn't in the middle of your living room, the achieved efficiency gets wrecked if you don't have a thermal purge scheme. If you have a place you don't mind dumping a little extra heat, doing a thermal purge seems to cover up a lot of the sins of oversizing.
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@fentonc So you're in the same boat we are…will be interesting to see what our season's oil consumption will be. We have good consumption records from past years so should have a good basis for comparison. Hopefully we save a few percent.
In addition to purge, I've increased our thermostat differential (swing) from 1.0 to 1.5 degrees, which should result in fewer total burns of longer duration, and have also programmed in a night setback that sometimes eliminates one overnight burn (depending on outside air temp) with a longer recovery burn in the morning, which should also increase system efficiency slightly.
Yes, it's just tinkering around the edges, but that's all we can do at this point without major replumbing.
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I have to say I'm skeptical of the whole "thermal purge" concept.
I guess it all depends on where those BTU's would go if they weren't purged. It's a leap to say they're all wasted.
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@DCContrarian said:
"I guess it all depends on where those BTU's would go if they weren't purged. It's a leap to say they're all wasted."
Agreed that they're not all wasted. Some will get pulled into the building through gravity circulation. Some will get drafted up the flue. It's a guess as to what the split is.
However, if you've read Roger's answers about Energy Kinetics boiler efficiencies, you'll see that one of the factors in their efficiency is thermal purging at the end of each boiler run into the indirect water heater. But since we don't have indirect DHW, we're just purging through the heating loop.
I'll be calculating our fuel usage at the end of the season and reporting back with results for the skeptics.
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"Some will get pulled into the building through gravity circulation. Some will get drafted up the flue. "
And some will go into the building just through conduction. The basement is part of the building, after all. And whether or not you purge, the basement is going to be losing heat to the outside, up the flue and through the walls. Does purging change that?
"I'll be calculating our fuel usage at the end of the season and reporting back with results for the skeptics."
I'm also skeptical that an efficiency gain in the neighborhood of 5% is something that can be seen on an individual level. There are too many variables — how cold the winter is, even how many fuel deliveries and their timing.
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@DCContrarian said:
the basement is going to be losing heat to the outside, up the flue and through the walls. Does purging change that?
The entire house is losing heat to the outside. As is your house and everyone else's. That's why we need heating systems. And the more efficient those systems are, the less they cost to run, all else being equal. Purging makes the system more efficient (in theory) by needing to burn slightly less oil over the season to obtain the same total heat output inside the building envelope.
We have done a number of envelope upgrades (attic insulation, better windows, air sealing) to try to reduce heat loss. At some point I'd also like to try an automatic vent damper on the flue to further reduce air infiltration/exfiltration.
Better insulation and reduced air infiltration are two ways to save energy. Another way is to improve the boiler efficiency, and that's what we're talking about here.
I'm also skeptical that an efficiency gain in the neighborhood of 5% is something that can be seen on an individual level. There are too many variables — how cold the winter is, even how many fuel deliveries and their timing.
I've calibrated our fuel tank gauges and I keep track of fuel levels at the start and end of each heating season. We get detailed printouts with our oil deliveries that I also track on a spreadsheet. So using the season starting and ending tank levels, plus the season's total deliveries, I can track how much oil we burned that season to within a few percent.
When I moved here and started doing the building maintenance, we were burning about 1350 gallons per year. With improvements like attic insulation, better windows, and some air sealing, we're down to 1150 gallons per year, so we've reduced consumption by 200 gallons so far.
Yes, marginal savings from things like thermal purge will be harder to measure. This year I've changed a number of small things (raised the thermostat differential, programmed a night setback, and set up the post purge) whose individual effect I won't be able to separate out, but if we see a 5-10% total difference, that cumulative effect would be measurable.
As for how cold the winter is, I download Heating Degree Day records for the season and calculate our HDD's per gallon for each heating season, which eliminates the weather variability factor. If HDD's per gallon go up, that means we've improved our efficiency regardless of whether any given winter was hotter or colder.
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You're still at roughly 150K input at 1.18 GPH.
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do you have an indirect connected?
