Boiler is oversized...so what?

Roddy

I've read a number of posts on the site about oversized boilers. On the absolute coldest day of the year here in the Chicago suburbs (about -20F degrees), my 40 year old Weil McLain boiler runs 50% of the time to keep up. On these coldest days, every room is nice and warm, and every radiator is just about fully warm across the whole radiator. 69 degrees is maintained near the thermostat. The system is single pipe steam, and I'm guessing the boiler is oversized based on what I just wrote, but what would be different if it were correctly sized?

On average winter days, my first level stays at 69 degrees (which is the thermostat setting and the stat is located on the first floor), and my upstairs, bedroom level stays at about 63 degrees. On these average winter days, all of my first floor radiators get warm about one third across all the columns, but up on my second floor the radiators get less warm.

Plenty of main venting, adjustable Hoffman radiator vents (or equivalent) on most radiators. Only thing I might do is insulate the returns. Upstairs radiators are fully vented, fully open to max. First floor radiators settings vary by room based on desired heat, etc. Furnace never cycles on pressure unless I bump the stat up by a degree or two, which is very rare.

So, while I guess my boiler is oversized (?), I'm not really complaining. My question is how would my heat be different if I were correctly sized? Would I be saving fuel with a different size boiler? I certainly don't plan on replacing my boiler until it fails. My last bill for 31 days (Nicor gas is our provider) was the highest ($171) ever as I recall, compared to previous years---but part of that is Nicor's rates have gone up this year, approximately 5%. By the way, my house is 100 years old, 2,000 square feet, two floors, plus a full basement.

Any thoughts, suggestions?

Thanks,
DRod, the interested novice

Hap_Hazzard

If your boiler was correctly sized, it could supply steam to all your radiators as fast as they can condense it and continue doing so until the thermostat is satisfied. From what you've said, your boiler may be undersized, but it sounds like your radiators aren't balanced as well as they could be.

The only way to tell if your boiler is correctly sized is to add up the EDR of every radiator and compare the total to the rating of the boiler. The EDR of the boiler should be on a ratings plate on the boiler. If you can't find it, and you know the model number, we can look it up for you.

To determine the EDR of a radiator, you have to know the make and model of the radiator and count the number of columns and sections, then look it up in a reference such as Dan Holohan's EDR book, which you can order through this site, or ask someone like me who owns the book to look it up for you. We can also help you identify your radiators if you post pictures of them.

Also, if the exposed distribution pipes in your basement aren't insulated, you need to count them as radiators, so you need to either account for that or insulate them, depending on whether you need to keep your basement that warm.

Jamie Hall

In answer to your direct question -- "My question is how would my heat be different if I were correctly sized? Would I be saving fuel with a different size boiler?" -- the answers are nit wouldn't, and not enough to notice the difference.

I'm not surprised that Nicor rates have gone up this year. They will go up more -- quite possibly a lot more -- next year, but that's because of recent political decisions on a national scale. Not much you can do about that now...

bucksnort

Frankly I'd be more worried about what it's going to cost you in general to live where you do. How long are you planning on living there? Do you have an exit plan like many others are doing? I heard taking a toilet out of a bath can be a real $$ saver.

EBEBRATT-Ed

@Roddy
an oversized boiler on steam will cycle on pressure a lot. Stopping and starting the burner puts ware and tare on the burner and rapid short cycling bothers people and wastes fuel. The longer the burn time the better for efficiency

But if your boiler only hits pressure control when coming out of night set back etc occasional cycling on pressure is ok

SteamingatMohawk

@EBEBRATT-Ed Do you have any idea how much might be saved if my Cyclegard was swapped out for a LWCO that doesn't have the intermittent level test for 90 seconds every 10 minutes?

What wear and tear occurs on burners cycling? No moving parts. I could agree there could be wear and tear of the gas valve. That's part of why I asked the question above.

jhrost

It depends on how oversized your unit is. If your Weil-Mclain was twice as big as it needed to be the burner would often come on to heat that gigantic reservoir of water and then kick off before a single radiator got warm. As it got colder out you would start to get a little warmth, but the radiators would never truly get hot until after a substantial night time set back. You would be paying a 40 or 50% higher fuel bill than with a modestly oversized unit, but never really feel comfortable. There would be less fresh air in the house as well, a tip off to this is the way the cellar door will rattle when the monster burners come on and start to suck in air.

