How to pre-heat ERV cold air?
I am installing a Panasonic FV-VEC1 energy recovery ventilator. I am in Zone 5 and if this year will be anything like last year, I am concerned about temps in the single digits for a multiple days, even weeks. The ERV is the cold climate version, which in my understanding, once temps below a certain temperature are detected, the unit shuts down for a while, then recirculates air to defrost, and then “tests” the air temp again.
From the manual:
“When the outdoor temperature is < -22°F (-30°C), the defrost cycle is initiated and the product will stop operating for 47 minutes, then operate in heat exchange mode for 4 minutes in order to sample the outdoor temperature. It then operates in circulation mode for 9 minutes in order to defrost (exhaust and supply air are 50 CFM). Defrost cycle overrides the ASHRAE 62.2 timing function.”
The above being said, when you have a week of single digit temps, the unit stops working. To ensure continued clean air during long stretches of frigid temperatures, if I understand this correctly, the strategy employed by Zehnder and others, is to install an air heater on the intake duct of the ERV that draws air from outside. This thermostat activated heating element insures the temp of the air in the duct heading for the ERV is at least above -22°F, to keep the ERV from going into shut-down/recirculation mode.
Can anyone suggest a manufacturer or device to preheat the incoming air? I found duct air heaters but they start at around 10 kilowatts, something like 34,000 BTU and run on 220V lines. Seems excessive to warm air above -22°F. Am I wrong? Any suggestions on the design and device to use to warm the incoming air (avoid recirc/shut-down mode?)
Thanks for any constructive input!
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Replies
Zehnder sells an inline heater. I believe it's made by Stelpro, however I don't see one on their website. It will probably be expensive from Zehnder (probably will be for ducts bigger than what you have as well). I think you're probably going to end up building something custom, then plug it into a controller like this:
https://www.amazon.com/Inkbird-Max-1200W-Temperature-Controller-Greenhouse/dp/B01HXM5UAC/ref=sr_1_7?ie=UTF8&qid=1536693427&sr=8-7&keywords=inkbird
Note that if you can find a 5kW, 240V heater you can plug it into 120V and it will be about 1.2kW.
Also found this on ebay, might be what you're looking for.
https://www.ebay.com/itm/Tutco-Duct-Heater-With-thermostat-and-boost-fan/263901708306?hash=item3d71c48012:g:0MoAAOSwo8pbMmAS
Thermolec is one manufacturer of small electric duct heaters. They start at 1 kw. https://www.eccosupply.ca/pdf/price-lists/2.%20Heating%20Equipment/Thermolec%20Electric%20Duct%20Heaters.pdf
Try searching google for "electric duct heater" and you should get lots of results.
Climate zone 5 shouldn't be seeing sustained temperatures in the - 22°F range. I'm in Halifax Nova Scotia, also climate zone 5, and we might see temperatures in the -5°F range for a few days to weeks per year, but -20s's is rare.
Single digits are warmer than -22F, so why would the unit stop working?
It wouldn't stop working, but air volume is limited between -22F and 14F. At 5F, the duty cycle is 80%. If your design airflow is over 80CFM, that's a problem.
Even if your design is less than 80CFM, it's still kind of a problem. Let's say your design calls for 80CFM. In order to be sure you will always get 80CFM, you would need to actually set the unit to supply 100CFM. That means sometimes you're getting the correct ventilation (when it's below 5F), and the rest of the time you're getting too much (and wasting energy). I guess you could try to manually change the settings as the temperature changes, but honestly most people are not going want to do that.
I expect that occasional periods of ventilation at 80% of normal levels is a non-issue. But 0% would be.
There is a lot that could be done to optimize HRV energy use - for example, less CFM when it's windy. Or more CFM when it's mild outside and less when it's very cold (adjusted to create the same net annual health effect).
I agree that ~1kW is about right for this application.
It depends on how occasional, and how long the periods are. It's not unusual for the temperature in zone 5 to be below 5F for several consecutive days. If your house is at 80% of its design efficiency for a week straight, are you going to suffer calamitous effects? No, probably not. However, it's still not something I would knowingly allow for when planning my ventilation.
I'm not sure you can average the ventilation over a long period and expect it will work out the same. I suspect not. For one, there is no benefit to getting more than you need. So the averaging only works if it occurs within a time frame shorter than it takes for the air quality in the house to change significantly. In my experience, that can be less than a day. Again, is it going to be a catastrophe if you occasionally have low air quality? No, but neither is it a catastrophe for your house to be cold enough that you have to put on a coat to be comfortable. And yet, no one would accept that as a non-issue, they'd be calling for the builder's or hvac designer's head. I would apply the same standard to air quality.
Thanks for the comments.
The Thermolec units serve the purpose nicely.
- Sal