That is a better place to store the thermal purge as it is well insulated. You need need and want dhw. The hotter the tank the more exergy
If it is overheating a room or space, is the juice worth the squeeze?
Maybe the jacket loss to the mechanical room, if it is within the building envelope is not so bad?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
Yes, but my math was for the actual burn time of the run during which I took the data, which was 29 minutes.
1.18 GPH x 29 minute run time/60 minutes per hr = 0.570 gallons input
0.570 gal x 140,000 BTU/gal = 79,800 total BTU input during that particular burn.
Then we're losing 18% dry gas losses and 7% vapor losses during combustion, for 25% total combustion losses. That means we're transferring 75% to the water and boiler, which works out to about 60,000 BTU's obtained from the burn after combustion losses.
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Yes, would be better if had a DHW tank to purge into, but we don't.
Purge into radiators may result in minor temp overshoot, but temp control isn't critical. I'd rather save a few thousand BTU's per burn if possible. Also, our new ecobee 3 lites are working well and continually recalculating burn times based on previous burns, so overshoots will be corrected in subsequent burns.
Boilers are well insulated, so there is virtually no "jacket" loss strictly speaking. The only way any significant residual heat gets out of the boiler after shut down is through water circulation and air convection up the flue.
We probably do get natural gravity circulation anyway after the burn, but the post purge accelerates it and maximizes heat transfer out of the heat exchanger and up to the pipes/rads.
I don't mind the pipe heat losses in the basement because that's useful space with laundry, etc, and the bare pipes are our only "radiators" down there. So any residual heat that gets carried out of the boiler by water is "useful" heat, even though some of it gets convected/radiated into the basement.
So the main goal of the purge is simply to extract residual heat as fast as possible from the boiler before it convects up the flue. Any heat that doesn't go up the flue I consider "saved."
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@DCContrarian - I agree it's difficult to tell minor improvements from a seasonal fuel-usage perspective (at a minimum, it really needs to be adjusted for heating-degree days), but my current setup tracks the estimated heat delivered (based on second-by-second supply and return temps for each zone when the circulator and zone valves are active), fuel usage and outdoor temperature on an hourly and daily basis.
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Since it sounds like you have a nice digital control setup, have you been able to calculate how much heat you're extracting from your post purge?
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I did some isolated benchmarking earlier:
Along with an elaborate simulator setup to try to get seasonal effects:
It's very difficult to do fiddly experiments when it's cold outside, as my house is extremely affected by incoming sunlight, and the heat loss is affected how warm various parts of the house are (so I can make changes that increase thermal efficiency, make the house warmer, but also increase fuel use in terms of BTU/heating-degree-hour, for instance).
One interesting aspect of my three zone system is that I've just been running the 'colder' zone (the main floor) with the thermostat steady at 68F, and I've turned on thermal purge to all 3 zones, so the upper floor and basement have been heated entirely from the residual heat from the main floor so far. According to my tracking system, my average thermal efficiency so far has been in the mid 60%s, vs <50% previously at these temps, so it seems to be working pretty well.
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@fentonc Thanks, really nice work!
One question on your bar graph in the first link (thermal efficiency vs. purge lower limit temp). You say your boiler is 84% AFUE, which means you're losing 16% in dry gas losses. But because it's a gas boiler, you're also losing about 15% in vapor latent heat. So after 31% total combustion gas losses, you're at 69% real efficiency.
But your last two bars in that chart show efficiencies with purge over 70%. How are you getting more than 69% efficiency when your combustion is only 69% efficient?
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I think you're double-counting the vapor latent heat. I recently had the boiler combustion-tested, and the actual combustion efficiency measured at 76% due to low gas pressure (or possibly a faulty gas valve), which is close to the max of what I was able to hit with the thermal purging in my benchmarking experiments (probably within measurement error of my setup).
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Combustion efficiency analyzers typically only measure "dry gas" losses, so a 76% combustion efficiency result on an 84% AFUE gas-burning boiler means your boiler was running a bit sub-par as you say, but the 76% dry gas efficiency still doesn't include the roughly 15% latent heat of vapor loss.
