Short-cycling with steam and boiler sizing

michael_15

What is "short-cycling"? Does it refer to when the boiler shuts off on pressure before heating all the radiators, or does it just refer to short boiler cycles in general? After all the radiators are heating, what is the typical duration of the on-off periods while the thermostat is still calling for heat?

My boiler heats all the radiators, and then enters into a 5-minute-on then 1-minute-off pattern until the thermostat stops calling for heat. (cut in 1 psi, cut out 2 psi) I generally only notice this from a cold start, which only happens once a day since I set my thermostat back at night. Is that too short? I do know that my boiler is terribly oversized (360 sqft DOE versus 160sqft radiators (pre-pickup)), so I wouldn't be surprised.

Would adding a hot water zone to heat some other spaces help ameliorate the over-sized-ness of the boiler?

-Michael

David Efflandt

Similar situation

I am in the same boat with 112k btu/hr input boiler rated 283 sq.ft. net, and 160 sq.ft. of radiators (reduced by covers on half of them). But mine runs only 2 minutes on, when warm enough to pressure cycle. That gives my vent damper a workout, and one reason I only use significant setback spring/fall (solar gain & quick recovery) and only slightly in winter (enough radiate when I wake up). Since boiler was just installed (by previous owner) 2001, it would not be cost effective to replace it with properly sized boiler until necessary.

I am not an expert, but some way to heat domestic hot water might make sense. Heating unused areas would just use more fuel.

I have been lurking here for awhile and have not noticed a definite answer to what constitutes short cycling or what is normal pressure cycling for a properly sized boiler. But I imagine in a perfect world, on the coldest day of the year, the boiler would run until thermostat was satisfied (maybe even continously) and every room would be the desired temperature.

Jim_35

am i wrong ,but a 283 sq ft steam boiler

for 160 sq ft of rads plus the pickup factor for the mains and risers seem just about right and not oversizeD??

I dont think youre oversizes.. any pro's care to comment?

michael_15

it's oversized

283 sq ft net means the boiler is actually a 376 sqft boiler.

-Michael

jim_14

michael I think you are wrong, a boiler rated for 283 sq ft

should be able to produce 283 sq ft.. not 376 sq ft

I think you are confusing btu input and output


any pro's care to comment???

DaveGateway

net means net, boiler is oversized

according to the boiler rating, net is the steam produced less 33% pickup factor for the pipes. --as opposed to gross which is the gross steam output.

so basically in steam systems, you have 2 efficiency losses - approx 20% for the wasted gas/oil going up the chimney and another 33% for heating up the pipes!

your boiler is oversized, but you should check out the anticpator setting on your thermostat. much of the year, a steam boiler will cycle on the thermostat and the anticpator setting can help you optimize the cycle length.

jim_14

im a litte confused here,

my boiler is rated at about 280 sq ft.. does that mean it produces more then 280 sq ft?

nj_2

ratings

the boiler is capable of generating enough steam to heat 376 sq feet of radiators. However, to get steam to the radiators btus are wasted heating the pipes. this is called a pickup factor. so the net rating means that the system is capable of producing enough steam to heat 283 sq foot of radiators + all the pipes to get there.

in actuality, your pickup factor may be different due to the length of piping and whether they are insulated.


this is as oppsed to the burner btu input and output which is the amount of gas actually consumed (input) vs the amount that is available to the boiler (output).

jim_14

oh I see what you mean, but arent there some boilers

that tell you not to add a pickup factor to their ratings?

From what I know about my boiler the rating is 270 sq ft and that includes the pickup factor, i believe the manual even tells you not to add the 30% to the 270 sq ft

michael_15

Reading the boiler label

I generally prefer to ignore the "net" values and look at the "gross output" or "DOE" of the boiler. Ironically, despite the "gross" label. this is not equal to the total fuel burned by the boiler. Here's a rough summary of the items you often see.

Input: This is the fuel used by the boiler. This is usually quoted in BTU.
Gross Output/DOE: This is the energy that the boiler can deliver to your home. The rest goes up the chimney or might get lost through the walls of the boiler itself into the basement (in which case, if your basement is heated, it may not be "lost", and what not, but I'll ignore that). This is usually quoted in BTU. Gross Output/Input roughly indicates the efficiency of your boiler.

Net Steam: This is often quoted square feet, but sometimes also in BTU. This is Gross Output divided by 240 divided by 1.33, where 240 is the conversion from BTU to square feet, and 1.33 accounts for the pickup factor. (If it's quoted in BTU, then omit the 240.)

There's also stuff about hot water which I won't bother to talk about.

I don't like using the net values because, well, I guess for the same reason I still like using DOS instead of Windows.

Your manual probably tells you not to add the 30% to the 270 sqft because:
  1) It assumes your pipes are insulated,

  2) It assumes you don't have missing radiators (and hence more pipe than expected),

  3) It assumes you've just determined the EDR of your radiators.

As such, you'd be working off of the "Net" rating and not adding back the pickup factor. So the EDR of your radiators should then equal the net steam capacity of the boiler.

If, for some reason, your pipes are not insulated (or the risers serve double-duty as first-floor radiators) or they're unusually big (removed radiators), you might need to apply your own pickup factor. Hence, you throw the "Net" rating out the window and compare (Radiator EDR*Your Pickup Factor) against the DOE/Gross Output of the boiler. You can divide this by 240 to get the gross output in square feet of steam.

