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Buderus/Air Handler short cycling

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Posted by Warm Canuck on January 12, 2006 at 12:26:52:

I've been running a Buderus Boiler GA124-17 (about 60k BTU out) in our house for 8 months, including what has been passing for winter in Southwestern Ontario. The boiler heats an indirect DHW Amtrol tank (with Enerworks Solar preheat) and a Lifebreath Air Handler for heat. A Tekmar 262 runs the whole show with indoor and outdoor temp sensing.

The house is a 4400 sq. ft. south facing ICF structure, about 89 on the Energuide scale, or about 50% more efficient than an R2000 house.

Right now, I have the air handler running on it's low speed all the time (about 250 CFM). This gives us filtration and humidity control through a Lifebreath HRV. The AH motor is an ECM motor and costs virtually nothing to run at this speed. The boiler temp required for a 14F outside temp is 125F, and the current boiler target temp for our around freezing temps is 106F. The Buderus lower operating temp is 104F.

So here's the issue. I'm running in continuous circulation mode. With the boiler pump running continuously, the Buderus is cycling 5 to 6 times an hour to maintain the target temp of 106. The Tekmar is set to auto differential, but looks like it's only running to +-10F of it's setpoint, though it can go to +-20F if set manually. The burn time is about 5 minutes per cycle right now.

Is it more efficient to run like this, or should I switch to thermostat control, where I would have less cycling but higher boiler temps ? I would energize the boiler demand terminals on the Tekmar with the thermostat and so the AH water pump would only run on a call for heat, maybe twice per hour depending on the thermostat I use. Buderus did not seem concerned over this cycling rate. The boiler prepurge is only 5 s, so you don't lose much heat during that.

There are people who think continuous circulation mode with an AH isn't great and that it's best used with slabs and radiant. I'm not switching to a modulating burner, as this is what came with the house.

Thoughts ?

I should mention that the DHW and the AH are on parallel takeoffs from the boiler supply manifold, that each runs a purge cycle (which for the AH is shorter than the time it cycle time anyway right now) and that there is DHW priority.

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