But maybe you used the "lower heating value" for natural gas of 910-950 BTU/cu ft, which already has the latent heat of vapor loss factored in. (As opposed to the "higher heating value of 1020-1050 BTU/cu ft, which includes the latent vapor heat BTU's). If you did use the "lower heating value," then that's where you accounted for the latent heat of vapor loss.
Anyway, thanks again for sharing your interesting data.
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As far as I know, all values (including my actual 76% and the nominal 84% if the gas pressure weren't too low) are using the 'higher heating value' of natural gas: ~100K BTU per therm (or 100ft^3), and that seems to match with what I can asymptotically approach via thermal purging and such when I try to estimate the actual output of my radiators (my setup measures the supply/return temps of the baseboards at 1 second intervals, estimates the BTU output via linear interpolation from the datasheets + baseboard length and then integrates over time). If you're not condensing, then I think the efficiency is easier to estimate because all of the latent heat is lost. If you are condensing, then you need to know what fraction of the latent heat you managed to recover (i.e. how much condensate and what the outgoing temp of the condensate is). If the only outputs from your boiler were room temperature exhaust gas and room temperature condensate, then I think you would be at 100% efficiency. Without condensing the exhaust gases, I think you're limited to <90%, and with realistic stack temperatures, I think you wind up at the 84% that most non-condensing CI gas boilers target (and if your gas pressure is low, like mine is, your excess air skyrockets and you wind up with ~70-something% efficiency).
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@fentonc OK, thanks. I agree that everything you said about combustion efficiency is correct.
The problem is that you're still missing the 15% latent heat of vapor loss in your calculations, and it's messing up your numbers.
Your boiler as now set up is only 61% efficient. It loses 24% of input BTU's as dry gas. And it loses 15% of input BTU's as latent heat of vapor.
Therefore it is thermodynamically impossible to recover more than 61% of input BTU's in your setup.
But there's an easy way to fix your numbers. I would suggest rerunning at your numbers by:
(1) Reducing your calculated boiler net BTU output by applying both the dry gas loss (24%) and the latent heat of vapor loss (15%) for a combined combustion efficiency of 61%.
(2) Re-running your % recovery numbers relative to that corrected (lower) net BTU output. Since your corrected BTU net output will be lower, your purge recovery % numbers will actually be higher, and in the limit should approach 100%. But they can't be higher than 100%, or something is amiss, because your boiler cannot extract more than 0.61 x 100,000 BTU=61,000 BTU out of a therm the way it is set up.
What I think you'll find is that, when you run the numbers this way, you'll get a recovery rate in the high 80%'s or low 90%'s with your 80-degree purge, the other 10% or less being lost to the piping between the radiator and the boiler, jacket losses, and flue losses during the purge.
Doing the math this way separates out the combustion efficiency from the thermal efficiency of your purge heat transfer, and makes it clearer what your purge is accomplishing. Then your graph will asymptotically approach 100% thermal efficiency as your purge temperature decreases. Meaning you are recovering close to 90% or more of the net output BTU's that your boiler extracted during the combustion process, in which 24% of the input BTU's were lost as dry gas, and 15% were lost as hot vapor, leaving 61% of the input BTU's as your "net output" starting point for your calculations.
Here are some simple examples with some made-up numbers:
Example 1
Therms input during burn: 0.1
BTU input during burn: 0.1 x 100,000 BTU/therm = 10,000 BTU total input
Combustion losses: 24% dry gas loss + 15% vapor loss = 39% combustion loss = 3,900 BTU total combustion loss
Net BTU output after combustion losses= 10,000-3,900 = 6,100 BTU
BTU's emitted by radiator during burn + purge: 5,500 BTU
Thermal efficiency with purge = 5500/6500 = 84.6% (with close to 100% possible)
Or, the other way to run the numbers is to ignore combustion losses totally, in which case you'd get (for the same 0.1 therm input):
Example 2
BTU input during burn: 0.1 x 100,000 BTU/therm = 10,000 BTU total input
BTU's emitted by radiator during burn + purge = 5,500
Combined system efficiency (combustion plus thermal) with purge = 5,500/10,000 = 55% (with close to 61% possible)
Note if you run the numbers this way, your losses include both the combustion efficiency losses and the thermal efficiency losses to piping, etc during the burn and purge, in which case your theoretical recovery rate cannot exceed 61%, because you lost 29% of the total theoretical input to combustion losses.
