Short cycling on pressure

matt_65

I've got a Honeywell FocusPro 6110 thermostat, and set the CPH to 1 as they recommended for steam. It doesn't really seem to be working like an anticipator and will call for heat too long and overshoot while the boiler keeps cutting out on pressure.

Maybe I should set it for 2 CPH so it won't stay on so long?

matt_65

Homeowner here, and I just moved into a 1930 colonial with the original one-pipe steam radiators and piping but a newer boiler. After getting Dan's book, browsing through "The Wall", and looking a the price of natural gas, I replaced the pressurestat with a vaporstat, got a low pressure gauge, insulated the mains, replaced all radiator valves, and flushed the filthy water out.

All the radiators will now heat well, but the boiler is cycling on pressure rapidly once everything is hot, if the thermostat is still calling for heat the boiler will take ~90 seconds to get to ~15oz, cut-out, and ~45 seconds later cut-in at ~4oz (though the pressure will be negative by the time the damper opens and the low water probe finishes its startup blinking).

Is it possible the pressure at the boiler is high because of the main vent? There is a gorton #1 at the end of the dry return, and I can hear it puffing and hissing but wonder if it is partially clogged. The radiator closest to the boiler (first off the main) also seems to "wheeze" considerably when the boiler cuts-off, could the mains be pulling air through it because the gorton isn't giving enough?

The boiler also has two side tappings, but one was left capped. The near boiler piping looks good, but without the 2nd tapping being used could the boiler be "seeing" higher pressure than really exists?

David Nadle

I've got the same problem.

By sharing experience maybe we can help each other solve it.

How long does the boiler run before the first cut-out on pressure? In my house the boiler runs at about 6 ounces while the radiators are filling, then after about 30 minutes the pressure starts to increase and I cut out at 9 oz. Most of the time the boiler does not run that long; the thermostat is satisfied first.

I have a Gorton #2 and a #1 side by side on my mains. I think they're both working fine. My rads are mostly Gorton #5's and #6s. I have one grossly oversized radiator in an upstairs bedroom that I have kept in check with a #4 vent and a TRV. Things are working well except for the short-cycling after a long run and the occasional "whooshing" finish you described. I don't get that every time.

This is just a theory. My reasoning is that my radiators are filling too slowly, and if I vented them faster I'd hit pressure faster and there'd be less condesnate coming back at that moment, there'd be less immediate collapse of residual steam and less of that strong vacuum on cut out. I might get a box of Vari-vents to test my theory, but if your rads are venting fast and my theory is bunk I'd rather save the expense.

When you get a chance, next time the boiler comes on, stand near your main vent and time the duration from the first hiss to when the vents are hot, and hopefully quiet. I'll measure mine and post as well. We want the mains to fill fast.

Then, record the time when all rad vents are hot and closed. This one is tricky for me because I have two TRVs upstairs and one rad that is so large it should never fill or it will be 80 in the room. So I'll do all but those.

If it turns out I can't fix the short-cycling with faster venting I have another plan. I will isolate the vaporstat with a delay-on-make relay. With this, even if the system loses pressure immediately and the vapostat makes contact right away, the relay will hold the boiler off for whatever time I dial in.

matt_65

I put heattimer varivalves on all my radiators, and most of them are set wide open. They do seem to vent the radiators themselves very quickly, so once the first section gets hot the other sections will also quickly fill within a few minutes.

I'll check the timings for the vents tonight.

The delay relay sounds like a good idea to stop wear and tear from cycling, do you think the extra off time will condense enough steam that the run times will increase appreciably?

David Nadle

I think the delay will let the radiators give more of their heat to the rooms, possibly satisfying the thermostat before the boiler has to come back on. It will also allow more condensate to return to the boiler, possibly preventing any gurgling vents or low water conditions. But mainly it will end the short cycling.

