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Posted by Heatpro on January 12, 2006 at 13:07:53:
In Reply to: Buderus/Air Handler short cycling posted by Warm Canuck on January 12, 2006 at 12:26:52:
"There are people who think continuous circulation mode with an AH isn't great"
That is something to discuss with the people; they might be the same people that newspapers use as 'undisclosed sources.' Continuous circulation can be expensive if air is cycled continuously through ductwork that is R6, 2-inches thick, instead of the R-19 suggested for walls; as much heat is lost from the larger duct surfaces in unheated attics or crawl spaces. Circulating it constantly through adequately-insulated and sealed duct would be the same as having warm air inside the home envelope.
Other than that, warm air from continuous air circulation heated by a coil is more comfoprtable than periodic bursts of cool air below skin temperature (like heat pumps) in ducts cooled during the off-cycle.
6 cycles/hr is normal for a furnace or hot water system, so no difference means no argument. Gas doesn't have as much of a mixing and atomizing sooting problem from rapid cycling as oil burners, so many heaters use gas pulse modulation with no large losses. The purge cycle is purging gases similar to moving some of the house air for some seconds, so isn't a large loss.
Continuous circulation for heavy slab heating is of little importance, as the heat is in the slab, so circulating water to take heat from a slab to warm the boiler during the off-cycle is strange. Letting the heated water purge from the boiler to the slab after the heat cycle, then shutting off the circulator makes sense, as that is the advertised feature of System 2000 to water heater tanks.
Higher boiler temperatures translates to lower efficiencies during off cycles. There is such a small difference in the efficiency of your boiler in the two situations with 110f temperatures, that worrying about whether you'll have the money to buy a few more or less magazine issues a year or one trip to BK is too much worry for the service time.
The raising of boiler temp to satisfy the 120F+ hot water demand is a greater efficiency concern, which is almost nil.