How small can you go for a steam boiler?

Kjmass1

I have about 300 of EDR currently, and at -10F last year, my decades old 192k/153k boiler was running at about 50% duty cycle, so output was 76.5k or so. Is it safe to assume my absolute heatloss limit at -10F is 72k based on the radiation? When the boiler is running 7 minute cycles, is there much a pickup factor that is at play?

Assuming 72k at -10F, my design day at 9F would be around 53k. That would be an EDR of closer to 220. Should I size my radiators closer to their room loads? There are 2-3 that could easily be half as small, especially since they are on the second floor. Any advantage/efficiency in doing something like that, or am I at the smallest size for a boiler anyways.

I need to replace my boiler in the spring (losing a lot of water, likely hole in the top), and wondering if I should downsize some for efficiency reasons.

Edit: Just went back and checked my ecobee from last night and looks like it ran 25% of each hour at 16F, so call it 40K at 16F (192,000*82.5*.25), or around 45K at 9F. That'd be a design day EDR of 188.

ethicalpaul

Your boiler clearly can be smaller. On design day it should be running continuously all day (if I understand this concept correctly).

My opinion is that you don't have to downsize your radiators unless you want to. On a call for heat, they will fill with steam until the call is satisfied and the boiler won't "see" the additional radiation.

Pickup factor is not a factor in my mind.

You're not at the smallest size for a boiler. Peerless goes down to about 230 sq ft and I think Weil-McClain goes down to about 200. Utica/Dunkirk also has a small 3 section boiler about that size.

Kjmass1

Thanks. Just went back and checked my ecobee from last night and looks like it ran 25% of each hour at 16F, so call it 40K at 16F (192,000*82.5*.25), or around 45K at 9F. That'd be a design day EDR of 188.

jesmed1

@Kjmass1 Are you in the Boston area? I live in the Boston suburbs, and we had about 15 F last night and our boiler ran a 24% duty cycle, very similar to yours.

Also, you mention a -10 F event last year. I'm guessing that was the Feb '22 polar vortex. We also got -10 F and 40 mph winds overnight, and our boiler also ran about 50% duty cycle on that occasion. So I'm guessing you live somewhere in eastern Mass.

We run hot water, not steam, but I'm interested in your heat loss numbers for comparison. Last night our heat loss was 1.18 gph input x 140,000 BTU/gal x 0.85 efficiency x 0.24 duty cycle = 33,700 BTU/hr to heat 2400 sq ft (one half of a condo building). So our heat loss/sq ft was 33,700/2400 = 14 BTU/ sq ft.

Would be interested to know square footage and age/construction details of your house for comparison. I'm still trying to get a feel for how our heat loss compares with other buildings in the area.

PS--I also just installed an Ecobee and am appreciating the run time data from beestat.io.

Kjmass1

jesmed1 said:

@Kjmass1 Are you in the Boston area? I live in the Boston suburbs, and we had about 15 F last night and our boiler ran a 24% duty cycle, very similar to yours.

Also, you mention a -10 F event last year. I'm guessing that was the Feb '22 polar vortex. We also got -10 F and 40 mph winds overnight, and our boiler also ran about 50% duty cycle on that occasion. So I'm guessing you live somewhere in eastern Mass.

We run hot water, not steam, but I'm interested in your heat loss numbers for comparison. Last night our heat loss was 1.18 gph input x 140,000 BTU/gal x 0.85 efficiency x 0.24 duty cycle = 33,700 BTU/hr to heat 2400 sq ft (one half of a condo building). So our heat loss/sq ft was 33,700/2400 = 14 BTU/ sq ft.

Would be interested to know square footage and age/construction details of your house for comparison. I'm still trying to get a feel for how our heat loss compares with other buildings in the area.

PS--I also just installed an Ecobee and am appreciating the run time data from beestat.io.

Exactly- just outside of Boston. Single family from 1940s, but insulated the walls with MassSave, spray foam in attic. I have some loose ends in the basement and a bump out addition that need better insulation and air sealing, basement is currently not insulated at all. New windows but nothing high performance. Basement is likely contributing 20-30% to my heatloss, but I need to address water issues before insulating.