David Hohengasser

Heat anticipator

I had the same problem. Someone on the wall suggested I adjust the heat anticipator on the thermostat. I have it set where the thermostat is satisfield when the vaporstat reaches about 12 ounces. My system almost never cycles on pressure. At most, it calls for heat three times an hour when it is very cold. Just a homeowner.

SusanC

Question about the TRV aspect

Can one use a #4 vent and a TRV successfully. I was told by my Wall-recommended installer that a #6 had to be used. My TRV apparatus is connected to the vent (of course) with a remote sensor since the radiator is in an enclosure. Now that we've been having cold weather the TRV is not holding, especially on 2° setback recovery, even when essentially turned off. It seems a long boiler run will result in warming of the radiator regardless; this is the first radiator off one of the mains and until the TRV, it had to be turned off resulting in a cold room - the guest room so at least guests didn't stay forever. I'm thinking, if the TRV itself is not shot, maybe a smaller vent will cut down on the breakthrough heat. However, I do believe the TRV worked last year (its first year) so chances are it is broken.

David Nadle

If your TRV kept the room confortable in October then it's probably working. The issue with a #4 is, it may fill the rad too slowly when you do want heat. My TRV can't hold forever either, one reason I haven't been able to set back. On a long run I'll get some overshoot, usually not more than 2 degrees. I used a #4 to limit this. You may find with a #4 your room will be cold in fall and spring. You could try a #5, also.

I do think stopping the short-cycling on pressure will make the TRVs work better. After I add the on delay relay I'll try a setback and see how the TRV fares.

SusanC

Boiler isn't actually short-cycling

Thanks for info. My boiler is pretty good about not short cycling (because now it's a bit underfired, I think). I find TRVs not to be too wonderful. Until now the only time I got heat in that radiator was on setback recovery so I would get a decent temperature (not hot like now) in the AM and then the room would maintain pretty well since the colder it was outside the more a little warmth would creep in through the radiator. I'm wondering if somehow the vacuum breaker in the TRV is not working like it did previously.

David Nadle

So you had a regular vent on that rad and it was too hot, now you have a TRV and a #6 and it barely heats?

First thing I would do is see if someone in your household reverted to old habits and shut the radiator off. Then I would remove and check the TRV assembly. Maybe it's as simple as the vent being clogged. What model TRV is it?

Fred Harwood_2

Sweet Spot

That's the anticipation sweet spot, David H.

matt_65

I added a second gorton #1 next to the existing one at the end of the dry return. The system was still warm but not hot when I tested, and from the initial puffs from the gortons to when they got hot and shut was only about 2.5 - 3 minutes. The boiler shows around 6 oz during this time. As the radiators heat up, the pressure will climb to about 10 oz and hold there for a bit, and then finally climb up to 15 oz and cut on pressure, and then cuts in less than a minute later. I didn't get a chance to measure when the radiators shut yet.

I picked up a delay-on-break/anti-short cycle relay from grainger today, and will probably put it after the vaporstat tonight to stop the short cycling. Its getting a little to far into winter to keep experimenting like this.

I still get a lot of gasping from two of the radiators, the bedroom one which is not too big and a big one downstairs. Do you think tapping them for a second vent would help?

David Nadle

Relay

I think you want a delay on make relay. I would run a seperate 24 from the transformer through the vaporstat and the relay coil and back to common, then use the relay switch as a substitute for the vaporstat.

Delay on make because the vapostat will cut back in too soon after cutting out and you want the boiler to stay off for the delay period.

Your mains clearing time sounds excellent.

matt_65

The delay on break relay I got can be run in series after the vaporstat, so when there is a call for heat the relay will initially be closed and the system will run just as before. If the vaporstat cuts on pressure before the call for heat ends, the relay senses the interruption on the circuit and opens and until the delay timeout is reached, and then closes again. If during the delay the vaporstat cuts back in, nothing will happen until the relay closes again.

This is the model I picked up:

http://www.icmcontrols.com/products/product.php?prod_pk=87

David Nadle

Don't think it will work.

