Is there a 'healthy' boiler on-time?

wildrage

I use one of those Nest thermostats. Overall, I think the thing is a piece of junk, but there are some useful metrics in it, and I like to be able to remotely monitor and change my thermostat.

Anyway, there's a 'history' tab, where you can view how long your boiler runs for. By 'run' it means, how long the thermostat is calling for heat.

Based on the image below (on Sunday I was messing with thermostat, because I was taking some radiators apart), I'm averaging 8-12 cycles a day, with approximately 30 minutes per call for heat, leaving me at 4-6 hours of 'on-time'. I keep the house cool - 60-64 when I'm here, and 58 when I'm out of town.

Temperatures are pretty seasonal. 20's and 30's at night, and 40's and 50's during the day.

I know there are a billion factors here, but I'm just wondering if it's possible for the boiler to be running 'too much,' and where I fit in with the average.

I'm in a 7,000 sq foot drafty Victorian with 15 foot ceilings and a 250,000 btu boiler. I have a second, 175,000 btu forced air furnace that covers part of the house, but I have that set to cut in only as a backup, typically doesn't run much more than an hour a day.

Jamie Hall

When I looked at the title, I thought "within a cycle"! Now I see. And the answer is... no. Look at it this way. The boiler has to run for a long enough time to heat the house. So and so many BTUs per hour (or per day!) leak out of the house, and therefore so and so many BTUs have to be added to the house from the boiler to keep the temperature constant.

So the boiler run time considered over a day is just a measure of how much heat the house lost over the day.

You can do some rather interesting (? frightening!) arithmetic with the information, though -- such as finding out what the apparent overall "R" value of your house is...

acwagner

I agree with Jamie that comparing between buildings does not really provide much enlightenment. If you keep the data over the years you could potentially see how the improvements you're making to house and the system improve your fuel consumption year after year.

However, I have similar data, so I'll play along.

Last year I got some help from some family members to program a Raspberry Pi to timestamp and log every time the gas valve on my boiler opened and closed. I was curious if doing temperature setbacks on our system saved fuel or not for our setup. So, I have very detailed gas usage data from January to May of last year.

My house is around 2,000 SF, build in 1928. The walls had been insulated and the windows and doors have copper weather stripping, so I'd say my house is above average in terms of insulation and tightness compared to other houses from the same era. The radiator load puts my design day at like -14 degrees based on my heat loss calculations. Not sure if that's crazy or not.

My steam system....was completely knuckheaded up. It was rated at 135K BTU input, but when I clock it it was closer to 118K BTU. I never figured out how they downfired it. Near boiler piping all wrong and undersized. No way to effectively skim the boiler, so it was a rollercoaster in there. Leaks, sagging pipes, poor main venting, water hammer at 2am....you get the picture. Yet, it worked.

Thermostat was either at 70 or 65 degrees depending on the setback.

Most calls for heat varied from 21-32 minutes. The highest usage I had was on a day that varied between 3-16 degrees F, and the boiler fired for 9 hours out of 24. Lowest amount was around 0.85 hours when it was 37-77 degrees F out (that's Michigan for you).

On a day like you're describing, the boiler would run for 4.3-4.8 hours in 24 hours.

Does that mean anything to you? I dunno. I wouldn't draw any conclusions on your house compared to mine and vice versa.

I got a new boiler this year and fixed all the said problems, so I'm curious to see if/how my energy usage changed.

mikeg2015

So many ways to approach this.

First you have steam, so the boiler should be sized to the radiant load. Typically there's about 1.5-2x the radiation above what is needed. So in very cold weather it might only run 50-75% of the time. Mild weather, maybe 5-20%.

How long it runs each cycles depends on too many factors to really list. But you'll find that a fair amount of heat goes the warming the header, exterior walls adjacent to the radiators, and warming the basement (which could be good or bad).

