Is there an "Ideal" Steam Heat Cycle duration?

wanttolearn

Hi-

i have been observing my steam heat cycle lately and i have noticed that the thermostat fires on quite infrequently.  On average I would estimate that it fires on about once every three hours (of course, this varies considerably depending on how cold a day it is).  However, once it does turn on, the cycle last quite long.  I havent timed it exactly, but I would guess approx. 45 mins to 1 hour per cycle (it might shut off briefly once or twice within such a cycle, but that is the total time of each heating cycle).  Then the thermostat shuts off again for another three hours or so before the  next long cycle starts.

I think the reason for this cycle pattern is that the thermostat is located in a room right above the boiler room, and the great heat from the boiler room acts like radiant floor heat to the thermostat room.  Thus it takes a long time for that room to "cool off" and for the thermostat to turn back on.

My question is: Is this an inefficient heating cycle?  am I better off moving the thermostat to a different room, where it might turn on every hour for 15 or 20 minutes?

The overall heating time per day would not change much between these scenarios, but I am wondering if it is more efficient to have more freqent, shorter cycles, or less frequent but longer cycles?  Or is there no real difference?

any opinions would be greatly appreciated.

Jamie Hall

More a matter of comfort...

than anything else.  The real question is, does the temperature of the rest of the building droop too much between cycles?  If it does, then yes you might want to move the thermostat (that boiler is insulated, and the piping, is it not?).  But if the temperature of the rest of the building is sufficiently constant to be comfortable, a longer cycle (provided that the burner isn't cycling on pressure too much) is at least as efficient as a shorter one, and possibly slightly (but probably unmeasurably) more so.



I know what you mean about the room over the boiler.  The cats resident in the building I supervise sleep most of the day in the room over the boiler -- on the floor, right over the boiler, and they're the best thermometers I've ever met.

Mike Kusiak_2

Cycle length vs. efficiency

As Jamie said, a longer cycle may give you slightly higher efficiency if you don't wind up cycling on pressure at the end.



Think of it this way. Say you run a very short cycle. You spend a considerable part of the time venting the air and heating the piping before any steam gets to the radiators to heat the rooms. Now consider a long cycle. The same amount of time is spent getting steam to the radiators, but a greater percentage of the cycle time is spent actually heating the radiators.



So a greater part of the heating cycle is used putting the heat where you want it. On the other hand, if the cycle is too long, you may cycle on pressure, or wind up overshooting the thermostat setting, both of which can hurt efficiency.

wanttolearn

follow up question

Both of you make reference to "cycle due to pressure"....what does that mean?  like i mentioned in my post, duering each heating cycle, the thermostat goes on and off a couple times....normally it stays off for a couple of minutes before turning on and continuin the cycle.  is this what you are referring to as cycling under pressure?

Brian_74

cycling due to pressure

Your system as a device (probably a pressuretrol, but possibly a vaporstat) that will shut off the boiler if it reaches a preset pressure. It won't turn it on again until the pressure is below a certain threshold. A boiler can reach the pressure limit before satisfying the thermostat. It will therefore turn on again once the pressure has gone down sufficiently. This is called cycling on pressure.