So if you run the numbers the first way, you can approach a thermal efficiency of 100%. Or if you run the numbers the second way, you can approach a combined combustion plus thermal efficiency of 61%. But the way you're running the number now, you're missing the 15% latent heat loss, which is screwing up your data analysis.
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Can you find a reference that shows the combustion efficiency / AFUE numbers don't already include the latent heat of vaporization loss? I think you're just double-counting the loss. My measurements seem extremely consistent with ~76,000 BTUs being transferred into my boiler+system and ~24,000 BTUs leaving via warm gases or water vapor for every therm of gas burned.
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@fentonc Yes, I explained it here in this old post below. As you'll see from reading the post, even a few pros are still confused and think the AFUE number includes latent heat of vapor losses.
My masters degree involved combustion lab work and analysis, so to answer this AFUE question for myself, last year I printed out the entire TSI "Combustion Analysis Basics" paper that I linked to in the thread below, and did all the manual combustion efficiency calculations for my own boiler by hand, using the TSI formulas, based on the flue temp and % CO2 that our boiler tech measured, and I got exactly the 84% or so AFUE number that his electronic combustion analyzer spit out. And those "dry gas" equations I used to get the same result DID NOT include vapor losses. There is literally nothing in the "dry gas" AFUE equations for vapor losses.
And as you will find in further reading, the combustion analysis instruments used by techs have no way of measuring the vapor content of the flue gas. So the instruments totally ignore the vapor losses.
So I can absolutely guarantee you that the AFUE numbers for non-condensing boilers do NOT include vapor losses.
What seems to be throwing you off is a combination of errors (I don't know which ones) that are giving you a plausible result. It's plausible, but wrong.
This is OK, because after you find and correct the errors, you may be doing better than you thought.
But as I said, if you choose to run the numbers differently, you can just use the "higher heating value" of natural gas, and then report your results measured against that theoretical maximum. But you cannot get more than 69,000 BTU per therm net out of your boiler the way it is configured (16% dry gas loss for 84% AFUE, plus 15% vapor loss).
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Hmm, i'll read through this and think about it. I'm really just tracking burned gas vs estimated delivered heat, and happened to have the 'efficiency' numbers to sanity check things (and the delivered heat seemed to asymptotically approach "burned_gas x efficiency"). If you're right, that would imply I'm systematically over-estimating delivered heat for some reason.
Here is a specific example from a few hours ago from my system:
2024-12-16 18:18:06.102049 HEATING_CYCLE_COMPLETE
BTUS Fuel 102269.02 Heat_est 77724.45
B 14234.84 1st 39428.92 2nd 22216.59 Eff (est) 0.74
Total_cycle_time 30656.15 23248.96 30627.40 23248.96
Burn_time 3068.07
Cycle_hist 335.97 79.16 297.93 80.70 293.89 81.19 295.91 78.69 300.03 80.18 305.83 77.85 301.67 79.13 301.93 78.02It's currently configured so that only the first floor has the thermostat on, and when the call is satisfied it turns on all three zones until the SWT reaches about 70F. So the circulator ran for 8.5 hours, there were 16 burn cycles, that burned approximately 1 therm of gas (102,269 BTUs), and delivered an estimated 75,878 BTUs to the three floors (based on length of baseboard, average water temperature for each zone, and air temperature in that zone, integrated on a second-by-second basis of the zone valve is open), for an estimated 74% of chemical-energy-to-delivered-heat efficiency.
You're asserting that my delivered heat is over-estimated by like 24%, right? (75.9/61=1.24)
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Let's work backwards. Let's assume your BTU measurements/calcs at the radiators are correct. So I believe you are getting those 76,000 BTU's or so at your radiators.
