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# Heat Loss Calculations

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Member Posts: 114
Hi again everyone,
So I used this heat loss calculator, https://www.builditsolar.com/References/Calculators/HeatLossOld/HeatLoss.htm

and this R value calculator for the walls
https://ekotrope.com/r-value-calculator/

I have balloon framed house with an EIFS exterior (which includes a layer of styrofoam) and mineral wool on the inside. To be conservative, I gave the assembly an R-value of 7.

To give an example, I have a bedroom with two exterior walls, 1 window, 1 sliding door, and the dimensions are about 14 x 16. It is on the second floor of a three-story house. I come up with 209 sqft of wall, 11 sqft of window, 32 sqft of door, and the calculator says the heat loss is 4460 BTU/hr. This seems to be plenty, as the room currently has a 4 foot section of fin tube, which I estimate to put out 4000 BTU, and the room heats just fine. So, that just confirms my R-value is on the conservative side.

1. Does that sound right to you? I read somewhere that a rule of thumb is the square footage x 40, which gives me 8960. Is it possible I am off by a factor of two, or is it just the difference between rule of thumb and actually calculating it out?

2. If I go through the house and replace some radiators with smaller ones based on the heat loss calculation, will that mess up the balancing for the old radiators that stay? Will I need to replace the air vents with different sizes to keep everything balanced? (I have one-pipe steam, BTW) My gut says no, but I wanted to check.

Thanks in advance, you guys are awesome.

~Jackie

• Member Posts: 8,578
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You can compare the heatloss from the SlantFin app, and see how close they are.
If you change the radiators for smaller ones, that will make your present boiler oversized, and less efficient. There could be some practical problems in replacing radiators, such as needing new inlet valves.—NBC
• Member Posts: 22,263
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Hi again everyone,
So I used this heat loss calculator, https://www.builditsolar.com/References/Calculators/HeatLossOld/HeatLoss.htm

and this R value calculator for the walls
https://ekotrope.com/r-value-calculator/

I have balloon framed house with an EIFS exterior (which includes a layer of styrofoam) and mineral wool on the inside. To be conservative, I gave the assembly an R-value of 7.

To give an example, I have a bedroom with two exterior walls, 1 window, 1 sliding door, and the dimensions are about 14 x 16. It is on the second floor of a three-story house. I come up with 209 sqft of wall, 11 sqft of window, 32 sqft of door, and the calculator says the heat loss is 4460 BTU/hr. This seems to be plenty, as the room currently has a 4 foot section of fin tube, which I estimate to put out 4000 BTU, and the room heats just fine. So, that just confirms my R-value is on the conservative side.

1. Does that sound right to you? I read somewhere that a rule of thumb is the square footage x 40, which gives me 8960. Is it possible I am off by a factor of two, or is it just the difference between rule of thumb and actually calculating it out?

2. If I go through the house and replace some radiators with smaller ones based on the heat loss calculation, will that mess up the balancing for the old radiators that stay? Will I need to replace the air vents with different sizes to keep everything balanced? (I have one-pipe steam, BTW) My gut says no, but I wanted to check.

Thanks in advance, you guys are awesome.

~Jackie

The load calc is an educated guess really, based on some assumptions as far as building construction and most importantly infiltration, how much leakage around the windows and structure. Assumptions on how the home or rooms are used, doors opening and closing, internal gains form lighting and appliances, etc. Most believe there is some fudge factor built into the programs also, maybe 10%?

The best indication of the actual load may be the current heat emitter. If these maintain all the rooms at the temperature you desire, stick with them or emitters of the same output.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 884
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"I read somewhere that a rule of thumb is the square footage x 40"

This is an old school rule of thumb, and you know what.....you will NEVER under size an emitter or boiler if you use it

Your 4k sounds pretty reasonable to me.
• Member Posts: 114
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@nicholas bonham-carter
Slantfin gave me 5120 BTU/hr, which is about 10% over my calculation. Good to know I am in the ballpark. Why would one need new inlet valves? Is it because they are probably so old that if I touch it, it will probably need to be replaced?

