HRV Control type
I just bought a house and I am considering having an HRV installed, but I would like to understand their control systems before I go any further.
The house is an end unit, 2 storey with a walkout basement, built in 2001 with R20 walls and half decent windows. It has a Lennox 50k BTU high efficiency furnace withs a PSC motor and a Lennox 10 SEER AC. I feel the furnace is oversized but I have not done a heat loss calc to confrm. I assume it is fairly airtight, but since I have a blower door it will have almost no air leakage when I am done with it.
I am a home energy auditor and I have seen close to a thousand HVAC installations before and after, although I rarely, if ever am on site with the HVAC techs, so I don't really get a chance to discuss any of the installs with someone knowledgable. From what I have seen, the HRV can be controlled either by a switch on the unit (high-remote-off), the remote setting obviously sending control to a remote located near the thermostat. There is also a humidity dial on some units, which I assume turns the HRV on when the humidity goes above the setpoint. Here is what I want to know:
Doesnt the furnace fan have to be on for the HRV to be effective? In the shoulder seasons, when the HRV is used most (least amount of air infiltration due to stack and wind effects), the furnace has a very low duty cycle, especially during the day when there is solar heat gain. In this case the HRV might run constantly, but it is not at all effective since the furnace blower is rarely on. Is it possible to wire the HRV to come on only when the furnace is on?
One idea I had to increase the duty cycle of the furnace blower is to have a 40k BTU furnace installed with an ECM motor, thus the duty cycle will be longer due to the lower capacity and the variable motor. In this case I could also leave the blower on continuously since the draw is so little.
Any comments or suggestions?
In my own home, my HRV and furnace fan are capable of being started together by one of four methods.
Timer wall switches in either of two upper bathrooms as well as the ventilation timer and dehumidify setpoints on my IAQ control. The dehumidifying portion of the IAQ can only be effective when the outdoor air is below 45-50F, otherwise it might be running 24/7 trying to remove what it's bringing in. So I keep the dehumidify setpoints set high RH during the late spring, summer and early fall and around 50% during the late fall, winter and early spring. I could hook up the A/C as the dehumidifier and winterize it at the same time, but who wants that utility bill at home? The HRV is quite adequate during the winter.
The two lower bathrooms have direct exhaust fans and do not require extra timers to bring on the HRV.
When I bought this home, it came with a standard Lennox 60,000. I, in my infinite wisdom, soon changed it out to a Payne high-eff two piper. Little did I realize just how much air that old Lennox used in the course of a season... I was soon installing the HRV out of necessity.
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