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Thread: hooking hot water heat thermostat to furnace in attic

  1. #1
    DIY Member estone's Avatar
    Join Date
    Mar 2008

    Default hooking hot water heat thermostat to furnace in attic

    I currently have hot water baseboard heat with two thermostats, one for the bedrooms zone and one for living room/kitchen zone. I elected to have the HVAC guy put a standard furnace in the attic when we put in A/C so that we could run heat from it too, plus it was only about 100 more plus running gas to it than just buying an air handler. He wants to just put in one thermostat for his system(a/c & furnace heat) and then leave the existing two thermostats just for the boiler heat. Would it be advisable to replace the two existing stats with stats that will run everything, hot water base board heat, attic a/c and furnace heat? I assume I will continue to use the hot water heat in the winter but if it ever went down it would be nice to have the backup system and just have two stats on the wall. Am I being to picky or should I just leave it as he says.

    Thanks in advance.

  2. #2
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    How would a single thermostat decide to fire up the hot air furnace rather than the boiler?

    At equivalent AFUE numbers, heating with the boiler is inherently more efficient than ducted hot air, due to FAR lower distribution losses. If the hot air furnace is located in an attic above the insulation layer, the difference is even greater, since none of the standby and distribution losses accrue to the house, only to the attic. It's enough of a difference that even an 80% AFUE boiler in a semi-conditioned basement would cost measurably less to run than a condensing furnace in the attic.

    Seems like a lot of trouble to go through for just a backup system, but just leave it as a separate system, and leave the AC/hot-air thermostat in cooling mode unless you ever actually need it. There's zero advantage to making a shot-gun marriage between the hydronic & hot air systems. I suppose you could do it with a multi-stage thermostat, making the hot air furnace the second stage, but it's just never going to be worth it.

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