How to select the right HRV system
This site comes up frequently on my google searches and am hoping to get some insight to my question. I am building a house in the Marine climate, SE Alaska. Not too warm, not too cold, lots of rain. I plan to build a very air tight house using the REMOTE system with 4″ of rigid foam board on the exterior. I will heat with multi-head Mini-split heat pumps. This will require an HRV for bringing in fresh air quality using it’s own dedicated venting. I have used the calculators which come out to 126 CFM on my two level house.
I initially thought that I should order a simple 150 CFM unit and call it good. Put an exhaust port into each of the three bathrooms and fresh air supply into three bedrooms and living room. Sounds simple until I started reading various comments, most from 5-10 years ago.
This HRV stuff is foreign to my small island community so my resources and local experience are non-existent. Any information I can obtain will be helpful in my decision.
Question 1. Low fan speed, high fan speed. If I buy a 150 CFM unit and it runs continuously at low speed it does not sound like I am getting the 126 CFM exchange that is called for. Is this good or do I have to find a unit that runs 126 CFM on low? Or is 64 CFM continuous more realistic?
Question 2. Over size or just the right size. Buying a 150 CFM would be simple. But I read that with a properly sized HRV I should not install individual exhaust only bathroom fans. If I divide all three exhausts to the three bathrooms only that will give about 50 CFM per bathroom on boost mode. Some bathrooms call for 80-100+ CFM. To size for the bathroom I would be looking at a 200 or 250 CFM HRV. What has been determined to be the best route here?
Question 3. If I only need 126 CFM and I install a 200 or 250 CFM unit for bathroom needs will I be over venting and throwing my energy away on a regular continuous basis?
Question 4. With three exhaust ports in three bathrooms, would there be a need to exhaust the kitchen and/or laundry room (thus lowering each of my bathroom CFM)? With the fact I am exchanging this much house air with fresh air at various locations I wouldn’t think it would be necessary.
Question 5. Selecting a unit. Broan 160TE claims 53-177 CFM, Efficiency of 83% and Electrical consumption of 24 watts. This looks like a very nice efficient unit. Looking over at Life breath there are three options but they are difficult to compare exactly with the Broan with given information. Life Breath says they have five fan speeds which might work good for finding a 125/250 split. Then I look at wattage and I see:
155MAX with 66 watts on low and 118 watts on high. 1.4 amp rating
205MAX with 64 watts on low and 96 watts on high. 1.4 amp rating
267MAX with 22 watts on low and 44 watts on high. 4.5 amp rating
What am I missing here? why not just go with the 267MAX with its low watt rating and high exchange option but set to low speed as a norm. What are the downsides to this oversize option?
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Replies
You don't have to have all your bathrooms serviced by the HRV. I find a bathroom typically needs at least 60CFM of exhaust flow, so if your design calls for 120CFM unit, I would duct only to the one of the most used bathrooms and use a standard exhaust fan for the other ones. This leaves you a bit of extra flow that you will need bellow.
You generally want one stale air pickup (65cfm) near your kitchen (as far as possible from your range), cooking generates a lot of particulates that escape the range hood and you want to capture those. It is also a good idea to put a small pickup in the laundry room (10cfm) as dryers tend to produce some very fine dust.
There is nothing wrong with getting an oversized HRV unit and you should always look for one with an ECM blower (the large Lifebreath is a good unit).
An oversized HRV allows you to run on boost when you need to quickly air out the house for things like wet dog smell or deep frying fish but run at lower speed thus higher efficiency for regular ventilation. The larger units tend to be pricier and need larger ducting, so it is a bit of a balancing act.
Be careful when comparing efficiency as manufactures make it a bit confusing. The apparent sensible efficiency of the Broan includes the bit of heating from the blower in the calculations, but its real efficiency is between 70% to 75%.
A good rule of thumb is you want a unit with hexagonal counter flow core as these tend to be 10% to 15% more efficient than the square cross flow cores.
P.S. The 120CFM design flow is more than enough for a 3 bedroom house, you could probably even run at lower flows. Get a CO2 sensor once the unit is set up to check.
Thanks for the reply. Great point about not all bathrooms have to be the same setup. My bathrooms wont need anything special, just more than 50 CFM I would guess. If I could get 70-80 CFM that would probably do the job.
You can set the volume of pickup at different points such as 65 CFM for the kitchen and 10 CFM for the laundry? I just looked over the install manual for the 267MAX and they list a "fully adjustable grille". Answers my question, interesting feature and something to factor.
It looks like I don't just branch off and run 4" duct everywhere. Is there a good duct sizing site/tool I can reference for my planing?
The 267 is a few hundred dollars more to start (actually cheaper if I eliminate bath fans) but appears to be more efficient than the 155/205. Both show 6" duct connections to the units. If I can use fan speed 1 or 2 continuous I will achieve my base flow. Speed 5 should vent bathrooms well.
What is the average static pressure (w.g.)? The references heavily use .2 and .4 but some go up to 1.0. For the 267MAX the CFM on speed 1 is 100CFM at .2 wg and drops to almost zero CFM at .4 wg. fan speed 2 drops to almost zero at .7 wg. Yet on the 155MAX the CFM holds up better on the lower fan speeds than the 267. This just gets more confusing.
One disadvantage I see with Life Breath over Broan is the filters. LifeBreath has a washable filter with no specs. Broan has replaceable filters capable of up to MERC 6/9 or HEPA.
Generally with HRVs you set up a trunk and branch duct configuration. If you size the ducts correctly your airflow will be in the ballpark. To get better adjustment, install a damper at the takeoff which you can use to fine tune the flow.
Generally speaking, you would run 3" to bedrooms and living spaces, 4" to a bath 5" to the kitchen. Using flex ducting made for high velocity AC systems is an easy way to snake supply ducts to each room as they fit inside a 2x4 wall. The only ducts that need to be insulated (provided all your ducts are inside the conditions space, which they should be) are the ones going outside.
For sizing ducts always use the flow and pressure capability of the unit on boost. Ducts that work there should work on low speed. Watch duct velocity anything above 900fpm will make noise and be fairly restrictive. I try to keep loss to less than 0.6" on boost. This will give you pretty low loss at ventilation flow rate which reduces the blower power.
With the large unit, you might have to up-size the main trunk to 7". Even though the unit has 6" ports, running long length of 6" trunk with a couple of bends or wall caps will add a lot of friction loss 1300fpm.
If you want better filtering, the better solution is an in-line filter box, these last much longer and generally filters are much cheaper than the specialty ones for the HRV. I run a Merv13 box on my ERV at home and does a pretty good job of keeping pollen out. If you do run one, make sure to take it into account for your pressure loss calculations.