Also, if my boiler ends up being oversized and less efficient, does that mean it will cost me more to heat, or just that the thing will by cycling on and off more than ideal?
• Member Posts: 8,578
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The connection between the inlet valve, and the radiator consists of a matched set of spud with union faces, and valve. The pieces are particular to each other, and will not seal in a mix"match situation. Removing the spud from the radiator is very difficult, so it is best to keep the radiators in place.
One property of steam is to only condense what is needed in the radiator, even though on a mild day, it may only fill halfway.
any overheating rooms can be adjusted using the venting-to some degree.--NBC
• Member Posts: 15,631
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Op stated copper fin tube.

The more fin tube you install the lower water temperature you can run =more fuel saved. Less fin tube=higher water temp=more fuel burner.......within reason.

If you replace the existing I would keep it the same size if your heating ok. I would not install less. keep the same amount in each room .... so you don't upset the balance. Or do a complete new heat loss and install fin tube accordingly
• Member Posts: 85
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If you really wanted to get exact with the heat loss calculation, you could do it in Excel and factor in a different R-value for the windows, walls, ceiling (if it has an attic above it) and floor (if cold basement below it).

At least that is what I did to get exact estimates because every off-the-shelf program gave me answers that seemed 1.5 to 2x too high (they were too high, it turns out).

R-7 for your wall sounds a little low...but a conservative value will ensure that you have enough heat.

But, the answer you are getting may be fine for what you are trying to do. If you are trying to perfect-size new radiators for each room to solve some issue with localized room overheating, or something like that, I would try to get more precise on the calculations.
• Member Posts: 85
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Just saw your other post...it's easy to setup in Excel if you are proficient in Excel.

Heat Loss = U * Area * Delta T

U = 1/R-value (the inverse of R-value). Units are BTUs per Hour per (Square Foot * degree F)
Area = in units of square feet
Delta T = Temperature differential inside vs. outside (in degrees Fahrenheit).

When you multiply them all together, Area units and Temperature units cancel and you are left with BTUs/hour.

Be sure you treat the walls, windows, floors, and ceiling separately. If you don't know exactly how good the insulation is inside the wall cavity, be a little conservative. Also, there are studs and headers in the wall that are not as insulative as the mineral fiber, so reduce by 20% to your perfect wall R-value calculation to account for framing wood thermal bridging.
• Member Posts: 11,077
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Is this 1 pipe steam or hot water?
• Member Posts: 395
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Just saw your other post...it's easy to setup in Excel if you are proficient in Excel.

Heat Loss = U * Area * Delta T

U = 1/R-value (the inverse of R-value). Units are BTUs per Hour per (Square Foot * degree F)
Area = in units of square feet
Delta T = Temperature differential inside vs. outside (in degrees Fahrenheit).

When you multiply them all together, Area units and Temperature units cancel and you are left with BTUs/hour.

Be sure you treat the walls, windows, floors, and ceiling separately. If you don't know exactly how good the insulation is inside the wall cavity, be a little conservative. Also, there are studs and headers in the wall that are not as insulative as the mineral fiber, so reduce by 20% to your perfect wall R-value calculation to account for framing wood thermal bridging.

I used Excel also after getting rather different results in a few of the online tools such as the Slant/Fin. The conductive heat loss was the relatively easy part. The harder part was making a good estimate of the loss due to infiltration and that has a significant affect on the total heat loss.
• Member Posts: 85
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Voyager said:

The harder part was making a good estimate of the loss due to infiltration and that has a significant affect on the total heat loss.

True enough in an older home. My renovated house has a 2.8 ACH50 before drywall, so I can basically ignore infiltration losses. If the home is not tight, adding 20% to the heat loss calculations would be a reasonable assumption.
• Member Posts: 114
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@JUGHNE, I should have clarified that it's 1-pipe steam. I have been thinking about this system for so long that I forget other types exist!