Honeywell stats use a P+I algorithm to control temperature. Unless I'm very mistaken the CPH setting is a maximum limit on the number of cycles, not a fixed value or average. I do not think increasing CPH is going to force the system to shut off early. The P+I algorithm determines that.

Let us know what happens if you try it. I think you'll find you can't select CPH=2 for heat. The closest setting would be CPH=3.

The Focus PRO might have a setting for "less agressive" temperature control in heat, which may help if you are regularly getting overshoot.

David Nadle

OK - cool! That's even easier.

matt_65

Originally there was a square Honeywell with mercury bulbs and a mechanical clock/timer on it, I wonder if it would give better control than the "new" electronic one...

David Nadle

I think the control of the new Honeywells is outstanding. The short cycling on pressure is simply not something that can be fixed by this kind of thermostat. But it wasn't designed for that.

Steve Garson_2

How do you determine how long to keep the burner off?

matt_65

I settled on 6 minutes delay for now. It seems to be enough that the radiators are still pretty hot when the boiler cycles back on to fill them again if the thermostat isn't satisfied, and enough that the boiler fires for at least a few minutes at a time (won't know how long exactly until the holiday, it was still going when I left for work).

I'm trying to figure out if the efficiency hit I get for letting things cool off a bit just to be reheated again is greater than the hit for constant short cycling on pressure. At least with the delay I'm not so worried that the damper motor or gas valve will die from the constantly short cycling.

David Nadle

Hit or Gain?

I think you (we, I'm doing the same thing this weekend) may get an efficency gain, because some percentage of the time the hot radiators will satisfy the thermostat within those six minutes.

SusanC

Re: TRV, which was a side issue in this thread

I realize I never responded to your question and also I guess I was not clear re: the behavior of the TRV. The TRV is on the first radiator off the main and it makes the room too hot without a TRV. What I notice with the TRV is that the degree the radiator heats seems to depend on the setting (of course) but also on the duration of the boiler run. Because the boiler has a long run upon setback recovery, I would have to set the TRV low for that run. To maintain the desired temperature I would then have to change the TRV to a higher setting for the considerably shorter maintenance runs and this setting (TRV) also would have to be modified some depending upon coldness of outside temperature and consequent duration of boiler firing. Since no one is in the room a lot of the time, I keep the room colder than the others and haven't experimented too much with the TRV.

When I wrote about the TRV in this thread, the weather had just turned cold and, with a fairly low TRV setting, the room got very hot on setback recovery. I had to set the TRV off or almost off to get to a suitable temperature over the course of the day so it seemed to me that it was not impeding venting of air, thus not functioning as designed. Since then it has been OK on recovery, but I wouldn't trust it on an extremely cold day. In general, whereas TRVs help, they appear not to be ideal.

As for model, it's a Danfoss 8565 I believe, with vacuum breaker.

David Nadle

That's a lot to take in. Some thoughts: The #6 vent is too fast for that rad. That's why you get way too hot without a TRV. The TRV also may not be closing fast enough to prevent overshoot.

On a long recovery you may be building pressure too high and compressing steam into the radiator despite the closed vent.

Finally, if you are short cycling at the end of a long recovery cycle you may be tricking the vacuum breaker into opening just as the boiler is blasting out steam again.

SusanC

Your thoughts make sense to me

Re: vent size, without a TRV, this radiator would make the room too hot with any size vent. It's a large radiator and the room, which is not very large, is (uncharacteristically for this house)pretty well insulated because of closet and storage space and mostly interior walls.

I would bet on the suggestion of pressure building on the long run so that some steam gets into the radiator despite presumed vent closing.

And I would also bet on the vacuum breaker opening suggestion as a possibility. Since my boiler was down-fired I'm doing very little short cycling but I could believe that on especially cold days the vacuum breaker might be tricked into opening allowing steam to enter as a cycle starts up again.

Thanks for your suggestions.