EBEBRATT-Ed

@SteamingatMohawk

For some reason I was thinking the OP had oil but I don't think he said. NEVER ASSUME LOL

With gas, I would say just the gas (atmospheric I assume LOL) valve. And those are probably tested for thousands of cycles.

I doubt you would save enough money to pay to buy a new low water cutoff.

Oil burners with a transformer and starting and stopping the motor was more what I was thinking

PMJ

@Roddy,

My boiler never runs more than 50% of the time to heat at -20F either. It is sized "properly" to the installed radiation or so they say.

You seem quite happy with your results, and at 40 years the boiler is obviously quite happy with the situation too. If you really aren't cycling on pressure much now meaning your steam delivery is all at low pressure, a smaller boiler isn't going to save you much. For systems reasonably well vented and balanced - which it does sound like yours is, you would save little in fuel going with a smaller boiler - certainly not enough to justify doing a replacement.

My advice is to enjoy your well made boiler as long as you can. Any replacement won't be of the same quality. I was advised by a boiler expert in 1992 not to replace my then 35 year old boiler because even back then he said "the one you have will outrun anything you can buy today even with all those years on it already". He was right. I still have it.

SteamingatMohawk

@EBEBRATT-Ed I agree with your reply.

The Steam Whisperer

I use the attached chart to work out possible fuel savings for installing a properly sized boiler. It sounds like yours is not so oversized as to cause short cycling on pressure, which is really hard on the boiler and efficiency. This chart assumes the boiler has no stack damper and is the typical atmospheric boiler installed in most homes in Chicago. Since natural gas has historically been so cheap in Chicago ( about 1/3 that of just over the border in Indiana) these inefficient designs are installed widespread.

Bucksnort... Real Estate in the city of Chicago continues to be about the cheapest of any world class city in the world. The suburbs tend to see much higher taxes than the city, but they also usually have better high schools. With the average cost of a single family unit over 1.2 million in New York and San Francisco with Paris and London being comparable, you can buy a very nice 2200 to 2500 sq ft brick single family detached home in a nice neighborhood for about $450,000 in the city. Lake front homes of similiar quality can be had for about $600,000, though the neighborhood nearby is somewhat sketchy here and there. These lakefront homes are only about a 20 minute drive into downtown Chicago.

Living in Major World Class cities is always going to be more expensive than a small town somewhere where values are so low that it is not even worthwhile improving or maintaining an existing home or building new. Also, with cheap real estate comes very low wages too. I've lived in both areas.....I'll take the friendliness of Chicago over the paranoia of the small towns I lived in anyday.

PMJ

@The Steam Whisperer ,

If I am reading your chart correctly the OP is at 50% load on design day and so let's say 25% load on an average day. So his average day is currently at 90% of the efficiency of a boiler running at 100% on an average day. If he were to downsize to a boiler that would be 100% on design day( the smallest boiler possible) his average day would run at 97% of the efficiency of a continuous average day boiler. So by your chart he is 7% worse off on efficiency being this oversize from the smallest boiler possible right?

7% annually on my bill with my big boiler would save me about $70. And this chart is with no flue damper which I will argue is a game changer with this. Not much doing here.

The Steam Whisperer

You have the idea, but its not quite that simple. The seasonal efficiency is the addition of all the efficiencies from when the boiler is off to the peak load over the entire heating season. When you do a bin data analysis taking into account the number of hours the boiler operates at various loads you end up with the seasonal efficiencies marked in pencil on the chart. The seasonal efficiency of a non-dampered boiler is about 70% or about 88% of the steady state efficiency....0.88x .8 = 70%, For a boiler doubled sized of peak heating load ( not radiation load) seasonal efficiency comes in at about 0.75 X .8 = 60%. This works out to a 15% reduction in fuel usage by going to the smaller boiler. From personal experience with a hot water boiler, a properly sized dampered boiler will save about 10% of fuel over a double sized dampered boiler.