@kenjohnson I play Excel like the piano (queen of the index-match and pivottable), so doing my own heat loss will be way easier than using these online calculators. Thanks for the tip!
• Member Posts: 114
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@kenjohnson what is the formula for infiltration losses?
• Member Posts: 609
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A word of caution to the excel people, the math is not hard, but the secret sauce in is in some of the assumptions (about leakage, construction quality, thermal bridging, etc) in the various models. That's why you see a 10-50% variation even when you specify the same size and construction details in the different calculators.

loadcalc.net is a good one, it's not pretty but seems to be more accurate in the real world than the slantfin one, IMHO.
• Member Posts: 395
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SuperJ said:

A word of caution to the excel people, the math is not hard, but the secret sauce in is in some of the assumptions (about leakage, construction quality, thermal bridging, etc) in the various models. That's why you see a 10-50% variation even when you specify the same size and construction details in the different calculators.

loadcalc.net is a good one, it's not pretty but seems to be more accurate in the real world than the slantfin one, IMHO.

I agree. The reason I ended up with Excel is because I wanted to see exactly what affect varying the assumptions would make. With the online tools, none told me exactly what they were doing or what assumptions they were making behind the scenes. With Excel, I put in the equations so I knew exactly what the variables were and I could do sensitivity analysis. The hardest was the edge loss through my slab as it is neither insulated nor open to ambient. It has a 1/2” asphalt expansion joint and then abuts an 8” hollow CMU. So, I guessed at a low R value and went from there. Infiltration loss was the second hardest assumption to make. I didn’t want to bother with a full blown measurement as I have only a workshop, not a McMansion, so I took a reasonable guess.

So far, things look pretty good. I designed for a 60 degree delta T and I am seeing almost exactly that. We had a few days where the temps were in single digits during the day and -15 at night with 30 MPH winds. The shop got down to a low of 52 with the boiler running flat-out. I am pretty happy with that as it will maintain my desired indoor temp of 60 in all but the coldest and windiest conditions and the gas usage is so low I almost can’t tell it is there compared to what my home has used historically. The added gas usage of the workshop is literally almost not noticeable. So, I am a happy camper and I just hope that the little TT boiler lives a long life since I decided to spend the extra \$1400 or so above what a water heater would have cost.
• Member Posts: 22,263
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WAG as far as the infiltration number. You could have a blower door test run, but again it would be just for that one point in time.

Good, better best is usually enough to get you in the ballpark for infiltration number. Try it several ways to see how much the load changes.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 23,450
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There is no usable formula for infiltration. As @hot_rod said, WAG is the way to go. To get any closer you'd need a terrific amount of computing power to model the aerodynamics of the building, and much more information about its construction than is ever available.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 609
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True, one bad defect in a building envelope can skew things mightily. And they aren't always obvious.
• Member Posts: 616
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If you have good records, you can use gas consumption compared to HDD to get a check on the program or spreadsheet methods.
Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread: http://forum.heatinghelp.com/discussion/154385
• Member Posts: 3,231
edited February 2019
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Your screen name being ForestHillsjd, I'm going to assume you're in Queens, NY. That "square footage x 40" RoT is for a part of the world colder than ours. I'm in Manhattan. Here in NYC we tend to use sq. ft x 30, or cubic feet x 3.5 (or "x 4" if we're feeling like there's excessive infiltration or low R-value to deal with). But be careful. All the rules-of-thumb tend to be geared toward standalone buildings. Not row houses. I've done actual heat loss calculations for attached Brooklyn and Manhattan townhouses and brownstones that have a heat loss of not more than 17-19 Btus per square foot. That means big (by our standards) 4-story \$6,000,000 homes being heated very well with 150,000 BTU boilers.
Contact John "JohnNY" Cataneo, NYC Master Plumber, Lic 1784
Consulting & Troubleshooting
Heating in NYC or NJ.
Classes
• Member Posts: 609
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Brewbeer said:

If you have good records, you can use gas consumption compared to HDD to get a check on the program or spreadsheet methods.

