Welcome! Here are the website rules, as well as some tips for using this forum.

# Boiler sizing from previous fuel usage

Options
Member Posts: 9
Hello, and sorry for the long post.
I’ve been trying to estimate the appropriate size steam boiler for my house. The boiler I would be replacing is rated for 175 input, 136 gross output, I=B=R Net 102, 425 sqft steam.
Using resources available online my radiators represent about 350 EDR, so by that standard alone the boiler is already oversized. Unless I’m missing something, in which case please let me know.
A look back at my fuel usage and heating degree days for last January, both full month and coldest 10 days of January seem to match up with each other in terms of heating load, but I may have goofed. Did I do this right?
January 20-29,
159 ccf, 468 hdd (heating degree days) @ 65 bp (balance point), 418 hdd @ 60 bp.
I understand the equation to be ((BTUs x Efficiency/HDD)/24hrs) x Delta T (using 3F as 99.6% design). And then times a fudge factor of 1.4. (I know I don’t need all these parentheses, but they help me organize my mind)
((15,900,000 x (136/175)/468)/24) x 65 = 71.5k btu load. If I multiply by 1.4, that’s 100k btu. Calculation without efficiency (since it won’t improve much) but without fudge factor = 92
((15,900,000 x (136/175)/418)/24) x 65 = 80k btu load. Multiplied by 1.4 is 112 k btu. Calculation without efficiency (since it won’t improve much) but without fudge factor = 103k btu

For the whole month, January 1-31,
453 ccf, 1297.5 hdd @65 bp, 1142.5hdd @ 60 bp
((45,300,000 x (136/175)/1298)/24) x 65 = 73.4k btu. Multiplied by 1.4 is 102.7k btu. Calculation without efficiency and fudge factor, 94.5k btu.
((45,300,000 x (136/175)/1143)/24) x 65 = 83.4btu. Multiplied by 1.4 is 116.7k btu. Calculation without efficiency and fudge factor, 107.3K btu.

HOWEVER, If I take the “safe” (x1.4) average suggested in the article of the four which is 107.8k btu, and divide by the 240 btu to EDR rule, I get essentially 450 EDR, which I know we don’t have.
If I take the uninflated average of 77K and divide by 240 I get 285 EDR, which I know we have significantly more than.
If I use the average of btu’s used without efficiency calc and fudge, it’s 99k, 413 EDR, again more than we have.
Of note, we have a gas stove and hot water. I did not attempt to remove that .5ccf/day or so from my calcs.
Also, I know there is a standard “pick up” multiplication for steam of 1.3 or so, but that is from Calculated EDR btu to make up for pipe heat up (?!)? Since I have actual load use data (which obviously satisfies any “pick up”) is this of any use? Also, what is the point of the 1.4 fudge factor, if we are determining heat loss per hdd from historical data? Also, any new boiler should be incrementally more efficient (3+% at min.)
Full disclosure, I had to google balance point, and understand it to be the temp at which an occupied structure no longer heats or cools when mechanical conditioning is not in use.
My question is, where should my head be at thinking about this? Should I get a boiler to fill the EDR that I have? Why if it represents a btu/hr higher than I calculated? Why, since the system obviously works most of the year below “max capacity” already? But if so, that would be something around 85kbtu, which for steam 1.3 pick up translates to about 110kbtu input. Should I shoot for something closer to the estimates derived from the coldest week/month last year? That would be closer to 77kbtu input. The house almost certainly could/should be made tighter as well.
Sorry for probably repeating things and talking over myself. Thanks in advance for any wisdom.
Iggs

• Member Posts: 23,303
Options
Mymy. This is steam? Way back up at the beginning you state you have about 350 EDR of radiation. So, this being steam, that -- and only that -- is what you size your boiler to. Not more than that, though a little under is fine. To make it even easier, steam boilers are rated by EDR, as well as an assortment of BTUh mumbo jumbo.

Just match the two together. No problem at all.

Now if it were scorched air or hot water some of that sizing to load would have to be done -- although there are easier ways to do it. But not steam. Green Building Advisor seems to have never heard about steam...