PMJ

I see @The Steam Whisperer .

Well however arrived at it seems that boilers sized to the radiation per the IBR rating run half the time on design day. Mine is sized for 1053 sqft steam and when I add it up I get 1000sqft. When I clock the meter it seems downsized as much as 25% and still only half of it is needed at -20F.

Anyway, the suggestion is that if I installed 1/2 my boiler it would barely get it done on design day and I would save 10% or $100 per season. I like the extra and the obviously better quality boiler.

bburd

@PMJ A boiler half that size might heat the building, but only the radiators close to the boiler would heat fully. The rest would be starved for steam, and your balance would be all out of whack.

PMJ

bburd said:

@PMJ A boiler half that size might heat the building, but only the radiators close to the boiler would heat fully. The rest would be starved for steam, and your balance would be all out of whack.

How did the coal fired boiler do it evenly then? On an average day it would have been making steam at half the maximum rate of a boiler half the size of mine?

ethicalpaul

Did it do it evenly?

Jamie Hall

PMJ said:

bburd said:

@PMJ A boiler half that size might heat the building, but only the radiators close to the boiler would heat fully. The rest would be starved for steam, and your balance would be all out of whack.

How did the coal fired boiler do it evenly then? On an average day it would have been making steam at half the maximum rate of a boiler half the size of mine?

Sort of true... except. The coal boiler was making steam all the time. All day. Provided somebody remembered to stoke it. Given time it would tend to even out -- but even so, I'd bet that when it was chilly upstairs it got fired harder. People did that...

The Steam Whisperer

PMJ. I have found that radiators are generally about 60% oversized for the heat load at design temperature if they were installed after the Flu Epidemic. Prior to the Flu they were much smaller. -20 F design must mean you are way north. Chicago is about 0F.
Since conventional one pipe steam systems need the boiler sized to the radiation, they take a hit in efficiency since the boiler ends up being about twice the size needed for design. This is why I recommend stack dampers on atmospherics or installing power burner boilers. The efficiency curve for a power burner boiler is quite flat until you get under about 20% load, and then it begins to drop.

With two pipe, I orifice the radiators down to design heat load and this allows me to put in a design load sized boiler... saving a bunch on the boiler installation cost and bumping up efficiency. Bigger jobs get 2 smaller boilers that are step fired, or a single large boiler with the burner set up on outdoor reset. With these set ups we see fuel usage reductions as high as 50% over the full on/full off boilers they replaced.

PMJ

The Steam Whisperer said:

PMJ. I have found that radiators are generally about 60% oversized for the heat load at design temperature if they were installed after the Flu Epidemic. Prior to the Flu they were much smaller. -20 F design must mean you are way north. Chicago is about 0F.
Since conventional one pipe steam systems need the boiler sized to the radiation, they take a hit in efficiency since the boiler ends up being about twice the size needed for design. This is why I recommend stack dampers on atmospherics or installing power burner boilers. The efficiency curve for a power burner boiler is quite flat until you get under about 20% load, and then it begins to drop.

With two pipe, I orifice the radiators down to design heat load and this allows me to put in a design load sized boiler... saving a bunch on the boiler installation cost and bumping up efficiency. Bigger jobs get 2 smaller boilers that are step fired, or a single large boiler with the burner set up on outdoor reset. With these set ups we see fuel usage reductions as high as 50% over the full on/full off boilers they replaced.

Actually I'm not north. Cleveland - but I've seen -20 here.

I've never been able to see how any radiation downstream of what is needed for the load on any given day has anything to do with anything. I've heard the flu analogy for radiation sizing many times - perhaps it's true. But I don't see how a continuous coal system could have been automatically adjustable with ultra sensitive pressure devices if the radiation was ever close to full or the boiler running anything but mid-range. And a big boiler at that so relatively small changes to the damper actually made a change relative to the system. My theory as to why these systems have so much "extra" anyway.

As I've said many times, I "size" the boiler by not running it so much. Haven't fussed with a radiator valve in years. I fill them all partly full and let vacuum take things from there - both the run/wait times and the balancing.

PMJ

I might also point out that "sizing to the radiation" assumes that all the radiation is needed on the coldest days, which in my case and most other systems of this vintage it clearly is not.