This works pretty well. Mine was pretty accurate once I accounted for the baseload consumption of NG (from my dryer, stove, and DHW).
• Member Posts: 11,077
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So you want to keep the existing boiler but downsize some radiation?
Just to clarify, is there fin tube in the future picture?
• Member Posts: 85
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If you know your ACH50, you could get a rough estimate of natural ventilation, see https://www.energystar.gov/ia/home_improvement/home_sealing/ES_HS_Spec_v1_0b.pdf

I made my house renovation super-tight, and the blower door test came in at 2.8 ACH50. I basically assumed no infiltration loss, but I also assumed no gain from background heat sources (refrigerators, lighting, people, etc.). If you read the link, my calculated natural ventilation would be about 0.15 air-change per hour - pretty low (and based on how my heating system is operating, it might be less than that). But this will change based on how windy it is outside.

If your house was really leaky (maybe 15 ACH50), then you might have 1 air change per hour. If your house was that leaky, you'd probably feel a lot of uncomfortable drafts. More likely, you are probably about 0.3 or 0.4. If you thought of it as 0.2, that would probably account for ignoring the ambient heat generation. That's why I said just assume 20%.

When you do your Excel calculations, make sure to separately calculate rim joist areas, treat the trim around windows as perhaps the same R-value as the windows (if there are leaky, uninsulated pockets), account for wall framing, etc. It was easy for me - I had open walls and could see what I had. Then, I modeled the walls with the exact wall framing factor. It was a lot of calcs, but I'm an engineer and I wanted to model it right. If you know Excel and are patient, it's not hard to get this right. And as another poster pointed out, you can change your assumptions and see the result, whereas the canned programs are just black boxes.
• Member Posts: 114
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@JohnNY, yup, you're right, I'm in Forest Hills Queens (but not the fancy part). It's a freestanding house. The previous owners put EIFS on the exterior, so it was pretty tight already, and now that all the walls are open, we've also stuffed mineral wool between all the studs (which are, unfortunately, only 2 x 4) My conservative calculations are averaging out to about 20 BTU/sf when I give the exterior wall an R-value of 10.

@JUGHNE, yes, I am keeping my boiler, but I'm replacing almost all the radiators. Assuming I get my heat loss calculations correctly, will there be any downside to reducing the size of my radiators? Will I notice a difference in comfort? Once I downsize the radiators, I figure that when the boiler dies (it's about 10 years old already), I can replace with a smaller boiler.

@kenjohnson Yeah, the EIFS (which, albeit ugly and not my first choice for cladding a house) makes the house pretty tight. I don't feel any drafts in the winter, so I agree that 20% is the right number for infiltration.
• Member Posts: 609
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You may be able to shuffle around many of your existing radiators to rooms that are a closer match to their size.
• Member Posts: 5,737
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I love that you're willing to replace/move your radiators in order to match your actual heat loss.
NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
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@hot_rod I always use poor, below average, and not so bad.
Steve Minnich
• Member Posts: 114
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@ethicalpaul I like that you put a sight glass on your riser! I'm glad there is place like this for folks to congregate. My journey started about four years ago when I moved into an old house with one-pipe steam and the realtor said I had to put a bucket under this tap and drain a little water out every week. I grew up on forced hot air, so I thought, "Uh oh, I better learn about this steam stuff," so I bought and read all of Dan's books, and started tweaking the things in my house. The only problem is that now when I walk into a steam building with poor balance or pitch or malfunctioning vents, my eye starts twitching.
• Member Posts: 114
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Hi friends,
So, I faithfully did my heat loss calculations for my three-story, 2700 square foot house. My boiler is Weil-McLain EG/PEG-50 with an output of 145,000 BTUs/hr.

It had about 80,000 BTUs/hr of connected radiators, but based on heat loss calculations, I reduced the total radiators to 50,000 BTUs/hr. (I was replacing the radiators with shorter ones so they could fit better in the built-ins, but since I was at it, I went ahead and sized them according to heat loss.)

