To Zehnder or Not To Zehnder That is The Question
CohesiveBuild
| Posted in Expert Exchange Q&A on
I’m in the beginning stages of designing a renovation in Ottawa, Canada, which is in the Zone 5a – 4 range. The house is 120+ years old and hasn’t been touched in new 50 years. I’m looking to do a deep energy retrofit on it and am looking for suggestions for a REALLY good ERV. Allergies are an issue in the house so a MERV13 filter is a must. Zehnder was a first suggestion from a Passive House frined of mine, but $10,000 for the unit, with the bits of a bobs seemed expensive. Some other suggestions would be greatly appreciated! Thanks.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
All I can tell you is our Zehnder hrv has been operating for seven years without me paying any attention to it, other than replacing filters twice a year.
The venmar N Series is a good choice. Auto balancing, ECM motors, made in Canada (I think) and you can get one for about 1500$ CAD. If you want more filtration option, you can add a Fantech air cleaner (CM 3000) which works with HEPA filters and cost around 500 $CAD.
Went through this recently and still in the process. Things to consider:
- zehnder is easier to install with their system (comfoflex +/- comfotube) and possibly quieter with the silencers etc
- code battles with building inspectors, so make sure to try and get them onside on the permit early if you do decide zehnder
When I priced out the difference between Zehnder and Vanee, the unit price difference was small (i recall perhaps like 1000 on a base of 5000?), but the biggest price difference is the zehnder system has the silencer, pre-warmer, and tubing.
Hard to do a direct price comparison if factor in cost of rigid ducting and labour if you do an indepedent ducting system for your ventilation. I was told Vanee would just insert into the existing HVAC system, which isn't the type of ventilation I was after.
So in the end I went with zehnder because I wasn't sure the cost differential was going to be actually that high, and for the dedicated run system.
That being said, I'm having some doubts now depending on the severity of building inspector...
How much is comfotube per foot? I love their system but heard some crazy numbers.
$2 usd/foot I think. They come as 50m rolls. Hesitant about them though. Not being UL listed and all ..
Yeah that's a bit expensive. I can buy semi rigid aluminium ducts for less than 1$ CAD/foot here.
It is pretty simple to do a Zehnder like homerun setup with couple of lengths of uninsulated 3" flex plus a log plenum out of a couple of wye fittings. Way less money, works just as well. 3" flex easily fits into a 2x4 wall.
The only issue is the 3" outlets are hard to find, your best bet is HE high velocity ones which look great but are spendy.
One thing that has always bothered me about the Zehnder setup is the pre-heater. These things are 90%+ efficient, this means that most of the heat you are putting into the supply airstream is sent straight back out. Makes no sense to me, there are much more efficient ways to keep the core frost free.
Post heater does make sense in very cold climate for comfort but that won't help with defrost.
Deleted
None of the heat delivered by the preheater is going back outside. Heat is going from the outgoing stream at room temperature into the incoming air, as long as that incoming air temperature is below the interior air temperature, the heat flow is one way. The heat added by the preheater is all coming into the house, 100%.
Let try.
Say the unit is 90% efficient, 585W (2000BTU/h) pre heater, 100CFM airflow, ignoring any humidity or phase change. 70F room 15F outside.
Without a preheater temperatures are:
-house supply (70-15)*0.9+15=64.5F
-outside air exhaust 70-(70-15)*0.9=20F
With pre-heater running
-fresh air supply 15+2000/(100*1.08)=33.5F
-house supply (70-33.5)*0.9+33.5=66.35F
-outside air exhaust 70-(70-33.5)*0.9=37.14F
So we put 2000BTU into the air stream:
-the house got extra 100*1.08*(66.35F-64.5F)=199.8 BTU/h
-outside air exhaust increased 100*1.08(37.5F-20F)=1980 BTU/h
So by pumping in heat before the HRV, a very small amount of extra heat is delivered to the house. By warming up the intake air, the HRV also has lower delta T to work against. So not only transfers less heat from house air but it also exhausts most of the heat we put in.
Unless my math is off, the COP of a pre-heater is 0.1 in this case.
