Backdraft dampers necessary?

wklopf

I'm considering installing a second water to air heat exchanger in my HVAC system. What I am thinking about would be an exchanger getting heat from a solar warmed system. The current source is a ground-source heat pump. What I'm thinking is to cut into the return plenum above the heat pump unit, add a filter, then the exchanger, an inline fan with capacity equal to the current system and discharge into the supply side ducting. I'm planning on having a shutter before the filter, but would I also need a back draft damper on the at the supply side of this system? Thanks..

wklopf

I am thinking that I will have tow parallel paths which the air could follow. When the heat pump would be supplying the heat I wouldn't want an open channel between the supply and return, so I thought hat I'd at least need a shutter to keep the warm air from going from the supply side through the new path directly into the return. My thinking is that when the heat pump is working and the new system is off, if I have an automatic shutter at the inlet (return air plenum) side, the slight vacuum on the return side should keep it closed. If there is also positive pressure back through the new path, that should hold the shutter cloesd. If I am right here, I would not require a back draft damper on the new system where it enters the supply plenum. Am I right here? Thanks.

wklopf

Thanks for the input. What I am thinking is that there is currently only one path for air to get from the return plenum to the supply plenum, which is through the existing heat pump heat exchanfer and fan system. Let's call that path A. Also, there is only one path for heated air to get from the supply plenum back to the return plenum, and that is through the house, back to the return registers. Now, if I introduce an alternative path by cutting into the return plenum and installing an alternative heat exchanger and fan system, system b, and putting that warmed air into the existing supply plenum, then there become alternative paths available. As I envision it, if system A is running, with no dampers or shutters, heated air could go the current route of out the supply registers and return via the return registers, or it could take the much shorter path of going in the reverse direction from the supply plenum, back through the non-running fan of system B and into the return plenum. Likewise, if system B is running, the heated air could pass in the reverse direction through the non-running system A. So, it seems to me that I need to block reverse flow for both systems. Is there something I'm missing here or am I over thinking this?

EdTheHeaterMan

Do you actually need another fan?

wklopf

Thanks much! That would be much simpler. I would have to figure out how to get the current system fan to run without engaging the first stage compressor, until the solar heat storage is depleted. Could it be as simple as having a temperature switch which keeps power shut off to the compressors until the solar storage drops below some set temperature?