So now I'm going to guess your boiler net output, without knowing anything else.
I expect you have thermal losses to pipes, etc, that you're not measuring those because you're measuring the radiator outputs only. Let's say there's 10% or so of those piping losses. It's impossible to say without knowing your setup. But those piping "losses" aren't really losses, because they're just the heat released by the pipes into your house, which you're not measuring.
So let's say you're measuring 90% of the total BTU's released in the house. The other 10% released into the house are actually being emitted by the pipes, and you're not measuring that (are you?)
So your boiler is outputting maybe 85,000 BTU's net, and you're measuring 76,000 of them, or 90%, at the radiators. So your "thermal recovery" efficiency at the radiators, with purge, is 90%.
So far so good.
So I think your boiler is putting out maybe 85,000 BTU/hr net, after combustion losses. Now you tell me your boiler is 74% AFUE gas. So I know its actual combustion efficiency is about 59% after latent heat of vapor loss.
So now I divide 85,000 BTU/0.59= 144,000 BTU gross input.
Or, in the best case, let's assume your radiator measurements captured 100% of the boiler's net output, instead of 90%. Which means your boiler input was 76,000/.59= 129,000 BTU.
Or, if your AFUE number is wrong, let's assume the mfr's spec, which is 84%. In that case, with vapor loss, your actual efficiency is 69%. In that case, your boiler input was around 85,000/.69 = 123,000 BTU. Or, if your recovery efficiency is 100%, your boiler input was 75,000/.69= 108,000 BTU.
So using a range of reasonable numbers, I would guess your boiler input at a minimum of 108,000 BTU (not far off from your reported 102,000, but this assumes an 84% actual AFUE and 100% of the net boiler output measured at your radiators, both of which are unlikely) and a maximum of 144,000 BTU (assuming 74% AFUE, and 90% recovery efficiency at the radiators).
So I'd say your numbers are off anywhere from 8% to 44%, but most likely in the middle somewhere at 25% or so off. It could be in your measurements/analysis, it could be in the BTU input rate, or (more likely) a combination.
If it were me, I'd start with nailing down the boiler BTU input rate, because that's highly variable depending on manifold pressure, etc. If you've had your boiler tech actually measure the regulator pressure at the boiler, and you know what that number is, you can get an accurate input rate. I think that may also be dependent on temperature, so on cold days, a given regulator pressure is giving you denser gas with higher BTU content per volume, if I remember correctly.
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The input is measured by clocking my gas meter (very reliably 120 cubic feet per hour when the boiler is firing, regardless of outdoor temp) for calibration and then monitoring the “flame” LED output on my boiler. It matches my billed gas usage with my utility within 1-2%, so I’m pretty confident in that number. The output is estimated by 1) measuring the length and type of baseboard in each zone, 2) computing the average water temperature for the zone based on insulated sensors strapped to the supply and return pipes for each zone, 3) measuring the room air temperature for each zone, and 4) using linear interpolation based on the datasheets for the baseboards, adjusted for the actual air temp (vs the data sheet numbers at 65F). Once a second or so I poll everything and accumulate the estimated btu output for each zone, along with how much gas was consumed. I don’t actually use the afue or combustion analyzer “efficiency” numbers at all other than the “heat_est” line that I log.
I think any error in my setup would need to be coming from my model of the baseboard output vs average water temp.0 -
OK, so you have:
Input = 120 CFH
Higher heating value = 1050 BTU/cubic ft
AFUE= 74%-84% (?)
Vapor loss = 15%
Total combustion efficiency = 59%-64%
So your net BTU output is:
120 x 1050 x .59 = 74,340 BTU/hr net output @ 74% AFUE
or
120 x 1050 x .64 = 80,640 BTU/hr net output @ 84% AFUE
And it looks like your total burn time was 0.852 hrs. Which gives you a range of BTU net output of 63,356 to 68,705 BTU depending on which AFUE you believe.
So your 77,724 BTU's measured/calculated is about 23% high based on the low AFUE, and 12% high based on the high AFUE.
