Pressuretrol short cycling

rexster

I have a single pipe steam system in my house. Once the boiler is on for a while, the Pressuretrol will trip. After a short period of time, maybe 30 seconds to a minute, the Pressuretrol will reset and the boiler will run for about 5 minutes. My question is whether that is normal behavior. If it isn't normal behavior, what are the likely suspects?

rexster

Here is a picture of the Pressuretrol

BobC

The boiler is probably bigger than the system it feeds. Once everything fills with steam the pressure will build til it reaches the pressuretrol setpoint. Are all the radiators hot when the pressuretrol cuts out?

If the radiators are not filled with steam the steam mains may not be adequately vented. Is the end of the steam main hot by the time the pressuretrol trips?

Bob

rexster

Yes, they are all hot and the mains are hot. We did do work last year and removed two radiators, one was a 3-4 long and the other was maybe 2 feet long. We moved into this house two years ago and I have a feeling the previous owners might have removed a few radiators as well (but not sure).

BobC

Do you know what the EDR of all the radiators is compared with the sq ft rating of the steam boiler?

What kind of boiler is this?

Bob

KC_Jones

What was the reasoning for removing the radiators? That is most likely part of the issue. On a steam boiler size is dependent on the amount of radiation connected to it. What you have effectively done is oversize the boiler, by how much is the question. As Bob already said you should do an EDR calculation and compare that to the boiler size. That would give you and idea of how oversized the boiler is. As far as the short cycling the only real solution is sizing the boiler properly to the load or possibly installing a 2 stage gas valve controlled by a vaporstat that would drop the firing rate when pressure started building.

rexster

Don't know the EDRs, have to research that. Its a Burnham, Input BTU is 175000 and DOE HTG CAP is 145000.

BobC

That boiler works out to about 600 sq ft of steam. If the boiler is too large for the system it will cycle on pressure. If it's oil it can be downfired a little, if it's gas it might be able to be downfired more.

Bob

rexster

Thanks so much for the help. Once I get the EDRs (most are Sunrads), I should be able to compare it to the 600 and see if the boiler is oversized. That would make a lot of sense. It is gas fired.

rexster

Doing some quick calculations, it looks like I'll be way under 600 sq ft. I think the ball park number is 300-400 sq ft.

vincer652

I may be in a similar situation, where my gas fired steam boiler is firing on and off every 5 minutes or so. I have the cut in at .5 and differential at 1.0, cut-out at 1.5. All radiators are venting and heating, and the t-stat is satisfied to within 2 degrees. Is the cutting in and out and associated firing detrimental to the system or dangerous? My first thought is that the piping system is not holding pressure for a long enough time, through a steam leak of sorts, but i don't think that is the case. What could be done to limit the on-off firing of the boiler?

Jamie Hall

rexster said:

Doing some quick calculations, it looks like I'll be way under 600 sq ft. I think the ball park number is 300-400 sq ft.

That would do it...

What is happening is that the boiler is simply making steam faster than the system can condense it. The only way the system can compensate for that is to shut the boiler off from time to time to allow the system to catch up. The boiler cutting off on pressure is a symptom of that, not the cause, and cutting back in quickly is to be expected -- as the steam out in the system condenses (warming you up!) the pressure drops quite rapidly.

There isn't much you can do about it, with that much mismatch.

It won't actually hurt anything -- except your pocketbook for fuel. The efficiency does drop off. Not drastically, but it does drop.