Is my new boiler short cycling?

bklein

Just had a 40 year old boiler replaced. Due to a future addition planned in the next few years, we installed a slightly larger boiler. (166k btu instead of 155k btu).

The new boiler is a Weil-McLain CGA Series 3.

The old boiler would turn on when the thermostat called for heat and the burner would not turn off until the thermostat setting was satisfied.

The new boiler turns on when the thermostat calls for heat, then takes a few minutes to reach the 180F setting on the boiler and the burners shut off, within about 1 minute the water temp is back to 160F and the burners turn back on, it take about 1 minute to hit 180F and then the burners shut off again. This continues many times until the thermostat set point is satisfied.

Is this considered short cycling? Or is this a properly installed and functioning system? I assume the former.

A few things to keep in mind is that the outdoor temperatures are still nowhere near the lows that they will be in the winter. So am I worrying too soon? I can't imagine even in the dead of the winter the cycles will lengthen that significantly.

Also the house is 3 zones and at this point it is usually just 1 zone calling for heat at a time (the water will of course take longer to heat if it is multiple zones at once).

So why is this happening:
1) Oversized - perhaps it is slightly, but even if I reduce the gas flow significantly such that I see the flame lower significantly, the water temperature still goes from 160-180 in under 3 minutes and cycles off/on. So I don't think that is the major factor.

2) Should the gap between the maximum water temp (180F) and the temperature which causes the flam to turn back on (160F) be wider? I don't know if I can even change that setting. But if I set the max for 190F and the low for 150F, it will reduce the cycling a little bit.

3) The way it is installed currently the pipe exiting the boiler is about 3 inches in diameter and then reduces to 1/2 or 3/4 for the individual zones. The old system had a 1 inch pipe leaving the boiler. Could this lead to the hot water pooling in the large pipe and not being drawn away from the boiler thus allowing the water to hit 180F much faster in the current setup than the previous setup?

4) Could the pump be moving water too slowly? I have no reason to assume it is, but that would be another reason for the water to heat up too quickly.

As a possible solution (even temporary), can I manually reduce the gas flow (lets say about 20%) by turning the ball valve controlling gas flow? This way the water will heat more slowly and take longer to hit the 180F point? Are there any dangers in doing this, or any risk to damage to the system?

Any other thoughts?

KC_Jones

Was a heatloss calculation performed on the house to size the new boiler, or was it simply referenced off the old?  Heat loss is the only proper way to size a boiler, but just to give a general idea of what’s going on, how big (sq ft) is the house?  4000?  5000?  Something a lot smaller?

That’s now how boilers are sized, but can give an idea of if things are even in the ballpark.

If the old boiler had significantly lower efficiency than the new then the actual amount of BTU’s available for the system could have been a fair bit lower and what you are seeing could be “normal” for the boiler they installed.  Or you could have other issues.  It will probably be asked so might be a good idea to post some pictures of the boiler and it’s piping also.

Hot_water_fan

How much gas did you use last year? And what’s your general location? 155kbtu is a lot of boiler. 

mattmia2

bklein said:

As a possible solution (even temporary), can I manually reduce the gas flow (lets say about 20%) by turning the ball valve controlling gas flow? This way the water will heat more slowly and take longer to hit the 180F point? Are there any dangers in doing this, or any risk to damage to the system?

NEVER DO THIS. It won't burn properly and the restriction won't regulate the gas pressure in a predicable way. The burner cycling on the aquastat won't damage anything.

mattmia2

Now that we've covered that, how hot is the return from the zone that is calling? What kind of emitters are on the zones and how much on each zone? That will give us an idea of how much heat a zone can transfer from the boiler. It may be possible to turn the cut in on the aquastat lower, but if the emitter is fin tube, there isn't a lot of output at 140 degrees and below.

HVACNUT

Are you using the Economy dial?
Are the thermostat and DHW wires spliced together, or do they go where they're supposed to go?
In any event, I would turn the temperature down to 160°. Only turn it up if a zone isn't satisfying. 

Jamie Hall

Thermo 101. If the temperature shoots up like that, you are adding heat much faster than you are losing it somewhere else. The boiler may be too big (and as has been said, don't try to down fire it yourself -- it's not a stove), but if it didn't do it before it won't do it much more now -- the size difference isn't that great. So the problem is in the cooling system -- that is, the radiators and how much water is going through them. Under pumped? Split into zones which are too small? If the system can't dump the heat through the radiators, it's going to cycle like that.

mattmia2

Or was the old system just quieter about it or did the old boiler have a lot more mass of water and iron so it acted as more of a buffer?

MaxMercy

mattmia2 said:

Or was the old system just quieter about it or did the old boiler have a lot more mass of water and iron so it acted as more of a buffer?

Probably.. From what I've read, the newest generation of even standard CI boilers carry less water these days.

Jersey2

Maybe use the economy mode. My manual says if I used economy mode, the boiler would turn off at a lower temperature than 180 depending on what economy setting used. I have a different boiler though. And maybe put a security camera to observe your boiler to actually see when it turns on and off.