Problem is, the radiators got delivered today, and they look so tiny compared to the old ones that it has me second-guessing myself about whether they are properly sized. Also, is it possible that even though the output is 37.5% less, the volume of these things is more than 37.5% smaller? Is there like, less air inside the tubes? They just seem so dinky! I had the 25" three column type and went to the 19" Gov-free ones. I'm trying to picture the order of operations.
My big-a\$\$ boiler will power on.
The properly vented system will fill with steam quickly.
The steam will condense in the radiators, sacrificing their latent heat which gets stored in the cast iron.
The soul of the steam (water) will slink back to the boiler, to be reincarnated later.
The radiators will give heat up via convection and radiation to the room.
Room heats up, thermostat will turn the boiler off. (Is there a universe when the pressure will actually cause it to cut out first?)
Room continues to heat because of the heat stored in the cast iron, causing the temp to overshoot (by a lot? a little?)
Room cools down.
Boiler fires up again.
Do I have that right?

But I'm assuming the boiler will cycle more frequently now because the bucket of heat stored in each radiator is smaller. If so, is that a good or bad thing? Will it take longer to heat up? Will it keep the temperature swings in the house within a narrower band? Is the cycling bad for efficiency?

Is there a disadvantage to having a boiler so big relative to the connected radiation other than it cost more, takes more space and needs a bigger vent?

Thanks in advance for listening to the rantings of a mad woman.

~Jackie
• Member Posts: 23,450
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What will happen is that your giant economy size boiler will cycle on pressure while the thermostat is calling. Probably, since it is very much oversize, frequently. Basically what the pressure cycling is doing is effectively reducing the boiler output -- size -- to what the radiators can condense.

You will still want the thermostat set to probably 2 cycles per hour anyway, but you'll have to live with the boiler cycling.

Do make sure that the pressure control on the boiler is set properly. 1.5 psi for the cutout is plenty high! Raising it won't help anything, just bun more fuel.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 1,194
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If EDR is accurate (I think it overstates actual output in many cases depending on shape, venting and installation details). It also assumes I think 65F room temp and base of radiator), then a 33% pickup for a uninsulated header (common on systems before 1920) will probably still hit pressure limit on a setback. I think 10-15% is more realistic and 235 BTU/sqft is a better number for most radiators unless you run >1psi normally.

But you have to manage the venting rate (easier on 1 pipe, hard on 2 pipe) or a smaller boiler will have balance issues. Main has to be very very well vented with a small boiler.

As for overshoot, it is reduced if everything happens slower. If the radiators take 20 minutes to fully heat ... or never fully heat from a warm start, instead of 10 minutes, then overshoot is reduced. Everything happens slower. dad from a radiator located 15’ from a thermostat, maybe take 5 minutes to reach the thermostat, plus probably a 1-2 minute averaging in the sensor.

Further, the smaller boiler runs longer, so less time is spend idle, releasing heat to a semi or unconditioned space. Flue dampers are not leak free, so less time spent in idle venting out heat. Also, smaller boiler has a smaller flue, lower water volume, so again, fewer losses.

One challenge with a smaller boiler is that you may need a little condensate storage on the return just below the normal water line.
• Member Posts: 1,265
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"As for overshoot, it is reduced if everything happens slower".

Correct. What surprises me though is how complicated people make doing the slowing down, quickly moving to worrying about the size of the boiler and/or the installed radiation. If the total steam supplied per hour equals the actual demand, there is no overshoot. Matter of fact there would be no satisfaction of the call at all - it would be endless. So the shorter the time from call to satisfaction, the more the overshoot. With steam systems these things are inversely related.

But it is fine for the boiler to be big and deliver the steam fast when it is being delivered; what is needed is just that the total amount be limited and delivered in more shorter pulses. This is, at the end of the day, what Tecmar and Ecosteam really do. A simple duty cycle timer can provide much overshoot improvement at a much lower cost. Certainly much less than changing hardware.

