short cycle maybe?

Dan_43

I have an 8 yr old boiler, (gas fired) I do not have the make an model at the moment will add it when I get home.
What has bothered me for quite sometime, is that the boiler reaches 1-1/2 lb shuts off and in about 1 min drops down to a 1/2 lb and starts up again taking about a minute to reach 1-1/2 lb and starts the cycle all over again. it seems like the pressure drops to fast. My question is...is this OK? I do not have the sizing at this moment, but I do know when the new boiler was sized the radiators were calculated.
Dan

KC_Jones

Short cycling is either oversized boiler, inadequate main venting or a combination of both. We see both as being fairly common around here. The pressuretrol is functioning the way it's supposed to in that it cuts off the burner when it reaches that pressure. If you post the EDR calculations and the boiler size we can confirm the boiler is sized properly. If you post what make and size of main vents you have as well as length and size of main piping we could verify if you have enough main venting. Don't assume they calculated correctly when they put the new boiler in. When I had contractors out they measured all the radiators and then just sized according to my old oversized boiler....so I didn't hire anyone. Get back to us with the information and we can help you out.

Dan_43

could my main vents be too much? I made a "T" arrangement at the end of my 2 returns. I have (3) main vent on each return.

vaporvac

Main venting can rarely be "too much". There is a limit determined by the "open-pipe" method, but any more than that just affects your wallet. In any case too much venting wouldn't result in the situation you're seeing. Your pressures indicate either not enough main venting or an over-sized boiler, or both as mentioned above.
How long are your mains and what vents do they each have ?

KC_Jones

Agreed you can't have too much venting. Did you size this boiler? I looked at some of your older posts and noticed you posted this on what I assume is a different boiler. "I measured all the radiators added a pick up facture of 1.5. Chose the boiler that was the next size above my sq/ft figure. " If you size like that you will end up MASSIVELY oversized by at least 50%.

ChrisJ

Where are the pictures?

Fred

The fact that it takes so little time for the pressure to drop is normal. The short cycling is the issue and as has been said, it's either an over-sized boiler or inadequate venting. If it runs for a few minutes, on start up (maybe 10 to 15 minutes) before the short cycling begins, Likely culpret is an over sized boiler.

Jamie Hall

Might I add, though, that if it runs for 20 or 30 minutes after it first makes steam before it starts to cycle... that's pretty normal for a typically sized boiler. Yes, you can get it to run longer before it starts to hit the pressure -- if you happen to get lucky and really get the sizing perfect. Who does (we have a few fanatics who do...)?

Also, the timing of your cycle is quite normal.

Dan_43

Here is some more information. The boiler is a Burnham (see sizing in attachment). I redid my EDR calculation. have attached doc not sure if its up loading or not. My EDR total 482 w/o pick up factor. If I remove from my calulations the 3 shut off radiators my EDR is 429.
It does not look like I'm grossly over sized. I may be more so when I have 3 of the radiators shut off. Here is a picture of one of my main vents and if you can see in the back ground the other vent is in the photo. I was getting water spitting that's why I have extended the vent piping.

BobC

Your boiler is rated for 542 sq ft of steam and you have a calculated EDR or 482 so the boiler is a little oversized so a little short cycling is to be expected.

Are the mains heated fully to the ends when the boiler shuts off the first time? How long does it take for steam to reach the main vents? Are the radiators getting hot at that time also?

What is the boiler piping like and how much bounce is in the sight glass when it's making steam?

Bob

Dave0176

It is oversized but not crazy. You def need more main venting, look into some Gorton #2 main vents. Also with the larger boiler, your system is perfect for a two stage gas valve. This will allow your boiler to cycle down to low fire and build steam slowly so you heat more evenly.

KC_Jones

How long and what size are your mains? I would take a wild guess and say you aren't even in the ballpark on your venting judging by the amount of radiation. Need to measure your mains until after the last radiator, length and size. Post that and we can suggest how much venting you need. I agree the oversizing isn't terrible, but having those radiators turned off is really probably hurting you more than helping. I would suggest turning them back on. If you aren't ever planning on turning them on or planned on removing them you should have installed the next size down boiler. With them off you are 20% oversized with them on you are 11% oversized. If you think you are saving fuel or something having them off, I would seriously doubt that. And the short cycling it contributes to isn't helping with boiler longevity.

Dan_43

The 482 does not include the piping . Is the 542 with or without the pickup factored in?
Also tell me more about the 2 stage gas valve, or point me to a past thread.
Thanks for all the feed back.
Dan

KC_Jones

When figuring the size of a steam boiler you add up your radiation, in your case 482 and compare that directly to the boiler rating. The boiler manufacturer includes a .33 pickup factor in that rating already. There has been some discussion on this topic around here lately and the idea that the pickup is excessive and we should be considering building heat loss a little more than just radiation sizing. In your case there are many on here, me included that probably would have gone with the next size down boiler. Since you have what you have a 2 stage gas valve is another option. It essentially allows you to fire the boiler at 2 levels. Starts on high and once you hit a certain pressure it cuts down to low fire so it can "simmer" longer and not short cycle. I have this on mine, but I never build any pressure so it currently doesn't do anything. Sound like you have more pressure than you need so it would probably be a good idea in your situation. I would highly recommend a pro for this addition as you are modifying the gas train of your boiler.

PMJ

For a system with a boiler truly oversized by a large amount the simplest and least expensive way to handle this would be to put an on/off timer between the tstat and the boiler. Both the on and the off times need to be adjustable. You limit the total run time and spread that out in equal amounts separated by equal off times. Spreading it out accomplishes the same thing as cranking it down.

Fred

Definitely needs more venting.