Short Cycling

Roland_4

Hi All,
OK so I have a new boiler,a Burnham PVG-4 with 30Gal indirect DHW. I have read ofter on this forum that short cycling is profligate and wastefull and shortens the life of the heating unit. How does one determine if the boiler is short cycling? I suspect that the temperature gauge on the supply pipe is inaccurate as the displayed temp is lower than what my IR thermometer tells me. Also, the silver dial on the boiler aquastat has rather coarse temperature divisions. Do I monitor the amount of time the boiler runs from a cold start to reach the set room temp? Or do I monitor the on-off cycles to maintain the set room temp? Thanks, Roland

Ken_40

Great question!

Some basics:

1. When only one zone calls for heat, short cycling is "normal."

2. When the boiler is larger than required, short cycling is a certainty.

3. When one zone is for D/H/W, and someone's in the shower, short cycling should be rare.

4. If the boiler is setup for cold starts, short cycling will be minimalized.

5. Short cycling can be seriously avoided with certain control strategies - most of these controls are relatively inexpensive e.g., delay timers, R.A. aqustats, etc.

What leads you to think it is short cycling or will?

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Roland_4

Short Cycling

Thanks for responding Ken,
I understand the concept of short cycling but I don't know how to recognise it in my circumstance. We havent had much in the way of cold temps where I live. I have only monitored the behavior of my system one time when heat was needed. To bring the room temp from 65*F to 72*F the boiler initially fired for 15 minutes, then cycled on 5 minutes and off 5 minutes three times until the room temp stabilized at 72*F. This is hardly a good sample on which to base a conclusion. I just want to understand what to look for as normal opperating behavior. Also, cleaning the convectors made a big difference in the amount of heat delivered. They were filthy. Probably hadn't been cleaned by the former owners in decades.

Ken_40

Dust Bunnies...

The compromise of heat output when HWBB fins have even modest amounts of dust, lint, hair (dog,cat and human) residuals, is one of the most underestimated factors of HWBB systems I'm aware of.

When W/M stated the closure of the louver atop the cabinets of their HWBB's diminshed output by 50% - I re-read the passage in dis-belief. We all know now, closing the little blade/valance is hardly a tight seal; perhaps a 90% closure at most. Then too, we saw more than a few homes complain of low heat when wall-to-wall carpet was installed. The installers choked off most of the lower slot intake and diminished air flow - reducing output substantially too.

Obviously, ANYTHING that restricts air flow accross the fins has a negative impact, what is alarming is how much output is lost - with so little restriction!

The losses are more than just comfort. The fuel wasted is akin to driving a car with one foot on the gas - the other on the brake - at he same time.

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coalcracker

Your boiler WILL cycle on and off because the circulator is returning cool water to the boiler from the room and causing the boiler water to dip below the low limit setting. After the water temperature in the pipes has stabilized, you will still have losses from the baseboard to heat the room.

If you watch your temp gauge you will probably see the cool return water draw down your boiler temp to anywhere from 1-10 degrees below the low limit aquastat setpoint and the boiler will start up again and run until it reaches the low limit setpoint.

TK03

To avoid short cycling in off seasons add a control to control short cycling such as a Tekmar or Beckett Heat Manager. I have had good luck with both

coalcracker

And Beckett guarantees that you will save a minimum of 10% in fuel costs with the Heat Manager.