A steam system automatically sizes itself to the load though the thermostat and the pressure controls on the boiler.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 2,703
Options
being that you're a steam system,
you don't size a future boiler to past gas usage, nor the house heat loss,
you size to your connected EDR, 350,
• Member Posts: 9
Options
Thanks for the responses. How much below measured EDR is it safe to go? If my EDR is (just to pull a number) 1.5 times what my house needs, why do I need a boiler capable of filling it? If 1/3 of my total radiant emitting surfaces are a mistake, must I and every occupant hereafter have to deal with that mistake by upsizing the boiler (short of changing radiators) every time it needs replacing? Assuming each radiator is filling at a similar pace, why must they fill all the way across. Or is the filling at a similar pace the bugaboo? If so, nothing to be done about it?
Jamie Hall, you said "A steam system automatically sizes itself to the load though the thermostat and the pressure controls on the boiler." Can you elaborate? I want to understand.
• Member Posts: 9
edited October 2022
Options
@Jamie Hall , just tagging you cause I should have in the comment above. Newbie to this stuff. Only got clued in to the @ thing because I got a badge for the @ 65 degree balance point.
• Member Posts: 2,703
Options
the boiler needs to make enough steam to fill and condense in all the connected radiation, the connected EDR, your, or you're, 350,
only if you knew you would be shutting off some rads in rooms that you don't care to heat would you consider a smaller EDR boiler, and then the boiler might not heat well when all rads and rooms are opened up again
• Member Posts: 2,703
Options
• Member Posts: 918
edited October 2022
Options
The main problem with an undersized steam boiler is that the furthest radiators from the boiler may heat insufficiently or not at all, while the nearby ones may fill all the way.  It’s difficult to correct for inadequate capacity by adjusting radiator vents.

By contrast, with a hot water or hot air system undersizing simply causes lower supply temperature throughout the entire system.

This is why replacement steam boilers are sized to fill the installed radiation, while replacement hot water boilers or hot air furnaces are sized to building heat loss.

Most of the time the steam system’s thermostat will be satisfied before the radiators are completely filled, except on an extremely cold day or while recovering from setback.

Bburd
• Member Posts: 9
Options
Thanks again all.
@bburd, you said, "Most of the time the steam system’s thermostat will be satisfied before the radiators are completely filled, except on an extremely cold day or while recovering from setback."  How is this different than using a boiler with an output lower than calculated EDR?  If my rads heat relatively evenly in October or April, could I consider my system well balanced enough to go with a boiler rated smaller than calculated EDR?  If I get steam to the end of my main befor heading up to rads, would the radiators really fill in order from nearest to farthest?  Dynamics engineer I am not...
I'm really not trying to be a pita, just trying to understand.
• Member Posts: 918
Options
This relates back to what Jamey said, the radiators will operate at reduced capacity under lighter loads assuming the system is well balanced.

When a steam system with inadequate boiler capacity is pushed hard, bad things tend to happen from a balance standpoint. I can’t really explain it in more detail, but this is the collective experience of those who understand steam heating and have learned from previous problems.

Bburd
• Member Posts: 15,526
Options
@Iggs

With steam the boiler is sized to the connected radiation. The MFG includes a 1.33 pick up factor in the calculation to heat the steam piping and cold radiators. Some think the 1.33 factor is excessive. The main question is are all your steam lines insulated or not?

If they are not insulated stick with the plan and and size the boiler to the connected edr.

If everything is insulated Then you could pick a boiler not more than 10-15% below your load
• Member Posts: 23,303
Options
It can be a little hard to visualise. However, you have to remember that you really have two different things going on.

The first is that the building is losing heat at a certain rate. That's your heat loss, and it varies with the outside temperature and wind, among other things. In any heating system, there will be at least one thermostat which senses the space temperature and, if it gets too low from the heat loss, call for heat from the boiler (or furnace or whatever, of course). The boiler -- or whatever -- will respond by heating water or air or, in the case of steam, creating steam, which is then transported to the building and warms the building. When the building is warm enough, the thermostat will sense that and shut off the boiler.

Now there are some modern hot water boilers which can modulate the firing rate -- based on outside temperature, but most boilers don't do that, so the best they can do is turn on and off, so the average heat output, averaged over time, is what is needed. Just for example, suppose you only need 25,000 BTUh to heat the structure, but the boiler puts out 100,000 BTUh. In that instance, the boiler will run one quarter of the time -- say 15 minutes out of each hour -- making the average 25,000 BTUh . This does result in some temperature variation -- but if the system is properly sized and controlled within itself, this will not be noticeable.

So you want a boiler which is capable of making as much steam as all the radiators working together can condense. If less heat is needed than that -- which will be the case most of the time -- the radiators won't heat all the way across -- it takes time for them to heat up -- and the thermostat will be satisfied before they have a chance to. Steam production ends, the radiators stop getting steam, and gradually cool off while keeping the room warm. The room gets a little cooler -- the thermostat asks for more steam, it comes to the radiators, and they warm up again.

