HRV Preheater in a Cold Climate
I am looking for input on running our HRV preheater as efficiently as possible in a cold climate. We are north of 55, have long, cold winters and very expensive electricity rates.
Here are the details:
HRV: Lifebreath RNC-205
House: 1275 sq ft, super insulated and well-sealed
Main heating source: Blaze King wood stove, passive solar
Typical winter temperatures to -40
HRV rep recommended setting the preheater to kick in at 15 C (59 F) to prevent condensation. This seems like overkill to me, especially since the unit is located in an unheated ICF crawlspace that’s normally around 10 degrees.
Wood is plentiful and almost free here. Electrical heat is crazy expensive. With an eye to reducing electrical costs, I’d appreciate your input on:
– What’s the best temperature setting for the preheater?
– What’s the minumum ventilation setting/fan speed we should consider in winter?
– Is there anything else we can do to reduce costs associated with preheating air for ventilation?
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Replies
Easy way to save electricity cost is to post heat the HRV.
Most units have controls to handle low temperature, they just spend more time in defrost mode as it gets colder. As long as the unit is sized to provide adequate ventilation when taking into account defrost cycles, there is nothing wrong with letting the unit go into defrost mode.
The reason you want to post heat is that any heat you put into the cold air intake of HRV, is sent back out (a 70% HRV, sends 70% of the pre-heat energy right out through the exhaust). Pre-heat only makes sense if it is fed by something like a geo loop where the heat is free.
Preheat is mainly for frost protection. H/ERVs handle this differently depending on whether it's HRV or ERV, and the manufacturer. An ideal system would monitor temperatures and humidities, and maybe even airflow rates, to see when the core is at risk of frosting up. I doubt the Lifebreath is that smart, but I don't know for sure. An ERV is less susceptible to frosting compared to an HRV, and with the humidity recovery may give you better overall indoor air quality, but it depends.
The recommendation of preheat <15C is nuts. Might as well not bother with an HRV at that point. Liquid condensation should be no big deal for any H/ERV, if they have a drain. Those that don't (typically only ERVs) apparently are confident enough that there's never any condensation. If a system can't handle liquid condensation I wouldn't use it in a very cold climate.
– What’s the best temperature setting for the preheater?
0C. Nothing above that makes the least bit of sense. As pointed out already, the only reason for pre-heating is to prevent frost build-up. You could probably get away with a little below zero, since the temperature of the core would be averaged between incoming and outgoing. But 0C would be pretty safe.
– What’s the minumum ventilation setting/fan speed we should consider in winter?
Whatever you need to keep below 1000ppm CO2, at worst. 800ppm is a preferable maximum target. The rate doesn't really change depending on season or temperature, except when it's mild enough to leave the windows open; then you could turn it way down or off, depending on your window layout.
– Is there anything else we can do to reduce costs associated with preheating air for ventilation?
Using the most economical heating source; if it's the same source as for heating your house, the effective cost of preheating is zero. If it's electric and you're heating with wood, then the cost is relatively high. You could use a ground source pre-heater, but the conventional wisdom on that is it's not worth it. I would say it depends; if you completely DIY it, it will have a payoff period sometime before you die. If you're buying engineered solutions and paying labour to install, not so much. I do have one of these, so if you are curious about it feel free to ask.
I don't understand where this need to prevent frost buildup even comes from. I run an ERV here and have no issues in 0F weather with frost buildup without any pre-heat.
All modern HRV/ERV are designed to handle cold weather and will defrost as needed. With units that have recirculating defrost cycle (which most units do) about the only thing that happens is that you loose ventilation during that time as it is just recirculating house air through the unit. So you do have to make sure the unit is sized to still bring in the correct amount of fresh air when the unit can spends a lot of time in defrost mode.
Unless you have something silly like an indoor pool or giant indoor garden, you'll never have enough humidity to cause issues with an HRV/ERV.
There is absolutely no need to pre-heat. Even in extreme cold weather frozen cores just don't happen:
http://cchrc.org/media/ERV_ColdClimates.pdf
P.S. ERVs are generally better in cold weather since there is moisture transfer between the fresh air and stale air. This reduces the amount of moisture that makes it through the core and there is less need for defrosting. Most ERVs don't need to defrost at all until the temperature dips bellow 15F.
I'd turn off the heater and then monitor the system.
Thanks, everyone, for your thoughts.
Digging around a bit further on the internet, I found this statement with regard to our unit:
With our recirculation defrost strategy the sensible heat recovery efficiency (SRE) at -25C (defrosting) can be within a few percent of the 0C SRE (no defrost) delivering significant energy savings.
So, seems safe to run without the preheater, at least to -25. Our system is oversized, so there's definitely room for some ventilation loss. Maybe install a Co2 monitor and see if very cold weather impacts air quality negatively?
The recirculation vs preheating strategies is basically a North American vs European thing. North American manufacturers lobbied hard to have the recirculation method virtually enshrined in most of the building codes.
The claim that the efficiency at -25 is virtually the same as at 0 is disingenuous. This is only true if you don't count the energy taken from the interior of the house to effect the defrosting. For the purposes of code compliance, this is how they've managed to define it. Meanwhile, if you get energy from the ground, which is genuinely free (minus the small amount of electricity to run a recirc pump), they do count that as energy used. War is peace, freedom is slavery, and energy is not energy, except when it's not and then it is.
Although the marketing strategy is not right, in most cases it is more cost effective to use house heat for defrost. Even in an all electric house with mini split, the heat you are using comes at a COP of 2 to 3, so significantly less power use than a resistance heater.
In the case of the OP, with essentially free wood heat for house, the operating cost of recirc defrost is zero. Even with my not too expensive hydro rate, a 300W to 600W preheater running 24/7 is a fair bit of money.
This is true, if only looking at a snapshot in time. However a system depending on variable defrost cycles should either base the ventilation rate using the highest level of defrost, in which case the year round efficiency goes down the toilet, or automatically increase the ventilation rate as defrost time increases. That's assuming the goal is actually good ventilation. In practice, neither of those things happens, which means the ventilation is substandard for a significant portion of the year (the portion when it's arguably the most important, when you'd never consider opening the windows). When the singular purpose of the system is ventilation, I don't think ventilation is the thing you should be sacrificing as a compromise.
> automatically increase the ventilation rate as defrost time increases
"fixed average flow rate" does sound like a feature HRVs should do on their own.
From what I can tell, the advice to use a pre-heater in very cold weather may be because the defrost system could potentially shut down all ventilation.
Also, this Canadian homeowner had a fan failure due to frost, even though his system is equiped with a defrost system. Maybe the advice for a preheater in very cold weather is just a belt and suspenders approach?
This is the Canadian homeowner I was referring to:
https://wolfeislandpassivehouse.wordpress.com/2019/02/08/hrv-fan-failure-the-importance-of-pre-heating-in-cold-climate-zones/
Zehnder, being the only European brand currently marketed in North America, is the one exception where preheating is a requirement. You wouldn't run into this particular problem with your Lifebreath.