Duct heater required with an HRV in a cold climate??
I'm in the beginning stages of building a super-insulated house near Ottawa, Canada. The foundation is completed and we're about to start framing. One of the last details / sub-contractsI need to tie up is the HRV system. One of the firms providing a quote insists a 1500-2000w duct heater is required.
I recognize that the fresh air is likely to be somewhat cooler than the temperature of the house. However, this suggestions a little over-the-top given that the house will have R50 walls, R80 attic, tripled-glazed fibreglass windows and will be tight at less than 1.0 ACH50.
I'd appreciate suggestions.
Thanks,
John Scime
Lanark County, Ontario
Asked by John Scime
Posted Mon, 01/16/2012 - 20:17
Posted Mon, 01/16/2012 - 20:17
Tags: Mechanicals
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I suppose the question is how cold the incoming air could be. Take an anticipated delta-T, multiply by the heat recovery efficiency, and estimate the incoming air temperature. For example if the indoor temp is 68° and the outdoor is 18° then the delta-T is 50°. If the HRV is 76% efficient, you keep 38° of that delta-T, so incoming air would be around 56° (that is, assuming my math is not way off here, and it wouldn't be the first time.) Is that too chilly?
Posted Tue, 01/17/2012 - 02:23
John,
In your climate, an HRV or an ERV needs controls that handle low outdoor temperatures to prevent the heat-exchanger core from icing up and to maintain comfort. Every HRV or ERV that I know of includes such controls. Usually, manufacturers include a temperature sensor that will shut down the HRV periodically in cold weather to allow the core to warm up; some manufacturers circulate interior air through the core periodically to keep the core warm. These controls work, and no additional electric resistance heater is needed.
In general, the need for mechanical ventilation decreases slightly in very cold weather, because the stack effect increases. Homes generally leak more air in very cold weather than in mild weather.
So, don't install the heater -- unless there is an unusual technical reason for continuous mechanical ventilation in your home.
Posted Tue, 01/17/2012 - 09:31
Thank you very much for the information. I will not install the duct heater. The unit is a Venmar EKO 1.5 ECM HRV, so it has all the controls I will need, as well as good efficiency.
Cheers,
John
http://yearofthehouse.wordpress.com/
Posted Tue, 01/17/2012 - 14:22