Crawlspace HRV
 

Hi,
I am considering a project, and I could use some advice on how to do it. My crawlspace has a (wood with insulation) "staple up" radiant heat floor above it. When the house was built, the inspector required open vents for the crawlspace.

But the problem is, the open vents created a very cold crawlspace and since heat goes to cold, the heat from the floor was mostly being lost to the crawlspace where it was vented outside. This caused high energy bills.

The solution to that problem was to plug and seal the vents. No more cold air blowing under the floor. I had the crawlspace "encapsulated" and when the guy was here he was telling me that something like 80% of the air in our homes comes from the crawlspace because of the stack effect.

This got me thinking: I could vent the crawlspace and possibly provide some fresh air to the house by installing a Heat Recovery Ventilator in the crawlspace (mini basement). Ducting would be short and simple with the existing foundation vent to outside and any ductwork could be exposed across the bottom of the joists. Another HRV would be used inside the house anyway, but recovering the heat from the crawlspace as it supplies fresh air would be a good thing. The crawlspace would and should be like any other conditioned area inside the building envelope.

The problem I see with any HRV is that they are only 75-90% (at best) efficient. In the winter they replace warm air with cooler air, placing more load on the heating system almost like an open window would. I would like to have some heat in the crawlspace. I believe this would increase the efficiency of the radiant heat floor above it. For example, with the area below the floor at - say- 30* and the area above the floor at 66* the heat from the floor will want to go to the colder area. The insulation slows it down; however, it would still make its way toward the cold area. I'm thinking that with a smaller delta T between the temperature above the floor and below the floor the more efficient the floor can be.

So, to provide some heat to the crawlspace, what I am thinking of doing is running a loop from the floor's manifold to a water to air heat exchanger, which I would mount in the supply duct from the HRV. I have seen this done with electric heaters, but I prefer to use hydronic heat for a couple reasons. One, with my heat pump I can produce heat cheaper than straight electric heat. Two, it would be easy to connect to the zone manifold. Three, since it is connected to the zone manifold it would supply heat at the same time as the floor. The thermostat for the floor turns the pump on and opens the valve which supplies heat to the manifold, which would supply heat to both the floor and crawlspace.

The question I have is: since the water temp is only 90* what size HX would I need? It shouldn't have to be too big since it only has to make up the difference (plus a little more) of what the HRV can't. What about flow rates for the water? I plan on using 1/2 PEX to supply the HX, would a HX with 1/2 tubing be a restriction, or should I get a HX with larger tubing (say 3/4") to have more "btu's" in the HX at any time?

Then controls: I would like for the HRV to run on low continuously then when the HX is hot the HRV should be on high. I think I should be able to rig something up with an aquastat and a relay, but any suggestion are welcome.

In the future I may consider an air source solar collector to help supply heat to the crawlspace during the shoulder seasons, but for now I want to get the HRV with HX setup first. Any ideas, suggestions or help would be appreciated.