Boiler is from 1970s but running at 83%, but massively oversized and no automatic damper.

3 floors, 300sf attic that has low ceiling height and angled roof but it is finished and has a radiator in it. Call it 2400sf based on the foundation outline, 2100sf without the attic. So around 19BTU/sf at design day. You have the benefit of shared wall which is essentially no heatloss assuming the other unit is heated.

Beestat is awesome.

KC_Jones

So if we look at the Weil Mclain catalog and use the classic sizing standard you'd be getting an EG-40.

Obviously this post isn't about classic sizing, but I wanted a baseline for what I'm saying.

Based on what you posted, I'd move down to at least the EG-35. Now here is the deal, and this idea works two ways, the EG-30 is the exact same boiler with a smaller burner tray. So if you got the EG-35, and it felt like you could go smaller, you actually could drop down to the EG-30 and it would be approved by Weil to fire like that.

On the flip side, you could just go right down to the EG-30 and if it just wasn't working for you, then it could be upsized to the EG-35 if needed.

Not sure how many other manufacturers have similar setups. @ChrisJ did this with his EG-45, which is now an EG-40. It's all factory parts to make the change.

I know which way I'd go based on what you posted, but it is your house and your decision.

jesmed1

Kjmass1 said:

Exactly- just outside of Boston. Single family from 1940s, but insulated the walls with MassSave, spray foam in attic. I have some loose ends in the basement and a bump out addition that need better insulation and air sealing, basement is currently not insulated at all. New windows but nothing high performance. Basement is likely contributing 20-30% to my heatloss, but I need to address water issues before insulating.

Boiler is from 1970s but running at 83%, but massively oversized and no automatic damper.

3 floors, 300sf attic that has low ceiling height and angled roof but it is finished and has a radiator in it. Call it 2400sf based on the foundation outline, 2100sf without the attic. So around 19BTU/sf at design day. You have the benefit of shared wall which is essentially no heatloss assuming the other unit is heated.

@Kjmass1 Very interesting, thanks. We are quite similar in size, etc. 1920's or 30's house that was built as a 4-unit, which is unusual around here. We also have an uninsulated basement, and we also lack automatic vent dampers. I'm thinking that should probably be our next upgrade.

As you say, we have the benefit of a shared interior wall, so we have less exterior wall surface than you do. OTOH, we have no insulation in the walls, so it's kind of a wash. We had the original double-hung single-pane windows professionally weatherstripped with spring bronze and silicone rubber seals, and that is working well for better air sealing. We had 10" of cellulose blown into the attic a few years ago. So we end up being very similar at 18-19 BTU/sq ft at design day.

We also have a massively oversized boiler. When it comes time, I'm planning to downsize to around 80,000 BTU/hr, and that will still have a lot of margin.

Kjmass1

KC_Jones said:

So if we look at the Weil Mclain catalog and use the classic sizing standard you'd be getting an EG-40. Obviously this post isn't about classic sizing, but I wanted a baseline for what I'm saying. Based on what you posted, I'd move down to at least the EG-35. Now here is the deal, and this idea works two ways, the EG-30 is the exact same boiler with a smaller burner tray. So if you got the EG-35, and it felt like you could go smaller, you actually could drop down to the EG-30 and it would be approved by Weil to fire like that. On the flip side, you could just go right down to the EG-30 and if it just wasn't working for you, then it could be upsized to the EG-35 if needed. Not sure how many other manufacturers have similar setups. @ChrisJ did this with his EG-45, which is now an EG-40. It's all factory parts to make the change. I know which way I'd go based on what you posted, but it is your house and your decision.