Don't get me wrong - I do like having extra of everything.

The Steam Whisperer

I disagree, sizing to radiation (steam) does not assume all radiation is needed on the coldest days. As I said before nearly all radiators are about 60% oversized for the peak heat load. The problem is getting the system to balance both in moderate and extreme weather. Venting mains properly and radiators at the proper sizes helps enormously, but at the extremes is where problems tend to occur. I have found that steam still tends to favor the closest radiators in extreme weather, so if you don't have that extra capacity to fill the all the radiators, you may end up with trouble. You can do a lot with venting in homes on one pipe steam to make undersized ( to the radiation) boilers work, but when you get to multifamily, people will mess with the radiator vent the moment you leave.

PMJ

The Steam Whisperer said:

I disagree, sizing to radiation (steam) does not assume all radiation is needed on the coldest days. As I said before nearly all radiators are about 60% oversized for the peak heat load. The problem is getting the system to balance both in moderate and extreme weather. Venting mains properly and radiators at the proper sizes helps enormously, but at the extremes is where problems tend to occur. I have found that steam still tends to favor the closest radiators in extreme weather, so if you don't have that extra capacity to fill the all the radiators, you may end up with trouble. You can do a lot with venting in homes on one pipe steam to make undersized ( to the radiation) boilers work, but when you get to multifamily, people will mess with the radiator vent the moment you leave.

We need to get the terms defined then to do this.

Let's start with your definition of "peak load" and go from there.

JakeCK

Design temp in Cleveland Ohio is 2.5f if I'm not mistaken, not -20. Can we get to -20? Maybe once every 20-30 years for a few hours. We're more likely to see a -20 wind chill. 

JakeCK

Actually after I looked it up Cleveland's recorded coldest temperature ever was -20 in 94'. Records go back to 1872. 

PMJ

JakeCK said:

Actually after I looked it up Cleveland's recorded coldest temperature ever was -20 in 94'. Records go back to 1872. 

Right. I ran my current boiler on that day in '94. Took a little over 50% on time to keep up.

I did not say it was design temp for Cleveland.

JakeCK

PMJ said:

JakeCK said:

Actually after I looked it up Cleveland's recorded coldest temperature ever was -20 in 94'. Records go back to 1872. 

Right. I ran my current boiler on that day in '94. Took a little over 50% on time to keep up.

I did not say it was design temp for Cleveland.

Sorry, when I read this that's how it sounded to me.

PMJ said:

I see @The Steam Whisperer . Well however arrived at it seems that boilers sized to the radiation per the IBR rating run half the time on design day. Mine is sized for 1053 sqft steam and when I add it up I get 1000sqft. When I clock the meter it seems downsized as much as 25% and still only half of it is needed at -20F. Anyway, the suggestion is that if I installed 1/2 my boiler it would barely get it done on design day and I would save 10% or $100 per season. I like the extra and the obviously better quality boiler.

PMJ

JakeCK said:

PMJ said:

JakeCK said:

Actually after I looked it up Cleveland's recorded coldest temperature ever was -20 in 94'. Records go back to 1872. 

Right. I ran my current boiler on that day in '94. Took a little over 50% on time to keep up.

I did not say it was design temp for Cleveland.

Sorry, when I read this that's how it sounded to me.

PMJ said:

I see @The Steam Whisperer .

Well however arrived at it seems that boilers sized to the radiation per the IBR rating run half the time on design day. Mine is sized for 1053 sqft steam and when I add it up I get 1000sqft. When I clock the meter it seems downsized as much as 25% and still only half of it is needed at -20F.

Anyway, the suggestion is that if I installed 1/2 my boiler it would barely get it done on design day and I would save 10% or $100 per season. I like the extra and the obviously better quality boiler.

I see how it might have.

Anyway, I'm still looking for some definitions regarding "peak load", "design day load" etc. Also what percent on time should be expected from a boiler sized to installed radiation to the IBR rating. This is still what I see recommended here.

I realize in many cases insulation may have been improved. In my case it has not so I am looking at original steam install in original load conditions.

Anyone who wishes to attempt to define something go ahead.