Now if the boiler is too big, the radiators -- even hot all the way across -- can't condense all the steam the boiler makes, and the pressure in the system must rise. There is a pressure control on the boiler which senses that and turns the boiler off -- thermostat or no -- until the radiators can catch up, and then turns the boiler back on if the space still needs more heat. This isn't ideal and it does reduce the efficiency of the system a little, so you try to get the boiler sized as closely to the radiation as you can. in most cases -- and on most days -- the thermostat will turn the boiler off before it gets close to that. The exception is if you use larger setbacks -- which really aren't recommended for steam systems and, for that matter, most hot water systems.

Rather long winded -- I'm sorry -- but does this help any?
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 9
Options
What I'm getting is it could be a real pain in the butt to balance a system with a boiler significantly less powerful than EDR calls for, due to finicky pressure differentials.  Thanks for all the effort explaining.
I did just find the "help" section, and saw the article linked below discussing sizing for oversized EDR.  Mentioned a burner that can modulate its output,  so I assume this I a custom job.  Doesn't seem like the thought has caught on with many folk, so maybe more finicky than most want to deal with.
Again, I really appreciate all the input.  This site is great.
https://heatinghelp.com/systems-help-center/taking-another-look-at-steam-boiler-sizing-methods/
• Member Posts: 469
Options
Are you sure you got the EDR right? Post some pictures of your rads. Front and side on each. Measure the height from the floor. It might be less than you think. Possibly more, but likely less. Won't be able to tell unless we see. What is the thickness of each section?
• Member, Moderator, Administrator Posts: 2,284
edited October 2022
Options
neilc said:
@neilc we're in the midst of a redesign on that area of the site to make everything easier to find. Thanks for your patience with us as we re-organize!
President
HeatingHelp.com
• Member Posts: 1,215
edited October 2022
Options
HI,

I wrote the article you are referencing and its hold true for smaller one pipe steam systems. However, larger one pipe systems are harder to keep in balance, so the more conservative method of sizing to radiators applies. However, if you have a two pipe system, these limits pretty much go out the door. Orificed supply valves can control the steam delivery to each radiator and provide system balance when the boiler is operating below the radiator capacity. This technique was lost in history around the Great depression. If you look carefully at the information from the big two pipe manufacturers from the 1930's, ( WEbster, Illinois, etc) you will see many had begun using orifices in thier two pipe radiator trap systems for the balance advantages. We see them in a number of systems here in Chicago in the original radiator valves. WE've set up systems that had 3 million btu's input boiler capacity and could run them in all but the coldest weather with a boilers running at only 500,00 btu/hr input.
I should add that some 2 pipe systems still need an extra boost at cold start up to get even distribution. Where we could see the piping, it often came down to the steam preferring the branch mains coming off the top of the system main over those that came off the bottom of the system main. On this system, we ran the boiler at about 2/3's of the peak building heat loss until steam was established at the ends of the main and then dropped the modulated burner back down to the capacity that matched the current heat loss of the building. This design modulated the heating plant directly proportional to the outdoor temperature. once we started getting into the lower 40's of so outdoor, the burner would run almost continuously, modulating to match the outdoor temperature.
• Member Posts: 9
Options
@The Steam Whisperer, thanks so much for dropping in! My system is, as I mentioned, approx 350 EDR, 1900 sq ft. If I believe that the average of my calc's comes out to 100k btus in, that would suggest I would get less than 80k net DOE out, which translates to 62k btu coming off the rads after 1.3 pick up (but since my pipes are insulated, maybe more like 1.15 pick up coming to 70k coming off the rads? Anyway, 260 -290 EDR called for, unless I'm screwing something up!) One pipe. One big mains loop around the basement of a 1920's brick veneer bungalow (had some kind of insulation blown into walls before we purchased). Two second story radiators in one big room, one more in bath. Depending on how I look at it, either 30-50+btu/sq ft, whether I consider output before or after pick-up. Would you consider this a small or large one pipe system. If small, how low would you go? And what sort of vents would you try (1A's?)? Again, thanks for dropping in!
• Member Posts: 9
Options
@random12345, here's  pics. 3 column, 38" tall, 6 section.  I'm calculating 30 EDR.  Am i on the wrong track?   American Radiator Co.
• Member Posts: 1,215
Options
I would consider that small. For vents we use Ventrite adjustables for low venting rates and balancing. yours rads are only 84,000 out, so something around 100,000 input should be pretty safe. You really have to balance the system and be sure it is cycling in the coldest weather, otherwise the steam will lean towards the closest radiators.