Thanks, great info. Would you downsize radiation to get closer to actual home heatloss? If the boiler is only going to run to satisfy the heatloss, how much savings/difference is there from a boiler that runs nonstop on design day, vs a boiler 2x as big but runs 50% less. Anything to reduce standby losses? 

dabrakeman

I'm at 14.3btuh/sqft 27% duty in an 1890 wooden Victorian (12ft ceilings minimal insulation or improvements but for some windows) over a recent 24hr period averaging 19F outside. I keep the house at 66F but do a nightly 7hr setback to 59F. IT was 19.6Btuh/sqft 37% duty over 24hr averaging 4F. I have an EG-65 which is 21% oversized per traditional method of sizing and with the two bedroom radiators off as I normally run it it is 40% oversized. The home oddly is one where I do not believe it is over radiated in fact because of additions and a previous owner likely moving and changing some radiators around part of the house is under radiated. Certainly would have downsized the boiler had I known then what I know now.

mattmia2

I assume this is 1 pipe? If it is 2 pipe you can use orifice plates (or existing metering valves) to reduce the steam each radiator consumes.

If it is 1 pipe I think if you go below the EDR that is connected(I don't think the pickup factor in a small house is significant so the baked in 30% is way too much), you will need to have everything well balanced, you may even have to slow down the venting of a main to make the steam reach the end of all the mains at the same time if you have multiple mains and they are different lengths. I think it can be done but you will have to do a lot of fiddling with venting to get it to heat all of the emitters.

Kjmass1

mattmia2 said:

I assume this is 1 pipe? If it is 2 pipe you can use orifice plates (or existing metering valves) to reduce the steam each radiator consumes. If it is 1 pipe I think if you go below the EDR that is connected(I don't think the pickup factor in a small house is significant so the baked in 30% is way too much), you will need to have everything well balanced, you may even have to slow down the venting of a main to make the steam reach the end of all the mains at the same time if you have multiple mains and they are different lengths. I think it can be done but you will have to do a lot of fiddling with venting to get it to heat all of the emitters.

Thank- yes 1 pipe. I’ve got all the venting dialed in pretty nicely now, and you are right it takes a lot of time that really only the homeowner can do over a dozen cycles or so. 

KC_Jones

I wouldn't downsize anything with the radiation. Again, feeding less steam the raidation will only fill partially and act like a smaller radiator.

Now, if you want to gain some space for reasons, you could downsize. For me that juice isn't worth the squeeze.

ethicalpaul

You don't have to worry about balancing mains precisely.

Imagine one main vent closes first. What happens next? The other main will get almost all the steam because its main vent is still open.

Yes, a little steam might start going up toward some of the radiators, but the vast majority of the steam will go toward pushing the air out of that second main vent.

Within minutes or seconds that second vent will close then all the steam will start heading to the radiators.

Kjmass1

KC_Jones said:

I wouldn't downsize anything with the radiation. Again, feeding less steam the raidation will only fill partially and act like a smaller radiator.

Now, if you want to gain some space for reasons, you could downsize. For me that juice isn't worth the squeeze.

I doubt any steam pro will install a boiler less than the attached radiation, so the thought was to get the EDR knocked down a bit closer to actual heat loss. Unless you are saying the difference in gas usage between a 220EDR boiler and a 300EDR boiler is negligible as they both satisfy the demand, just with different run times/cycles.

mattmia2

if it doesn't cycle it will be more efficient, there are some losses in it shutting down and cooling off some then having to heat up again. those losses are likely only a couple percent. the wear in the gas valve and especially vent damper if it has one are more significant.

ethicalpaul

I doubt any steam pro will install a boiler less than the attached radiation

Many of them don't even look at the radiation. The real struggle is getting them to install one smaller than the existing boiler

delcrossv

ethicalpaul said:

You don't have to worry about balancing mains precisely.

Imagine one main vent closes first. What happens next? The other main will get almost all the steam because its main vent is still open.

Yes, a little steam might start going up toward some of the radiators, but the vast majority of the steam will go toward pushing the air out of that second main vent.

Within minutes or seconds that second vent will close then all the steam will start heading to the radiators.

