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Cold Climate Retrofits: Reusing Hydro-Air with Central ASHP

system_smart | Posted in Mechanicals on

I am interested to explore and understand the “cons” of a design concept for low-carbon heating and cooling that utilizes hydronic (Hydro-Air) coils for supplemental heat, instead of electric strip heaters in the air handler (AHU). Background:
*Cold climates 5A-6A
*existing homes circa 1850-2000 with deep retrofits completed and/or scheduled (R49-60 attic, R13 cavity dense pack, R19 bsmt ceiling, 7ACH@50Pa, double or triple-pane vinyl windows, HPWH in unfinished basement with mixing valve to tamper 140F output to domestic use, ducts sealed to <5% leakage and uninsulated, hot water pipes insulated R2) 
*existing centrally ducted AC with Hydro Air coil to be replaced with ccASHP to meet heating loads and cooling loads with compressor in 90-95% of conditions, and supplemental heat during the rest.
*simple design schematic included
*existing (fossil fuel) boiler is near end-of-life and only serves HydroAir coil and previously served DHW (either via indirect tank or directly). 

What calculation would determine when the HPWH tank should be >80gal (beyond normal off-the-shelf product)? 
When would the indirect tank be more useful (capacity, HPWH COP, etc) than the heat exchanger? If it already exists, it seems obvious to reuse.
What changes are required to the ASHP control setup to enable or integrate with the HydroAir pumping loop?
What should I be asking about?

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  1. paul_wiedefeld | | #1

    The schematic makes it seem like the HPWH is supplying the backup heat via the hydro air handler? That's a bad idea - it's both inefficient and leads to poor performance. You're cooling the house to then heat up the house. I'm pretty sure the performance would work out to be less efficient than resistance.

    There's not really a compelling reason to use hydro air handlers for a single zone. A furnace is much more common and cheaper. Electric strips are fine too. You want a system future contractors will understand and have parts for.

    1. system_smart | | #2

      @paul_wiedefeld Yes, given this is for existing building retrofits, hybrid furnaces may be suitable, but our region includes many oil/propane customers that cannot access gas or don't want it. Basements are typically isolated reasonably well from this "pretty good home". If basement air ~50F for a HPWH operating >2.5-3.5 COP, and ASHP is operating >1.8 COP @15F, the HydroAir still seems better than strip heating. If bsmt air drops to 40F at 100yr storm, water still moves heat better and is better than OAT. Might make the rationale to insulated ducts better. Am I missing something else?
      What part of the system or resident comfort is experiencing the poor performance?

      1. paul_wiedefeld | | #3

        Propane and oil furnaces exist, so access to natural gas is not a barrier.

        You're missing how HPWH's work - they extract heat from air. So an unconditioned basement would not be 50 degrees if it was used as central heater - it'd approach outdoor air temp pretty quickly or steal heat from the rest of the house. Second, the capacity of a HPWH is generally low - a Rheem heat pump water heater has a max of 12,000 btu/h for the highest capacity offering, which decreases as surrounding air temp decreases. So you're getting low efficiency if the basement is perfectly air sealed as the temp decreases or you're getting low efficiency if the heat pump is stealing heat from upstairs. The poor performance is the even slower DHW recovery time as well as the low central heating capacity.

        All-in-all, this is a bad idea and we have several better, mainstream options.

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