HRV preheating with hydronic coil.
I am a Building inspector in Yellowknife NWT – -52 celcius design temp. I recently had an email discussion with a mechanical contractor who puts heating coils on the cold air inlet of the HRVS he installs, mostly venmar constructo’s. it went something like this.
ME. Heating coils on the outside air intake to the HRV. This destroys the efficiency of the HRV, as the temperature differential of the intake and outlet air is very close, so there is less potential to remove heat from the outlet air.
If customers want to temper the air coming out of the supply, this can be done after the HRV.
HIM: Our reasoning behind having the coil before the unit is from past engineered jobs that we have done. I just spoke with — —-from —— Engineering about the work in question. It is in our experience that when there is no coil, and its -30 (or below), the HRV can not make up the differential which in turn ends up blowing cold air into the residence. In turn the boiler (or
furnace) is on more to heat this cold air mixed through out the house. Or, the home owner, turns the unit off all together (most often the case) eliminating the air exchange all together.
When asking Jon about installing the coil on the supply side he said in the cold weather the core is going to freeze up. His thoughts are the unit would be in defrost (recirculating air not exchanging it) mode more than actual true HRV mode.
To sum this whole thing up, he says there is no easy answer, but in his opinion having the coil on the supply air entering the unit is his preferred method of piping an HRV in the cold weather climate. I hope this answers why we have done it this way. What is your opinion?
ME: I see where you and the engineer are coming from with the HRV preheater. And I agree that there is no easy answer. However I firmly believe that the intent of HRVs, to scavenge energy from the outgoing air to the fresh air, means the heater should not go before the HRV. Yes the HRV will be in defrost more often, but they are designed to do that, some better than others. Otherwise, we might as well go to using balanced inlet outlet fans. The buildings energuide rating relies heavily on the use of an HRV. The point about people turning off the units and then having no ventilation, that is very true, and so I applaud your use of coil heaters to temper the air, which are optional in the building code. I just believe they should go after the HRV.
What is the general concensus on this issue? Here in Yellowknife all new construction must meet Energuide 80.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Luke,
Your questions are excellent, and your analysis is logical. You are building in an extreme climate, where temperatures are often too low for efficient HRV operation.
Have you talked to any technical reps at Venmar? They probably have an opinion on these issues.
Luke,
Here are two sources of information on ventilation in very cold climates:
A Prototype Alternative Ventilation System for Retrofit, Rehab and Renovation of Rural Alaska Houses
An HRV System Overview
One interesting fact here in yellowknife is that HRV's probably go into recirc much more often then necessary, due to the temp probes, but not taking into account the very low humidity here. I almost never see water in the drain tube, and allow a bucket to be used as a drain.
Luke,
In the Passivhaus world it is common to utilize earth to air heat exchangers (not earth tubes) to defrost the incoming HRV air. In Europe they are called brine-to-air heat exchangers, we call it a closed loop glycol-to-air ground heat exchanger. The only energy penalty is a few watts to run the pump. The system is "passive" in the sense that no heat pump is utilized. The goal is to just increase the temperature enough to keep the core from freezing. In the summer there is the added benefit of offering some cooling and dehumidification (HRV w/ bypass capabilities recommended).
The Zehnder ComfoFond-L is an off the shelf closed loop earth-to-air heat exchanger that is readily available in the US through Zehnder America. They will help size the loop system to match the climate and ventilation load.
http://www.zehnderamerica.com/products/comfofond.asp
Thorsten Chlupp is a builder in Fairbanks that has used this technology with great success on his projects:
http://www.reina-llc.com/resources/videos/
Here are some more resources on the topic:
http://www.sole-ewt.de/solepumpen-regler/?lang=en
http://www.passivehouse.us/bulletinBoard/viewtopic.php?f=6&p=917
http://passivehouse.greenhaus.org/laying-the-pre-tempering-ground-loop-of-pex-t
http://www.aecb.net/PDFs/Passivhaus_JPW_2.pdf
A less expensive alternative is to go with the Warmflo defrost system offered through Ultimate Air.
From their website: "UltimateAir has provided a pre-heat - pulse modulated solution that allows for no decrease in ventilation. This is the most efficient solution which ensures the fresh air required for the health of the occupants in the home. This is one of many other methods that best keep the incoming air at or above 12 F."
http://www.ultimateair.com/blog/bid/82344/Mechanical-Ventilation-and-Defrost
http://www.ultimateair.com/products/accessories/warmflo-defrost-system/
http://www.ultimateair.com/products/accessories/warmflo-defrost-system/
Skylar,
Thanks for posting the useful information and links.