Not necessarily. Hence all the "the last radiator on this main never warms up" threads. Timing mains is important, especially if there is a large disparity in main lengths.

ethicalpaul

If the last radiator on a main never gets hot, it’s not due to some tiny time difference between when the mains fill with steam. Read and reconsider what I wrote above.

it’s an overall balance problem or more likely a sag-caused water trap

Steamhead

ethicalpaul said:

If the last radiator on a main never gets hot, it’s not due to some tiny time difference between when the mains fill with steam. Read and reconsider what I wrote above.

it’s an overall balance problem or more likely a sag-caused water trap

Well, this is definitely possible, but 99% of the time we find this problem, upgrading the main vents cured it.

mattmia2

Probably will have to be very careful to slow down the radiators closest to the boiler.

ethicalpaul

All the radiators should be slow of course (this is what helps the mains fill first), but really in a residential system, do we think that the distance to the boiler is really a factor?

What would it be about an additional length of main plus an additional length of 1-1/4" or 1" feeder pipe that would make a difference? Steam is so fluid, those pipe resistances are nothing.

ethicalpaul

Steamhead said:

Well, this is definitely possible, but 99% of the time we find this problem, upgrading the main vents cured it.

That's a big number. I would ask: how?

If some venting failure like main vent is failed closed, then sure. But otherwise I can't see how.

How long does it take for even a too-small main vent to let the main fill with steam? Even if a main takes 5 full minutes longer than another main to fill, as I wrote earlier (and if you think about it, you should see that I make sense), when one main vent closes, then that's all the more steam going into the other main.

Any main vent (and I mean a real main vent, not one of those old tiny Hoffmans) is going to be more attractive than a radiator vent (again, unless a bunch of size D vents are on the radiators) to the steam.

Then the mains are full and there's another 20, 30, or even 40 minutes of firing time for steam to get to the radiators. How could any radiator not get steam in that much time? And the last radiator on the main doesn't matter--the steam doesn't mind flowing wherever the pressure is lowest--and that's not determined by distance in a residential system.

Here's how it can happen:

- main vent failed closed, causing a whole side of the house to be slow to heat
- some radiators with too much venting, causing a main vent to not be attractive enough to steam, and starving some other radiators
- some radiators with failed closed vents (obviously)
- A sag/water trap in a main or radiator runout

tl;dr: trying to match the venting rate of mains is a waste of time

mattmia2

ethicalpaul said:

All the radiators should be slow of course (this is what helps the mains fill first), but really in a residential system, do we think that the distance to the boiler is really a factor?

What would it be about an additional length of main plus an additional length of 1-1/4" or 1" feeder pipe that would make a difference? Steam is so fluid, those pipe resistances are nothing.

The issue isn't about flow resistance, it is about the steam not progressing in the main until it heats the main just ahead of it to steam temps. That takes some time that varies with the length and size of the main. If there isn't enough steam to fill all of the radiators then they all have to start heating at about the same time or the radiators that heat first will pull more steam to them as they condense the steam that has arrived at them.

delcrossv

The issue isn't about flow resistance, it is about the steam not progressing in the main until it heats the main just ahead of it to steam temps. That takes some time that varies with the length and size of the main. If there isn't enough steam to fill all of the radiators then they all have to start heating at about the same time or the radiators that heat first will pull more steam to them as they condense the steam that has arrived at them.

This. No sag, etc., but the steam is filling, and condensing in the fast main and its radiators and never gets to the end of the slow / long main, unless you put the thermostat in the room at the end of the slow main, then everyone else roasts.

Or, you fix the timing.

https://forum.heatinghelp.com/discussion/185113/fast-return-slow-return/p1

ethicalpaul

mattmia2 said:

ethicalpaul said:

All the radiators should be slow of course (this is what helps the mains fill first), but really in a residential system, do we think that the distance to the boiler is really a factor?

What would it be about an additional length of main plus an additional length of 1-1/4" or 1" feeder pipe that would make a difference? Steam is so fluid, those pipe resistances are nothing.

The issue isn't about flow resistance, it is about the steam not progressing in the main until it heats the main just ahead of it to steam temps. That takes some time that varies with the length and size of the main. If there isn't enough steam to fill all of the radiators then they all have to start heating at about the same time or the radiators that heat first will pull more steam to them as they condense the steam that has arrived at them.

