Dunkirk Q90 short cycling?

Kevin Shaw

Dear Professor,

I just installed my first high efficiency boiler and have a few questions. I have been timing the boiler for a couple of hours per day to see how it is running. It is a natural gas hot water system, the boiler is a Q90 -100. 100,000 btu in 90,000 out. It runs on average 7 minutes and is off 7 minutes. I though this was pretty short. The aqua stat is set from the factory at 180 my water temperature gets up to 140, the return line is much cooler no gauge, but I can hold onto it with no problem. My programmable themostat had a separate setting for High efficiency boilers. I changed this yesterday for a high efficiency system. The boiler runs much longer, but the off cycle is about the same. After changing the dip switches the boiler will run for 23 - 30 minutes with a 6 to 9 minute off cycle. The only real difference that I noticed was the water temperature got up to 155 and the return temp seemsed to be about the same. As I mentioned I don't have a gauge on the return line so I don't know for sure the return temp. I set the dip switches back to where they were and the boiler went back to 7 minute on and 7 minute off cycle while lowering the water temp to 130 and still satisfying the thermostat. When I sized the boiler I did the heat load 2 different ways one by sizing the radiators the other by the wall and material makeup. One came out 92,000 BTU one was 87,000 I bought the 100,000 btu boiler so I don't think it is oversized by any means. It seems like it will cost less to run the shorter cycles than to keep the boiler on 2 or 3 X longer. What do you think the problem is? Do you think 7 minutes on and 7 off is OK?

kevin

Steve M_2

short cycling

Most boilers that use an aquastat as a control device do short cycle. A modulating gas valve combined with reset control lessens the possibility of short cycling. I haven't put in a Q90 in a few years and I don't think they have that feature. If you have room, consider a buffer tank. Your Q90 and it's pricey ignitors will love it. You can also use reset controls on the tank for even more savings and comfort.

Mark Hunt

What was the outdoor temp?


And when the boiler was "off", was it off on limit or was the t-stat satisfied?

You have a fixed firing rate boiler that you sized according to worst case or design day. If you were not at that design day temp, the boiler will short cycle. It is putting out more than the house needs and it is oversized for those days. Nothing you can do about that.

The low water temps lead mre to believe that the outdoor temp was not near design day and the house was satisfying with 155 degree water.

Mark H

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