Still Short Cycling

JohnCT

I've taken the following steps, but the boiler is still short cycling:
1. Insulated all the mains and risers.
2. Replaced radiator vents with appropriate Gorton vents.
3. Bumped main venting to 2 G2s and 2 Big Mouths on 90' of 2" main.
4. Added a low pressure gauge and dialed in .5 cut in/1.5 cut out.
5. Increased the thermostat swing setting to +/- 1 degree and only use a 2 degree evening setback.

The system works pretty good now. The mains heat up pretty quickly, the main vents close in a few minutes, the radiators heat up pretty quickly and seem to be well balanced. But it will go through a few short cycles before the thermostat shuts it down. Runs for a while with the pressure gauge pegged at zero, then starts climbing for a couple of minutes to 1.5, cuts out, drops to .5, cuts in, and repeats this until the thermostat shuts it down.

I'm wondering if the boiler is just way oversized for the 1,800 SF house and I'm stuck with this. 8 radiators, used to be 9 before a remodel, and two of them are kind of small. Not sure what the boiler output numbers mean, lower one is 170,000.

Any thoughts?

Jamie Hall

Typical oversized boiler behavior. Not much you can do about it, unless someday you can see if the boiler can be downfired… which maybe yes, maybe no.

ImYoungxD

Seeing that your previous comment says "my oil company", I assume you're using oil. I suggest you ask them to install a smaller nozzle to lower the BTU. My boiler is 130k BTU which is oversized for my 1500 sqft home with 8 rads and they used a .75 nozzle to lower the BTU. It runs longer to reach the correct temp. Rarely short cycles unless it's less than 10 degrees outside.

ethicalpaul

Steps 1, 3, and 4 probably made the short cycling worse, but on the bright side, at least you're not burning fuel for nothing any more. It's short-cycling for the right reason (oversized boiler).

So short-cycling worse = nicer fuel bills for you.

Do what I do, and think of each short-cycle being one cycle closer to when you can replace the boiler with one that is correctly sized and piped thanks to the amazing help given on this forum.

KC_Jones

If you do the EDR calculations for your radiators you will find out how over sized you are.

Take a picture of the spec plate on the boiler for reference.

HVACNUT

Oil or gas?
The pig tail is clean?

gfrbrookline

Try setting your Tstat to 2 cph. It will tighten up the temperature variance so the boiler will run in shorter burst which may resolve your short cycling.

Fred

You do need to calculate the total EDR of those 8 radiators and compare that to the Sq. Ft. Steam on the boiler plate. That will tell you how over sized your boiler is. You did all the right things to tune your system but even that won't be a 100% fix where the boiler is grossly over sized.
Also, do you have a Cyclegard Low Water Cut-off on that boiler? That unit will shut the boiler down several times during a heating cycle to check the water level. That adds to what is perceived as short cycling. If you have that unit, changing it out for a Safegard will help.

JohnCT

ImYoungxD: I like the downsize idea. I understand there's more to it than just adding a smaller nozzle, so maybe I'll try to find a steam pro (by the way, didn't see any in CT other than a rad restorer, if anyone knows of any) instead of asking my oil company to do it. Thanks!

ImYoungxD

JohnCT said:

ImYoungxD: I like the downsize idea. I understand there's more to it than just adding a smaller nozzle, so maybe I'll try to find a steam pro (by the way, didn't see any in CT other than a rad restorer, if anyone knows of any) instead of asking my oil company to do it. Thanks!

Correct. If you lower the GPM of the oil, the burner needs to be adjusted correctly. Confirm with the company that they will do combustion test after changing the nozzle, adjusting the burner and ensure the results are good. Get a copy of the results.

JohnCT

HVACNUT said:

Oil or gas?

The pig tail is clean?

It's oil, and I'm assuming the pigtail is clean because the low pressure gauge is working and pressuretrol adjustments are changing the operating pressure. Bad assumption?

JohnCT

gfrbrookline said:

Try setting your Tstat to 2 cph. It will tighten up the temperature variance so the boiler will run in shorter burst which may resolve your short cycling.

I don't have a CPH option on my Lux thermostat. I can only control the swing temperature. It's set at 1. I could lower it to .5, which might result in more cycles but less short cycles. Do you see an advantage to that?

JohnCT

Fred said:

You do need to calculate the total EDR of those 8 radiators and compare that to the Sq. Ft. Steam on the boiler plate. That will tell you how over sized your boiler is. You did all the right things to tune your system but even that won't be a 100% fix where the boiler is grossly over sized.
Also, do you have a Cyclegard Low Water Cut-off on that boiler? That unit will shut the boiler down several times during a heating cycle to check the water level. That adds to what is perceived as short cycling. If you have that unit, changing it out for a Safegard will help.

Fred: Not that brand, but thanks for letting me know to check for that.

ImYoungxD

JohnCT said:

gfrbrookline said:

Try setting your Tstat to 2 cph. It will tighten up the temperature variance so the boiler will run in shorter burst which may resolve your short cycling.

I don't have a CPH option on my Lux thermostat. I can only control the swing temperature. It's set at 1. I could lower it to .5, which might result in more cycles but less short cycles. Do you see an advantage to that?

I have a Lux. I set it at #4 which is 1 degree difference. Works fine for me. I set it on #3 when outside temp is above 50