@foresthillsjd , I'm struggling to picture a scenario where I would ever consider reducing my excess installed radiation. I simply break up the burns in smaller pieces and limit the total amount of steam that can be produced per hour and never fill those big radiators so full.

1926 1000EDR Mouat 2 pipe vapor system,1957 Bryant Boiler 463,000 BTU input, Natural vacuum operation with single solenoid vent, Custom PLC control
• Member Posts: 7,573
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The shorter cycles will reduce overall boiler efficiency.
Oversized radiators in a steam system will have a tendency to overheat the space. It is not like a HW system where you can reduce the temp. Steam is Hot!
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 1,265
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> @Zman said:
> The shorter cycles will reduce overall boiler efficiency.
> Oversized radiators in a steam system will have a tendency to overheat the space. It is not like a HW system where you can reduce the temp. Steam is Hot!

My system demonstrates every day that neither of these things need to be the case. The length of the call is a direct measure of how much overheating is happening. With regard to efficiency, time from fire to steam actually being delivered to the rads is the "lost" time. That total time is actually shorter with more cycles but each of them starting with much warmer pipes.
1926 1000EDR Mouat 2 pipe vapor system,1957 Bryant Boiler 463,000 BTU input, Natural vacuum operation with single solenoid vent, Custom PLC control
• Member Posts: 9,546
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PWM for steam is how I'm understanding PMJ's theory in control. Small bursts of energy over a period of time into a big collector (rads). Verses one, or two big fills.

• Member Posts: 1,265
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> @Gordy said:
> PWM for steam is how I'm understanding PMJ's theory in control. Small bursts of energy over a period of time into a big collector (rads). Verses one, or two big fills.

Right. Wide range of adjustment possible. Operation with no pressure. Really does not have the losses generally assigned to more cycles. Clearly more efficient with natural vacuum between the pulses.
1926 1000EDR Mouat 2 pipe vapor system,1957 Bryant Boiler 463,000 BTU input, Natural vacuum operation with single solenoid vent, Custom PLC control
• Member Posts: 114
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Boiler Output: 145,000 BTU/hr
Actual Heat Loss: 46,691 BTU/hr (excludes basement)

Also, there is a hot water loop in the basement (area: 1000 square feet) with fin tube. I didn't include that in the calculation because I have never turned the heat on down there.

How many years do these W-M boilers last? I don't know how old it is. (Is there a place on the boiler that would show it?) It has to be at least 4 years, because that's how long I've had the house. When it dies, I will replace it with one that is more appropriately sized.

So that leads me to another question - if the radiators are oversized to the heat loss, when I go to replace it with a new boiler, can I size it to the actual heat loss (plus a factor of 30%) instead of the connected radiation?

I know it's a lot of work to change these radiators, but the whole house is down to the studs already, and the old radiators have already been removed. (If anyone wants any of the 3 column radiators, come to Queens and you can have them for free.)

And one more question -- on my gas range, I can just turn a dial and the flame goes down. Why can't boilers do that so you can throttle the output? Wouldn't the most efficient way to run it be at a constant simmer that perfectly matches whatever that day's heating requirement is?

@mikeg2015 Why would one need condensate storage on a smaller boiler?

@Zman How much of a problem is inefficiency from short cycling? Can you explain more about the losses both from the boiler sitting idle and also the firing up?
• Member Posts: 609
edited April 2019
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You have to size a steam boiler with the radiation in mind (not just the heat loss), otherwise the oversized radiators will steal the heat/steam thru the path of least resistance, before it can make it to the farther reaches of the system leaving you with an unbalanced system and cold rooms. It's better to have a adequately sized steam boiler that cycles more. The 30% pickup factor seems one that can be reduced quite a bit depending on whether your pipes are insulated and run in exterior/interior walls etc. 30% is a probably a bit of a worst case number.
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@superj in that case, could the balancing problems be solved by slowing the venting on the closest radiators?
• Member Posts: 11,077
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If you post a picture of the nameplate on the boiler someone here could give you the manf date from the serial number.
The plate with the BTUH ratings etc.