I assume your math is fine, but you're making a couple of false presumptions. First, recovering less heat than it otherwise would have is not the same thing as sending the heat back outside. This may seem like a nitpick, but it matters. The purpose of the pre heater is not to deliver heat to the house, it's to protect the HRV core from freezing. So the scenario of 15F outside air and 20F air is not valid. You can't operate the HRV like that, so you can't compare it to the preheating scenario. And yes, as you said, there are more efficient methods of frost protection, but the increase in efficiency isn't that great and comes with another cost. Recirculating works on basically the same math, the only difference being the heat source isn't electric. You're using heat from inside the house to heat the HRV just like the preheater, but if the house is heated by mini split you're getting that heat back at maybe 2:1 at 15F. However, the penalty is lower effective ventilation. So you have to increase the ventilation rate during those times, because your ventilation requirements didn't go down just because it got cold outside. I guess you don't have to, you could choose to have lower air quality, which is what most people do. Because none of those HRVs are smart enough to automatically up the flow rate, and it also means you have oversize your whole HRV system to account for this. In fact some just shut it off if it gets cold enough. How much sense does that make, you've got a device whose sole purpose is to provide consistent fresh air, and it immediately sacrifices that as soon as the temperature drops?
A more minor thing is that the pre heater doesn't just turn on and go at max whenever the outside temp goes below freezing. It modulates to maintain a safe exhaust air temperature. This means the incoming air only gets heated to close to freezing for the HRV, and quite a bit lower for the ERV.
"sending the heat back outside"
With a passive heat exchanger, whatever heat you put into a stream gets transferred directly to the other stream so the pre-heat is sent right back out. No way around this.
"Recirculating works on basically the same math, the only difference being the heat source isn't electric."
I haven't got all this fully sorted in my head, but I think recirc doesn't use any extra energy. The energy to create the frost on the core in the first place was transferred to the fresh air stream and sent to the house. During defrost, you exactly the same amount of energy that has already been sent to the house is need to remove the ice buildup. Overall there is no energy lost. I'm still not sure what to do with condensate from defrost cycle.
The increased ventilation airflow with a pre-heater is a good thing just not sure if it is worth the operating cost. The slightly oversized recirc unit would not be all that much more expensive plus give higher boost flows for clearing a bathroom.
Currently going through the process of shopping ERV systems as well across the River.
I have a year old quote on a Fantech system that I need to update. It was for an independntlay ducted system with their HRV before they had ERVs with ECM available. It was substantially cheaper than my Zehnder quote. Something like 25% of the price with installation included. It was a decent system.
We're in a retrofit situation and the ease of install might me go for Zehnder anyway since I will probably need to open fewer walls since the comfotube can be fished and I like the controls of the Zehnder better, but it is a serious splurge. It doesn't make economic sense for sure. Their redeeming factor is the design service. Most companies I spoke to didn't understand why I would want an independtly ducted ERV and some wouldn't even bid on the project.
Still, finding an installer for Zehnder was quite the challenge in Gatineau. You do have more options, so it might be easier in Ottawa.
Venmar has some very decent looking ERV, look at their Xseries (gold for vaneEE). Not much less efficient than the Zehnder systems. You can also shop around as some companies sell vanEE and others Venmar, at least on the Gatineau side. They are basically the same units and it might allow you to have a bit of negociating leverage.
If you find someone willing to install a fully ducted Venmar or Vanee system, I think it would be the wisest choice.
I hope this helps. Keep us updated on the status of your project. If I end up installing my system before you purchase yours, I could give you an updates.
Fantech got an HERO ERV? I thought they had only HRVs. Do you have a model number?
Sorry, I looked to quickly on their website. I had a quote for a HERO ECM HRV. I thought I saw a new ERV in their product list, but it was one of their older lines.
I have. Zehnder but agree that it was very expensive. In my case in my small narrow house I had very little extra space for full size duct runs so the Zehnder system was ideal.
The summer bypass mode when it gets cool outside is handy in my climate with very cool summer nights - reducing A/C loads a bit.
If I were designing a house from scratch or had a lot of space to spare for duct runs in a retrofit, I would try to get a different less expensive system, though.