• Member Posts: 469
Options
That does indeed look like a Peerless 3 column from American Radiator. 30 is right. Do you have a bunch of those? When was your house built?
• Member Posts: 9
edited October 2022
Options
@The Steam Whisperer, could you clarify what you mean by "make sure the system is cycling"?  Do you mean running long cycles, making sure the thermostat isn't satisfied too soon, or?....

@random12345, yeah, got mostly those 3 column in a couple different heights with varying #s of sections, and a couple odd ducks.  House is Mid 20s.
• Member Posts: 1,215
Options
Making sure that the thermostat is cycling the boiler...that way you know that the boiler is steadily filling the radiators during the whole heating on cycle.
• Member Posts: 9
edited October 2022
Options
@The Steam Whisperer I am probably being dense.  I assumed the boiler turns on when the thermostat falls below set temp, unless its above pressure setting, in which case if temp is below setpoint, it will turn on when pressure falls below setpoint.  And that it turns off when temp is satisfied, or pressure cutoff is reached.
And that if the system is functioning properly,  the pressure cutoff will very rarely, if ever, come in to play (like after winter power outage and huge temp rise is called for...)
• Member Posts: 23,303
Options
that's basically it.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 131
edited October 2022
Options
I understand the idea of "right sizing". I changed from 80kBTU hot-air (PO got 'a good deal') to 39kBTU warm air (what I figured the house needed).
In retrospect, don't try to cut it too fine. An oversize furnace/boiler will burn more fuel, but in modern designs not a LOT more fuel. It will come to set-temp quicker and shut-off (stop the fuel and fire) sooner than the "right size" fire. Then it comes to standby-losses. These were large on some of the old systems which had heaps of coal and hot cast-iron wafting up the chimney. They don't make them like that today.
In steam the side effect of barely-enough steam is one room hot and another room cold. "They say" that restrictors and vents can balance the system but this may be a lost art, and tedious to re-discover (I tried in my last house).
Remember that Oct Nov Dec Mar Apr you do NOT need your design-max 100kBTU (or whatever) because it is not zero degrees out every day and night. So your fire is *always* "oversized" except that one night in February. Letting it be a mite oversized for that one night won't have a major impact on your annual fuel bill.

And in retrospect, I now wish I'd got the one-size-up furnace. They were the same price (same fan, same finning, just one more \$20 gas nozzle). The 39kBTU is ample all but a few days a winter, but those days it can run 20 hours straight and not come to planned indoor temp. Especially if the dogs have to go out every few hours.
• Member Posts: 1,215
Options
PRR said:

I understand the idea of "right sizing". I changed from 80kBTU hot-air (PO got 'a good deal') to 39kBTU warm air (what I figured the house needed).
In retrospect, don't try to cut it too fine. An oversize furnace/boiler will burn more fuel, but in modern designs not a LOT more fuel. It will come to set-temp quicker and shut-off (stop the fuel and fire) sooner than the "right size" fire. Then it comes to standby-losses. These were large on some of the old systems which had heaps of coal and hot cast-iron wafting up the chimney. They don't make them like that today.
In steam the side effect of barely-enough steam is one room hot and another room cold. "They say" that restrictors and vents can balance the system but this may be a lost art, and tedious to re-discover (I tried in my last house).
Remember that Oct Nov Dec Mar Apr you do NOT need your design-max 100kBTU (or whatever) because it is not zero degrees out every day and night. So your fire is *always* "oversized" except that one night in February. Letting it be a mite oversized for that one night won't have a major impact on your annual fuel bill.

And in retrospect, I now wish I'd got the one-size-up furnace. They were the same price (same fan, same finning, just one more \$20 gas nozzle). The 39kBTU is ample all but a few days a winter, but those days it can run 20 hours straight and not come to planned indoor temp. Especially if the dogs have to go out every few hours.

That sounds like a description on an undersized equipment if it can't maintain temperature at the required design temperature. Also, typical forced air systems will create additional heat loss (due to added air leakage) beyond normal design standards due to the effects on the pressurization and depressurization of the building the system can cause. DOE and other reseachers have all found this out with typical U.S. installed "modern" forced air ductwork systems. It's probably one of the reasons why " ductsealing" is a big piece of energy savings programs.