You might take another look at your assumptions for the baseboard heat output BTU's per degree. There's some dependence on flow rate. If the flow rate is too low, laminar flow can develop that reduces heat transfer and thus BTU output. Turbulent flow at higher flow rates is better. I forget what the GPM number is for laminar flow developing in baseboards, but I want to say it's under 2 gpm. So if you know your flow is under 2 gpm per loop, that could be a factor in reducing the actual radiator output for a given water temp. (Pros please correct me if that 2 GPM number is wrong. I know I read it somewhere but now can't find where.)
I would believe a number that says you're getting 90% or higher thermal efficiency. So your calculated recovery at the radiators should be somewhere around 60,000 BTU, I would think.
Then 60,000/63,356=95% thermal efficiency with purge assuming 74% AFUE. I think that would be reasonable.
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I read through the TSI document you posted:
Two heating values are typically assigned to fossil fuels depending upon whether the latent heat of the
water formed during combustion is included, or excluded. If the latent heat of water formation is in-
cluded, the heating value is referred to as the fuel’s high heating value or HHV. This is the total fuel
energy determined using a calorimeter. If the latent heat energy is not included, the fuel’s heating value
is referred to its low heating value or LHV. High and low heating values are both used for calculations
of combustion efficiency. Because these heating values can be significantly different, especially for
fuels that have a high hydrogen content, it is important to know which heating value is used. Generally,
in the United States, the HHV is used whenever efficiency calculations are performed. In Europe, the
LHV is often used. Contact your local regulatory agency to determine which value to use.and
NOTE: In Europe, combustion efficiency is often calculated without the latent heat loss from the
formation of water. The dry gas alone is subtracted from 100 percent, and sometimes referred to as
the gross efficiency. In addition, flue gas loss is commonly calculated using the Siegert forumula.
Also refer to the following discussion in “4. Combustion calculations using the Siegert formula” on
the following page.@jesmed1 As a resident of the USA, it still seems like the efficiency numbers I've been discussing are based on the higher heating value of natural gas, and thus do include the latent heat as part of the losses (which, for a non-condensing boiler, you can be confident you are losing all of it). I didn't see anything in that TSI document that disagrees with that.
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@fentonc said:
"As a resident of the USA, it still seems like the efficiency numbers I've been discussing are based on the higher heating value of natural gas, and thus do include the latent heat as part of the losses..."
No, you've got it backwards because of some confusion about what the word "includes" means here.
The HHV is the "high" value because it "includes" ALL the BTU's that can theoretically be extracted from a given quantity of fuel. That's why it's the "high" heating value. It's the maximum number of BTU's you can extract from that fuel, "including" those BTU's tied up in latent heat of vapor.
And the way you extract the maximum number of BTU's from that fuel is by condensing the vapor, which carries about 15% of those BTU's (for natural gas). So if you don't condense the vapor, you are not recovering those 15% of total BTU's. You have now "lost" those 15% of total BTU's. You have not "included" them. They were "included" in the theoretical maximum (HHV) that you could have recovered, but you "lost" them by not condensing.
So your non-condensing boiler lost 15% of the HHV by "losing" the vapor BTU's "included" in the HHV, and it lost 16% of the HHV by "losing" dry gas BTU's as well. So your boiler actually lost a total of 31% of the HHV BTU's.
Now to the LHV.
The LHV already has the vapor losses deducted. So if you use the LHV, then you do not need to do the 15% vapor loss math yourself. It's already been subtracted from the LHV (Note that different references give different values. Some LHV's may only have 10% deducted for natural gas vapor loss).
So if you use the LHV, THEN you can just apply your dry gas loss number (say, 84% AFUE) and multiply it by the LHV, and you will get the correct net BTU output. The LHV already has the vapor loss deducted, and your calculation further deducts the dry gas loss.
I always prefer to start with the HHV and deduct both the vapor loss and dry gas loss myself, because it makes clear where the vapor loss is being accounted for.
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- 374 Solar
- 15K Strictly Steam
- 3.3K Thermostats and Controls
- 50 Water Quality
- 40 Industry Classes
- 47 Job Opportunities
- 17 Recall Announcements