It seems like you didn't see or are ignoring my main point...repeatedly. I'll say it one more time just in case I wasn't clear enough about it:

When one main closes, the vast majority of all steam is going to be pushing into the other main, resulting in a very short time difference in the mains filling with steam.

PC7060

jesmed1 said:

@Kjmass1 Very interesting, thanks. We are quite similar in size, etc. 1920's or 30's house that was built as a 4-unit, which is unusual around here. We also have an uninsulated basement, and we also lack automatic vent dampers. I'm thinking that should probably be our next upgrade. As you say, we have the benefit of a shared interior wall, so we have less exterior wall surface than you do. OTOH, we have no insulation in the walls, so it's kind of a wash. We had the original double-hung single-pane windows professionally weatherstripped with spring bronze and silicone rubber seals, and that is working well for better air sealing. We had 10" of cellulose blown into the attic a few years ago. So we end up being very similar at 18-19 BTU/sq ft at design day. We also have a massively oversized boiler. When it comes time, I'm planning to downsize to around 80,000 BTU/hr, and that will still have a lot of margin.

Very common situation. My original cast-iron boiler was 140 KBTU, replaced it 80 KBTU Modcon that run at 50% capacity at design day temperature. 

dabrakeman

The steam will go where resistance is least, yes, but this is not only controlled by the main vents (or the remaining unclosed main vent) but also the sum total of the radiator vents on each main and once steam does reach a radiator the condensation rate. The higher the main venting rates of both mains the least influence those other two factors will have in upsetting balance. I.e. if someone has two mains and one closes a few minutes before the other yes the slow main (say it had one Gorton 1 on it) will catch up quick if the radiation and radiation venting on both mains is fairly equal and slow but if the fast main also had a few radiators with heat timer varivalves wide open then some of those radiators on the slow main may never see heat. In this example balancing and increasing the mains would improve the situation but of course replacing those varivalves would also go a long way

ethicalpaul

delcrossv said:

mattmia2 said:

The issue isn't about flow resistance, it is about the steam not progressing in the main until it heats the main just ahead of it to steam temps. That takes some time that varies with the length and size of the main. If there isn't enough steam to fill all of the radiators then they all have to start heating at about the same time or the radiators that heat first will pull more steam to them as they condense the steam that has arrived at them.

This. No sag, etc., but the steam is filling, and condensing in the fast main and its radiators and never gets to the end of the slow / long main, unless you put the thermostat in the room at the end of the slow main, then everyone else roasts.

Or, you fix the timing.

https://forum.heatinghelp.com/discussion/185113/fast-return-slow-return/p1

No, you fix your radiator venting (balance). The only reason I mentioned flow resistance is because someone mentioned "the last radiator".

All you need to do is make sure that mains have reasonable venting (meaning much more than radiators), and then the radiators can be balanced.

This obsession with making mains be exactly matched is unnecessary is all I'm saying.

109A_5

Hello @Kjmass1,
" How small can you go for a steam boiler? "

My old boiler is 300 Sq. Ft., my EDR is 347.

I would leave the radiation alone and just best match the new boiler size to the EDR. Boiler Sq. Ft. = EDR. So no pressure cycling issues. My system runs at less than 1 Once pressure, the house heats fine.

delcrossv

ethicalpaul said:

delcrossv said:

mattmia2 said:

The issue isn't about flow resistance, it is about the steam not progressing in the main until it heats the main just ahead of it to steam temps. That takes some time that varies with the length and size of the main. If there isn't enough steam to fill all of the radiators then they all have to start heating at about the same time or the radiators that heat first will pull more steam to them as they condense the steam that has arrived at them.

This. No sag, etc., but the steam is filling, and condensing in the fast main and its radiators and never gets to the end of the slow / long main, unless you put the thermostat in the room at the end of the slow main, then everyone else roasts.

