Short Cycling

Timco

pressuretrol does not care what temp your room is, only boiler pressure. What type system? 1 or 2 pipe? If 2 pipe, traps working? the pressure is going somewhere...but your description of the cycles does not sound too bad...

Tim

Tim_64

Burner Short Cycling

I undertstand what it is, but how much is to much? or how short is to short?

i.e. if I set my thermostat to 70 deg, the boiler comes on and burner runs, makes steam.. then the pressure increases until the pressuretrol cuts the burner, room hasnt reached 70 deg yet.

The pressure decreases (few minutes) and the boiler cycles on.. and runs for about 3-5 minutes until the pressuretrol cut in again... temp is almost 68 now..

Pressure decreases (again a few minutes) and then the boiler cycles on and runs for a few minutes... etc..

This cycle repeats until 70 deg is achieved..

Is this normal? or is this short cycling?

I have the pressuretrol set at 1 psi, and the differential set at 3 psi, so the system runs up to 4 psi, and shuts down, then cycles on etc.

nicholas bonham-carter

short-cycling

try reducing your pressure to under 16 oz. max. and see if the problem was caused by the vents operating improperly with too high a pressure keeping them closed longer than optimum.--nbc

Tim_64

Short cycling

The system is a 90 y.o. one pipe steam system. Over the past summer I've installed a new boiler (Weil-Mclain SG04) replaced the wet return, replaced all the radiator and main vents per the "Balancing" booklet, made sure all the radiators are pitched properly, replaced a bunch of the leaky radiator valves,re-insulated the mains,etc. I still have 2 valves to repack or replace.

What I'm trying to find out is if the behavior I tried to describe is normal.. Say it take 30 minutes to go from a cold boiler (60 deg) to the 70 mark I set the thermostat. During this 30 minute heating cycle, the burner cycles on and off 6-7 time in 30 minutes ( because it bumps the pressure stop). Is that normal behavior? or is it cycling to mush?

Tom_115

short cycle

Your boiler may be fired larger than your sq feet of radiators. The ideal setup is to hit the balance of all radiators filled with steam as the temp rises to the stat set point. This balance assumes the right amount of radiator sq ft in the room with the stat. Also assumes the boiler matches the load (total Sq Ft or radiation) of radiators. The fewer times your boiler starts and stops the more efficient your system will be.
It sounds like you could lower the fire rate of your burner a bit and see how it works. If you add up the EDR of your radiators it should be about the same as the Net IBR sq ft rating of your new boiler. Boilers step up in size by the number of sections inside them, (Each section will heat so many more radiators) so you pick the best fit for the amount of radiation you have. Not to small and not too much too big. you can fine tune the size by lowering the fire rate of the burner. Warning. have a burner pro with the proper analizer to set the air settings. and do not go too low or your boiler will simmer and not steam. Smaller settings should save some fuel. If your system is oversized you may never hit the single burn goal but you can make it better. Also lower the pressure as low is it will go. Steam pressure is costing you fuel. Best of luck and I hope this helps

Tim_64

boiler size

I already know the new boiler is a bit oversized. I calculated the EDR of my radiators and came up with 340 sf. The SGO3 is rated at 350sf. I chose the SGO4 because I will be adding an extension on my house in the near future and need the extra capacity. The SGO4 is rated at 450 sf. Im also going to connect an indirect DHW tank in the next week or so.

I'm really enjoying leaarning how the system should work. I took for granted the old system in my house (original National Boiler) because it seemed to work. Now I'm learning about fine tuning and optimizing the system.

I'll get someone to downfire and adjust the burner in the meantime.

Tom_115

Sounds like it is doing what is to be expected. be sure to add the amount of EDR you oversized when you do the addition. If the run time becomes excessive you have gone too far with the burner down size

Jamie Hall

optimist

It would be nice to get the boiler sized and fired like that...

Actually, one can come pretty close on a simple temperature maintain, although one does need to take into account the pipes as well as the radiator EDR when setting boiler size and firing -- those pipes, even if insulated, do soak up a fair amount of steam. But as I say one can get close; the pressure will start to rise as the space warms, but ideally the space temperature will shut off the thermostat at the same time as the vapourstat shuts off the boiler.

However, recovering from a setback? Different story. You have a much longer run time (how much longer depends on the setback). If you have enough boiler/fire to warm the whole system up (all radiators hot) the pressure will rise and the vapourstat will shut the boiler off... and on... and off... just as you described. Until the space is warmed up again. What you are describing is normal.