New Gas Boiler "short cycling"

Ismellelephant · 4:51PM

I had a new Weil McLain Cgi PIN 4 gas boiler installed and it appears to be "short cycling". After 3 or 4 visits from the installer they are now claiming this is normal. Recently in 20 degree weather the house only reached 67 degrees despite the thermostat being set to 70 degrees, and all through the night the boiler would shut off and on every 1 to 2 minutes.
Shouldn't this boiler run constantly until the set thermostat temperature is reached?
Purging the air in the baseboard system and a thermostat replacement didn't solve the short cycling. Thanks for any info.

neilc · 5:34PM

did you get air?

is the circ running?

post some general pictures,

system pressure, and temp at boiler?

pecmsg · 5:42PM

Is there a 1st and 2nd floor?

kcopp · 6:03PM

Running and firing are 2 different things. Sounds like the boiler is firing but will only give up some heat and not satisfy the zone.

Could be air. Could be improper circulator/ flow.

Ismellelephant · 6:35PM

Thanks for the replies.

There is no air in the system, a purge was done.

It is a one floor ranch house, one zone and about a 1000 sq ft house.

Running and firing, when the boiler reaches 160 degrees, it fires up, in about a minute or two it reaches 180 and fires off. The circulator is very quiet but I believe it continues to circulate when the firing is in its off cycle. It just continues this over and over again.

In 40 degree weather the house will reach set temp even with the 1-2 minute cycling, but in 20 degree weather it fails to reach set temp.

I understand the firing off and on to satisfy the 160 to 180 deferential. But when there is a call for heat shouldn't the boiler continue to fire until the thermostat set temperature is reached instead of firing off and on?

This is the system pressure meter

Jamie Hall · 6:50PM

At first impression, either there is still air — in the boiler circuit — or the boiler pump isn't circulating throug the boiler.

neilc · 8:49PM

can we get a wider shot of the boiler/

floor to ceiling, showing circ, expansion tank, all in one, maybe from a couple angles

Ismellelephant · 9:31PM

I am not at the home right now but will post pics.

I do know everything was replaced, water regulator, backflow valve, new vents, new circulator, expansion tank. I also asked the installers if the boiler might be oversized and they said it is not.

Maybe this isn't a yes or no question, but when there is a call for heat shouldn't the boiler continue to fire until the thermostat set temperature is reached? The boiler replaced "seemed" to operate this way and would run for 10 minutes or so till thermostat temp was reached.

neilc · 10:03PM

the boiler is hitting the high limit at 180(adj), and shutting the burner down as it should, you're not getting the boiler heat into the house,

it doesn't sound like you have proper circulation, or there's a bypass, or ??

did the old heat work to setpoint before the new boiler?

pecmsg · 10:30PM

water temp in and out of the boiler?

Ismellelephant · 10:34PM

I know of no bypass and when in the 40's it will reach thermostat temp. I have no info to offer on the old boiler other than is would run for 10 minutes or so when heat was called for.

So if the boiler is hitting the high limit at 180 and shuts the burner down is how it should work, then at this point I'll probably have to wait until we are in the 20's again and see what happens.

I really appreciate all the info.

hot_rod · 10:59PM

Could be the boiler is oversized. How many feet of fin tube is connected?

CGi 4 is about 88,000 output

Typical fin tube is about 500 btu/ft. So you would need about 176' of fin tube to match the boiler

a WAG for 1000 sq ft of home would be under 30,000 btu/hr needed. A load calc would tell a better story.

But the boiler could be 3 times the size required?

So on the coldest day it will short cycle on milder days the cycling will be even shorter.

kcopp · 11:01PM

Simple question…. Is all the piping hot? Supply out to the system and return?

Go out 10 feet from the boiler… still hot ?

109A_5 · 11:06PM

If the old boiler worked as you would expect, not short cycling, and heating the home as expected. And the new one is short cycling and NOT heating the home as expected. The question is what significantly changed. Assuming the new boiler is not significantly oversized.

It appears system presently can't move the boiler's heated water throughout the house correctly, to heat the house appropriately, trapped air, circulator issues, bypass not set correctly or some restriction in the piping can cause this situation.

Since the boiler is not heating the house correctly the boiler's water heats up too quickly reaching the high limit 180 degrees and that shuts the boiler' burner off. It is normal if the boiler's water temperature gets to 180 for the boiler's burner to shut off.

The question is what significantly changed.

New Gas Boiler "short cycling"

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