Or, you fix the timing.

https://forum.heatinghelp.com/discussion/185113/fast-return-slow-return/p1

No, you fix your radiator venting (balance). The only reason I mentioned flow resistance is because someone mentioned "the last radiator".

All you need to do is make sure that mains have reasonable venting (meaning much more than radiators), and then the radiators can be balanced.

This obsession with making mains be exactly matched is unnecessary is all I'm saying.

Well after several rounds of balancing radiators with poor result, followed by excellent results from timing the mains, I'll have to disagree with you on this one.

That said, the timing doesn't have to be "exact" just "close enough". I do think this never came up with coal fired boilers as the system was kept warm- so having mains with vastly different EDRs and lengths didn't cause issues.

LRCCBJ

Kjmass1 said:

KC_Jones said:

I wouldn't downsize anything with the radiation. Again, feeding less steam the raidation will only fill partially and act like a smaller radiator.

Now, if you want to gain some space for reasons, you could downsize. For me that juice isn't worth the squeeze.

I doubt any steam pro will install a boiler less than the attached radiation, so the thought was to get the EDR knocked down a bit closer to actual heat loss. Unless you are saying the difference in gas usage between a 220EDR boiler and a 300EDR boiler is negligible as they both satisfy the demand, just with different run times/cycles.

The difference is non-existent if you maintain temperature of the space. The larger boiler will simply shutdown earlier.

However, if you do setback, and the larger boiler develops pressure, and shuts down on pressure, it will definitely increase gas usage due to the constant restarts while the thermostat remains closed,

Jamie Hall

I wonder. It seems to be an item of accepted lore that the fuel usage will be greater if a boiler shuts down on pressure and restarts quickly. Sorry. At least for oil, I don't buy it, not one little bit, based on observation of a couple of moderately sized oil burners. What I see with them is this: the pressure rises (slowly, since they are sized correctly) and, eventually, reaches the cutout (in the one I observe most closely, 6 ounces). The burner shuts off, and there is a 15 second post purge. The pressure drops to the cutin during the post purge. The vapourstat recycles and, after a brief pause, there is a 30 second pre purge, during which the pressure drops further, but never to zero and the boiler does not go off the boil (residual heat). The oil burner starts, and within a matter of seconds (literally) the flame is stable again The system never stops steaming. The flame has perhaps two to five seconds of unstable ignition, if that. Which amount to 0.0009 gallons of fuel oil for that burner, at the most, wasted. 0.002 dollars worth of oil at today's prices.

Now I can see different control schemes where there may be some wastage on cycling -- but only if the boiler is allowed to cool between cycles. Otherwise, no. Sorry. Don't buy it, won't buy it.

LRCCBJ

Jamie Hall said:

I wonder. It seems to be an item of accepted lore that the fuel usage will be greater if a boiler shuts down on pressure and restarts quickly. Sorry. At least for oil, I don't buy it, not one little bit, based on observation of a couple of moderately sized oil burners. Otherwise, no. Sorry. Don't buy it, won't buy it.

If your conclusion is valid, you'll have to refrain from recommending that people carefully size the boiler to the radiation. If they don't, the boiler is "too large" and will absolutely cycle on pressure. Since you will not buy the fact that this reduces the efficiency of the boiler, there will never be any need on HH to carefully size a boiler to the installed radiation and any oversized boiler will function with the same efficiency as a properly sized boiler. It will simply cycle endlessly on pressure without loss of efficiency.



Sorry, but I don't think you will get any support for that position.

Jamie Hall

LRCCBJ said:

Jamie Hall said:

I wonder. It seems to be an item of accepted lore that the fuel usage will be greater if a boiler shuts down on pressure and restarts quickly. Sorry. At least for oil, I don't buy it, not one little bit, based on observation of a couple of moderately sized oil burners. Otherwise, no. Sorry. Don't buy it, won't buy it.

