Short cycle on pressure

[Deleted User]

Reid Kaplan

Short cycle on pressure

Hello all,

I just had a new oil fired Weil Mclain heating system (SGO5) installed to replace an ancient 1937 boiler. The house is a 2 story, 7 room colonial of about 1500 sq. ft. with original radiators with new air vents. The steam piping is black iron, well insulated and has 2 legs in the cellar. Only the longest leg has a main vent. OTW, it is constructed exactly according to the designs given at this site.

After first installation, the water was really dirty and there was explosive hammering at the end of the heating cycle at the very end of the main and the nearest radiator riser to that. This has now been mostly cleaned up (still a bit murky pink), although the steam is still somewhat wet (by observing the top of the sight glass). But no more hammering.

The problem of short cycling happens on cold mornings when the pipes and rads are hot but the temp at the t-stat has not yet reached the set point of 69 (the air temp is 67). The pressure rises from 0 to 3PSI (indicated on the dial) in about 5 minutes, The 404A pressuretrol shuts off the system, the pressure drops to 0PSI in about 30 secs (but the restart delays some) and the cycle repeats until the temp reaches 69 (about 4 or 5 times over 45 minutes). I set the pressuretrol at 1PSI main with 1 PSI Differential (the installers had set it at 2 and 1). I also installed a new no-name main vent today and verified that air was coming out of it quickly (the old one was very restricted). I then set the pressuretrol at .5 and 1, as was suggested here, but that only shortened the cycle time. I have not been able to find a Gorton #2 main vent locally, but have emailed Gorton.

Sorry for being wordy, but what should I do?

Unknown

If the vents are hot

when it cycles off, you are making steam faster than you are condensing it.

If the vents are cold when it cycles off, you have not enough vent.

If it only does this when you change the thermostat setting (up), don't worry about it. It is caused by running it for an extra long cycle time.

If it happens when the thermostat was left alone, the boiler might be a little big. In mild weather, the thermostat would satisfy before pressure built, on a properly sized boiler.

Noel

Reid Kaplan

It happens when the radiators and vents are hot and the system has run about 1 1/2 hours from a cold start (5:30AM -7:00AM), about 60, and the air temp downstairs at the t-stat is 67 and the setting is 69. At this time, the air vents seem sealed (they are thermostatic, aren't they?) and do not release air.

If my boiler is too large, then there is nothing to do about it, except grit my teeth, right? As I said, it is a Weil McLain SGO 5, and I have about 1500 sq. ft in 2 floors with 6 rads on the 1st floor and 5 on the second. Does that sound oversized - I have no way of knowing?

Thanx,

Reid

Unknown

Vents are open unless they are HOT or broken.

A one and a half hour cycle is not normal.

Just to avoid confusion, let's not talk about symptoms when you turn the thermostat to a different setting. The symptoms that turning up a thermostat, or setting one back for a period of time cause, cannot be confused with a system running along at the same setpoint.

When it runs at a constant room temperature, does it short cycle? Are the vents hot, if it does?

Your room sizes are important if we are selecting radiators. Once the radiators are selected, the radiator size and room temperature desired are all that matters. The boiler needs to fill the radiators with steam, and all of them. Too much or too little steam is determined by the radiators sizes.

Again, if during a normal cycle, the radiators all heat together (not nessesarilly all of the way across)and the thermostat satisfies without much burner cycling, the boiler size is pretty close.

Noel

[Deleted User]

thats why i

teach my customers to not set back the thermostat on a steam system..

Reid Kaplan

No short cyle at constant temp...

It only short cycles on heating from a 9 degree night setback when it is about 20 degrees, or colder, out. It does not short cycle if the outside temp is about 32 outside.

When it does, all radiators and vents are uniformly hot across their length

So, it sounds as if the boiler *is* properly sized and it is the recovery from setback at low ambient temps that is causing the pressure short cycle. (However, my old 1937 monster *never* short cycled with setback.)

However, if I don't setback, it is uncomfortably hot, upstairs in the bedroom while sleeping (if it is the daytime 69 degrees), - unless I sleep uncovered and naked :-) Also, doesn't a setback reduce oil consumption, hence $$$?

Thanx, Noel, you (and this entire site) have been very helpful for a civilian, like me.


...Reid

Unknown

A properly sized boiler

with a low water content can boil pretty fast. It is sized to heat all of the radiators with the piping hot, PLUS 33%!!

That 33% is for bringing the system up from cold quickly. Once it is all hot, the boiler is effectively oversized by 25% (Four thirds of the actual load) so it will begin to build pressure.

If it were coal, no thermostat would satisfy, and it wouldn't need to be oversized. Also, there was no burner to shut off, so an electric pressuretrol wouldn't have been there.