Is the boiler sized right or oversized? In a steady state, a 400k BUT boiler running for 10 minutes consumes .67 Therms same as a 350k running for about 11-1/2 minutes or a 200k running for 5 minutes. This is why one strategy with an oversized boiler is to use a timer to reduce the duty cycle to effectively downfire it.

The greater concern is short cycles. You want it to run I think at least 8-10 minutes. Then it should use that stored energy for 20 minutes to maybe 2-3 hours depending on outside temp and inside temp.

EBEBRATT-Ed

as far as I am concerned the longer the run cycle the better (assuming the boiler is sized correctly and the system works properly) short cycling and standby loss is a killer. Your refrigerator and refrigeration walk in boxes are designed to run 16 out of 24 hours in a day.

most heating equipment is grossly oversized. A lot of installers are so scared to size it properly.

they do a heat loss and fudge it upward
they size the heat emitters and fudge it upward
Then they pick a boiler and if between sizes they go up again
then comes the pick up factor 1.15 for water and 1.33 for steam

Now, when everything (boiler and piping) is hot on the coldest days and the boiler is running more you really don't need much of a pick up factor

By the time your done with everything it can be quite a bit oversized.

RomanGK_26986764589

Wow. 30 min is pretty long call for heat. My boiler on time is usually 10-15 min per cycle during shoulder temps. When it's really cold out it may run for 18-20 min tops.

acwagner

Maybe. He has a 7,000 SF 3-story house. Lots of big main pipes to vent, I'm guessing. Doesn't mean there isn't room for improvement.

@wildrage I don't recall if in one of your posts if you mentioned what kind of main venting you have and how long and what size the mains are.

nicholas bonham-carter

If you have a low pressure gauge, you can see the resistance (backpressure), to the escaping air.
Two ounces or less during the venting phase, is what your main venting should function at. Slow venting on the radiators will ensure the simultaneous arrival of steam at all radiators. Any slow rads at the top of a tall riser could later be equipped with vent one size larger.—NBC

Fred

I have5K sq. ft + 2K in the basement and I run about 15 to 20 minutes a cycle, like @RomanGK_26986764589 too.
Are the mains and header insulated?

wildrage

Thanks for all your input. I know it was a bit of an 'open ended' question. So many variables.

I have no insulation...well, the only insulation I have is insulation installed behind the vinyl siding, which I assume doesn't do much. I do have double storm windows (inside and outside), because my house has some very decorative windows.

On a 30 degree night, with my heat set to 65, I lose about 1-2 degrees an hour, which I don't think is too terrible. By the time the thermostat is satisfied, the warm cast iron radiators continue to heat, so my offset is about 3-4 degrees, which means on a 30 degree night, my boiler clicks on every 2 hours or so, which seems rather normal to me.

A 30 min call for heat doesn't seem too crazy to me. 15 radiators on 3 floors, 15 foot ceilings on all 3. Takes about 8 mins for the first radiator to get super hot, all hot probably 15 mins into the cycle...then you've got to figure the time it takes the air around to trigger the thermostat to turn off, and I end up 3-4 degrees over the call for heat by the time the radiators get cold.

I've got very fast venting on most radiators, so I'm working on that, and poor (if no) steam main ventilation. I also have an elaborate series of steam traps, and a Watchman condensate pump (which I'm told I don't need).

Looks like my ontime is similar to @Fred which makes me feel somewhat consoled .

wildrage

@Fred I'll have to time exactly how long its taking for a cycle...I estimated 30 mins. I'm lacking a lot of insulation on basement mains. There are 2 50 foot ones (i'd estimate) with a combo of Asbestos, Fiberglass, and (no) insulation. Basement is heated by forced air (and partially the mains, i'd guess!).

Fred

@wildrage , You have that temp over run because the Nest just isn't good at allowing for the latent heat emitted by cast iron radiators. It manages the temp like it was forced air where there is no residual heat produced. Your boiler would probably run for 20 to 25 minutes a cycle if could anticipate that residual heat being emitted.