If your conclusion is valid, you'll have to refrain from recommending that people carefully size the boiler to the radiation. If they don't, the boiler is "too large" and will absolutely cycle on pressure. Since you will not buy the fact that this reduces the efficiency of the boiler, there will never be any need on HH to carefully size a boiler to the installed radiation and any oversized boiler will function with the same efficiency as a properly sized boiler. It will simply cycle endlessly on pressure without loss of efficiency.



Sorry, but I don't think you will get any support for that position.

No I don't think I'll get much support. Not a popular position. But I'm used to that -- I've been making comments based on observation and experimentation and actual physics for over 60 years now, and it's hard to buck general knowledge, even when it's manifestly wrong.

There is a good reason to specify right size equipment, however -- whether you are doing a boiler, a pump, a vehicle, or whatever. On the specific case of a cycling boiler, there is a very good reason to avoid needless cycling -- but it's not efficiency. It's wear and tear on all the starter components and to a certain extent thermal stress on the burners. Bigger tends to be more expensive, sometimes you are off the best efficiency or power setting for the equipment, whatever. It's called "doing it right" -- and that's none too popular these days, either.

Jamie Hall

I might add -- as a VERY general comment, not confined to heating at all, but applying to life in general -- doing the right thing for the wrong reason is never defensible; nor is doing the wrong thing for the right reason. The general rubric is "the end never justifies the means". Each action must be justified on its own merits.

Also a wildly unpopular opinion these days.

dabrakeman

Maybe I am missing something but doesn't it take more energy to get a bigger boiler steaming than a smaller one and during that time 18% of the energy is going up the chimney?

mattmia2

How much heat is carried in the post purge and pre purge?

Back to @ethicalpaul s comment. If all the steam is being consumed by the radiators that are heating before it reaches the last radiator, that radiator will never heat. They all have to start heating at about the same time if the boilers output is a radiator or more less than the connected edr

Jamie Hall

dabrakeman said:

Maybe I am missing something but doesn't it take more energy to get a bigger boiler steaming than a smaller one and during that time 18% of the energy is going up the chimney?

That gets really complicated, but in general, yes it does. That doesn't help overall efficiency -- but it doesn't apply to very short (two or three minute off cycles). It is why long off cycles are to be avoided -- but that has to be balanced by the need to avoid large temperature swings in the conditioned space.

Simple answers just aren't there -- but overall a "right size" boiler is to be preferred!

jumper

This question is do you lose or do you lose. If boiler is too small to adequately heat the place @10° below zero then it is too damn bad during twenty days of sub zero. If boiler can handle that extreme condition then it will cycle when weather is normal. Historically we tolerate cycling but some folks complain when radiator goes cold.
With HHW some use hot water temperature reset determined by outside temperature. Some folk then complain that radiators don't heat up fast enough.

dabrakeman

People are the problem! Always complaining

ethicalpaul

mattmia2 said:

How much heat is carried in the post purge and pre purge? Back to @ethicalpaul s comment. If all the steam is being consumed by the radiators that are heating before it reaches the last radiator, that radiator will never heat. They all have to start heating at about the same time if the boilers output is a radiator or more less than the connected edr

If some of the radiators can steal all the steam from other radiators, then the system is dramatically undersized.

that’s not what I’m talking about.

i am talking about spending unnecessary effort matching the main venting of multiple mains. It’s very difficult to slay all the straw men that are born here, I have learned.

regarding what @Jamie Hall said, I support your statement 100% for what that’s worth haha!

There is almost no lost fuel/efficiency for an oversized steam boiler (gas even less so!)—due to cycling. All the net BTUs are going into the home regardless

But correct sizing is still good for several reasons I and others can think of:

- smaller boilers are less expensive with less/smaller/less expensive near piping 

- the additional cycling wears out a few components such as gas valve/blower motors

- people hate hearing cycling, it pisses them off and makes them think the install was wrong (it was to be fair)

- here’s one I haven’t heard before that I recall: by building pressure, you are prone to overshooting the thermostat more (even 2psi is a lot of heat energy stored in the system)—something those of us with correctly-sized systems don’t see

- not related to cycling, but above someone said it takes longer firing to boil the water initially—this is valid of course