It's still the same SYSTEM, with a new BOILER, and compromises need to be made. Would you have it set up to run by a thermostat, and maintain temperature? Or would you have it set up so that it ran indefinately by a switch, which you turn up and down to be comfortable, manually?

You have an automatic system, that reacts properly when you run it by hand.

Noel

Reid Kaplan

I read *all* the Wall posts on this subject...

...and I will follow your suggestion to compromise on the setback, although it seems, from reading everything, that it is not a compromise, simply doing things smarter. For which education I thank you, Noel, and all the others who wrote cogent posts.

Also, thank you for answering a newbie's question directly. It is often the case, in other forums, that expert folks get bored when the same subject is brought up again and again, and just never bother to deal with it anymore. This is a great place.

Cheers,

Reid

scrook_2

steam setback

So, if you have a steam system (I have HW) and you prefer to sleep in a cool environment (I like ~60-62F) but like warmer when awake (I like ~65-68F) what should/would you do/recommend and why:

a: rip out the steam and convert to HW (or worse, you know what [GRIN])?

b: be too hot when sleeping?

c: be too cold when awake?

d: live w/ short cycles on return from setback?

e: use less than the standard 33% P/U factor?

f: try to find a high/low fire oil (or gas) burner and downfire once some slight pressure is reached?

g: other?

Unknown

D

It's already been solved. The pressure control is what they came up with. It keeps the steam pressure where it needs to be. Nothing is wrong with this plan. It only does this once or twice a day, and it really doesn't cause any harm.

If you are thier service person, it causes your phone to ring at 6:15 AM.

Noel

[Deleted User]

scrook, could always

put a thermostic radiator valve or vent on the radiator in the bedroom.

scrook_2

steam, setback & short cycles

My point was that one may desire setback at night and may also own a steam system -- the result is that the boiler short cycles during the return from setback. Result: the overall efficiency is not quite what it might be if it ran steady state, but offsetting that, a bit of fuel is saved during the setback period, but most importantly one does not have to sleep naked w/o a blanket.

In an ideal world the steam system would have a modulating burner say 2:1 (50%), or at least 1.5:1 (67%) turndown, coupled to a steam pressure transducer (in addition to an on-off pressure switch), so as steam pressure rose the burner would downfire. That way it (like the old coal system in the "What Is It" post, above) could full fire to heat the cold pipes and rad's, then throttle back to only what was needed to maintain the connected radiation while staying below the p-trol's off pressure. But, such a thing is not available so short cycling on return from setback is compromise we make in trade for an aotomatic fired boiler on a system built by The Dead Men for hand fired coal.

David Efflandt

Some thermostats can ramp up

Honeywell has thermostats with Smart Recovery that bring up the temperature up gradually from setback. They compare both wall and air temp to learn when to start ramping up to arrive at the set temperature at the set time, instead of trying to bring it up all at once after the set time.

That may kick the boiler on more often, but for a shorter period of time, which may reduce pressure cycling. But the water is still warm, so it should start steaming quickly.

Mark Eatherton1

How to mess with a Smart Recovery thermostat...

have a Vision2 logic down stream...

My poor 8600 is going to lose it's mind before it figures out whats going on:=)

The experiment continues...

ME

Dale

Cold pipes

Had a large building on steam that wanted setback but the turn on racket was terrible. Made it alot better by putting in a low limit aquastat that kept the boiler at 180 which didn't heat but kept the pipes warm and expanded enough that the turn on morning noilse wasn't too bad, costs a bit but simple to try. If you have a somewhat insulated basement the heat isn't completely wasted.

Drod

Short Cycling/Set Back

Had a similar problem. I solved it by stopping the night set back altogether (thermostat set at 69 ongoing), plus I changed the vents in the upstairs bedrooms, but not the bathrooms. This keeps the main floor a couple degrees warmer than the upstairs/bedroom level. This seems to keep everyone happy, except my wife who'd like it at least 75 degrees in every room.

Drod

I'm not sure that's what Smart Recovery Does.

If I remember correctly it takes all the readings you indicates, then determines what time/how early to start the heating process to get to the new, higher temperature in the morning. I don't think it "ramps up". I think it simply turns on and let's the Pressuretrol cycle off and on until the desired temperature is reached---by the time you've set as the desired time.

Unknown

No, the Smart ones do ramp up

they cycle off to let the temp coast up the last few degrees. They "learn" how much to lead or lag for the current weather, once they are set and left alone. They might, for instance, stop 3 degrees before setpoint, then come back on after a few minutes to touch up the room temp.

Not ALL of them do this, just the nice ones.

Noel