Example 1: I have 160 sq ft of radiators but a lot of piping (the system serves only the 2nd floor) that isn't fully insulated. I've decided to use a 1.5 pickup factor, meaning I want a boiler with a GROSS OUTPUT of 240 square feet. I ignore the net values on the boiler. My boiler says "Net steam: 271 square feet," but I'm ignoring that and working off of the "Gross Output: 86000 BTU" (or so) line. 86000 BTU equals 360 square feet. Since I only need 240 square feet, my boiler is oversized.

Example 2: I have 260 square feet of steam in my radiators, which has fully insulated piping and no removed radiators. This means that the 1.33 pickup factor is probably pretty good. I could either: 1) compare the 260 sqft to the "Net Steam" on my boiler (which is 271, so the boiler is pretty much the right size), or, multiply the 260 sqft by the 1.33 pickup factor (346 sqft = 83000 BTU) and compare it to the Gross Output of the boiler. This should help me arrive at the same conclusion.

Hope that helps,

-Michael

michael_15

Reading the boiler label

I generally prefer to ignore the "net" values and look at the "gross output" or "DOE" of the boiler. Ironically, despite the "gross" label. this is not equal to the total fuel burned by the boiler. Here's a rough summary of the items you often see.

Input: This is the fuel used by the boiler. This is usually quoted in BTU.
Gross Output/DOE: This is the energy that the boiler can deliver to your home. The rest goes up the chimney or might get lost through the walls of the boiler itself into the basement (in which case, if your basement is heated, it may not be "lost", and what not, but I'll ignore that). This is usually quoted in BTU. Gross Output/Input roughly indicates the efficiency of your boiler.

Net Steam: This is often quoted square feet, but sometimes also in BTU. This is Gross Output divided by 240 divided by 1.33, where 240 is the conversion from BTU to square feet, and 1.33 accounts for the pickup factor. (If it's quoted in BTU, then omit the 240.)

There's also stuff about hot water which I won't bother to talk about.

I don't like using the net values because, well, I guess for the same reason I still like using DOS instead of Windows.

Your manual probably tells you not to add the 30% to the 270 sqft because:
  1) It assumes your pipes are insulated,

  2) It assumes you don't have missing radiators (and hence more pipe than expected),

  3) It assumes you've just determined the EDR of your radiators.

As such, you'd be working off of the "Net" rating and not adding back the pickup factor. So the EDR of your radiators should then equal the net steam capacity of the boiler.

If, for some reason, your pipes are not insulated (or the risers serve double-duty as first-floor radiators) or they're unusually big (removed radiators), you might need to apply your own pickup factor. Hence, you throw the "Net" rating out the window and compare (Radiator EDR*Your Pickup Factor) against the DOE/Gross Output of the boiler. You can divide this by 240 to get the gross output in square feet of steam.

Example 1: I have 160 sq ft of radiators but a lot of piping (the system serves only the 2nd floor) that isn't fully insulated. I've decided to use a 1.5 pickup factor, meaning I want a boiler with a GROSS OUTPUT of 240 square feet. I ignore the net values on the boiler. My boiler says "Net steam: 271 square feet," but I'm ignoring that and working off of the "Gross Output: 86000 BTU" (or so) line. 86000 BTU equals 360 square feet. Since I only need 240 square feet, my boiler is oversized.

Example 2: I have 260 square feet of steam in my radiators, which has fully insulated piping and no removed radiators. This means that the 1.33 pickup factor is probably pretty good. I could either: 1) compare the 260 sqft to the "Net Steam" on my boiler (which is 271, so the boiler is pretty much the right size), or 2), multiply the 260 sqft by the 1.33 pickup factor (346 sqft = 83000 BTU) and compare it to the Gross Output of the boiler. This should help me arrive at the same conclusion.

Hope that helps,

-Michael

Rudy

short cycling means the burner going on and off frequently.if you turn up the thermostat way above room temperature and the burner is still short cycling it is something else.i would guess it is cycling on steam pressure.try drop the firing rate of the burner.please be absolutely sure the low water cutoff is not bobbing around.if the thermostat is the culprit and is not an electric one the heat anticipator pointer should point to.4 or .5. let the burner cycle on the thermostat.if the burner short cycles turn the thermostat all the way up.

Rudy

steam boiler short cycling

if the burner is short cycling on the pressure control drop the firing rate of the burner to lower input

jim_14

shouldnt the boiler cycle on steam pressure??

Im talking about after the rads get hot on the first long cycle and the pressure builds and cuts out at say 2lbs.. the pressure should then drop to 0 and then the boiler should turn on again for more steam to rads but much quicker and hit the pressure of 2 again, am i right??????

doesnt this repeat over and over until the tstat is satisfied?

jim_14

ive read your explanation a few times

and I think I understand what you are saying.

So let me get this straight, my boiler is 105,000 BTU input with an output of 87,000 for roughly 80% effiecency.

The IBR rating is 271 sq ft of steam.
Does this mean my boiler is producing more then 271 sq ft?

My pipes are insulated and I dont have missing rads, and I believe the combined EDR of all my rads are 170 sq ft.

With a 1.33 pickup factor that gives me 226 sq ft

Since the smaller boiler is rated at 158 sq ft (62,000 btu in, 51,000 out), the next size up is the 271 sq ft. Now Dans book says when in between always go up to the next size.


Am I missing something here??

BTW, thanks for your explanation and patience!