Thermodynamics of ERV/HRV Preheat
I’m interested in improving cold weather ventilation in our 100 year old home, with extensive energy efficiency retrofitting. As the building envelope has tightened up, air quality (monitored primarily with CO2 monitoring) in the depths of winter has been an issue with CO2 levels rising as high as 1500 ppm in upper floor bedrooms.
About six years ago, I installed a Panasonic FV-04VE1 (basically a spot ERV) with good(ish) results (we run it at 40 CFM, 24/7). A few data points:
1. We are in Northern Ontario, with winter temps dipping below -35C.
2. The FV-04VE1 is not breaking any efficiency or ventilation records as temp drops. I’ve got a new ERV core on order as I’ve damaged the original by washing it with water (my bad) so the current 50-60% efficiency numbers are from a damaged core. Cold weather notes on this spot ERV from Panasonic:
WhisperComfort incorporates built-in frost prevention technology to avoid core freeze up. • Above 32ºF both exhaust and supply are fully functional • From 32ºF to 20ºF the mechanical damper on the supply air closes for 30 minutes, then opens 1 hour at the set high/low mode. The cycle repeats itself to avoid core freeze up. (Exhaust only 30 minutes per 90-minute cycle) • Below 20ºF the mechanical damper on the supply air remains closed for 1 hour and opens 10 minutes at low mode. After 10 minutes the cycle will repeat or change if the temperature has risen above 20ºF. (Exhaust only 60 minutes per 80-minute cycle)
3. I’ve set up temperature logging on the supply/exhaust (via some remarkably affordable home automation bits) to see just how things map out. Temps right now are not very cold at night. This is the live data page of the ERV feeds:
https://www.hundredgraphs.com/public/denwood
ERV sensible efficiency is recalculated and displayed whenever any of the supply/exhaust temps change.
My question is regarding preheating the air to the ERV and the thermodynamics of doing this. To me, as long as you raise the outside fresh air intake to temps well below the interior supply air, you are only raising the ~ 10F delta (from my data so far anyway), so the net energy difference of say 500 watts preheat, or 500 watts post heat would make no difference in the temp of fresh air entering the home…again as long as the delta between inside stale exhaust air and outside fresh intake air remains large.
Remember, the goal would be raising intake air temps to increase ventilation duty cycle times. Let’s say taking -30C air to -7C (20F) air. Yes, using a $9000 HRV with ground loop preheat would be awesome, but that is impossible in the home currently as tying in to the existing forced air system would be extremely difficult.
This is a snapshot from today’s data used in the efficiency calculations (link above for live data). So, for preheat before the ERV core or post heat…is there actually any difference in net energy used to achieve a given return (fresh) air temp? You can also click the magnifying glass on the graphs in the link above to get a closer look at the data.
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Replies
The spot ERV is not the best for discussing pre-heat VS post heat. If you look at the data sheet, at 40CFM exhaust it runs unbalanced as the supply is only 30CFM. Combine with the low efficiency of the core, most of the heat you add to the intake will be delivered to the house.
In your case, since you are looking to keep the unit from going into defrost too much when cold, a pre-heat is not a bad idea. This will be less of an energy penalty than running the unit in exhaust only mode for extended period to defrost the core. A bit of warmer fresh air supply as a result is also a bonus.
Akos, I appreciate your reply. I've been doing further research and more or less decided that solar won't do much for preheat and ground loop is out of the question. This leaves electric as the only option, but with a high operating cost. The NRC has done some research on novel ventilation concepts for cold climates and tested the RGSP 300 which I've seen zero discussion around so far : https://riada.ca/wp-content/uploads/2020/12/RGSP.pdf
This is a brief of the testing:
https://nrc.canada.ca/index.php/en/certifications-evaluations-standards/codes-canada/construction-innovation/canadian-centre-housing-technology-evaluation-dual-core-heatenergy-recovery-ventilation-systems
"Technology 1, dual core ERV RGSP 300
The CCHT testing of Technology 1 showed much higher apparent sensible and total efficiencies, respectively over 10 and 5 percentage points higher than the conventional single core ERV. Additionally, the Technology 1 operated continuously in the winter test period – no "defrost cycles" were required. Due to its damper cycling design between two heat exchangers, the dual core technology shows very high potential for continuous delivery of outdoor air in extremely cold climates. Technology 1 with much higher heat transfer effectiveness provided a supply air at a temperature up to 6°C higher than the air temperature supplied by the single core ERV, leading to a significant decrease in whole-house energy consumption (heating and ventilation) compared to the conventional single core ERV (6.2% saving on average)."
This is essentially a ventilator taking the twin LUNOS concept up a few notches with aluminum heat storage/exhange modules and a very interesting rotating damper to alternate between intake/exhaust on alternating modules. It looks like they are packaging it into a floor standing unit. It has me very intrigued.
There are certainly better option than the spot ERV but they cost more. Plus if you can't buy it, it doesn't matter how nice the technology is. I would like to have some better options for ERVs in cold climate. Geo pre-heat loops are simply too expensive and resistance pre-heaters are a power hog.
In your case, if you are only looking to pre-heat say 0F air to 20F at 30 CFM, you are only looking at 20F*1.08*30CFM=650BTU(190W). Assuming 60% efficiency, 40% of that heat will be deliverd to the house so about 76W. Using my hydro costs, that is about $0.7 per day, so not a huge cost but not free either.
Assuming without the pre-heat the unit runs in defrost most of the time, the heat would have to be supplied by your furnace. So in this case you are looking at a higher delta T so:
(70F-0F)*1.08*30CFM=2268BTU. So about 3.5x of what the pre-heat requires. In this case, pre-heat makes a lot of sense plus some of that pre-heat is actually delivered to the house in the form of warmer fresh air supply.
Akos, thank you...your analysis of preheat vs furnace makes sense :-) I figured about 1800 BTU to take -35F air to 20F or so at 30 CFM. A 500 watt preheater should do it. 12kWh here is about $1.50 /day accounting for time of use billing. That would be max cost, assuming our lowest night temps.
With respect to tech on the ERV/HRV side, here's more info on Tempeff's dual core setup. NRC has tested it with zero defrost required for Arctic applications. I had a great chat with their national sales manager yesterday:
This is the Tempeff system they tested:
https://www.tempeff.com/products/#rgspk
It's not cheap, but would come in 30% less than a full ground loop system. The system is basically two 48" long heat storage/release units that operate in push/pull mode alternatively. Their cores are aluminium plate, made in-house. The smaller units are tested at 92% sensible, 70% latent, at 0C and 100 CFM. No defrost strategy is required, even in "Artic" conditions so the unit can operate continuously, with 0 recirc, 24/7.
The center flap unit reverses flow in the heat storage pods (like a big, self contained LUNOS) every 60 seconds or so. The setup makes free cooling mode pretty straightforward too. Based on my conversation with their sales manager, the systems saw a lot of uptake for schools (Covid upgrades) as they could isolate 2-3 classrooms with respect to 100% air exchange, without upgrading heating systems.