HRV v. ERV v. fan
I need an extra 57CFM / 24 hour continuous ventilation in my 1965 New Hampshire (Zone 6A) 1200 sq ft 2-level house, according to my blower door test. The energy auditor recommended the Panasonic Whispergreen. I asked if an HRV (or ERV) would save money & do the job instead, including venting 2 bathrooms which are looking a bit moldy (currently 70CFM each – if no continuous venting were required I’m advised I’d need at least 100CFM). The auditor didnt know the answer.
Aside from the HRV-ERV question (which, having read the excellent blogs, I’m still confused about), I’m wondering if there are really significant energy savings to be made by using an HRV (or ERV) instead of a Whispergreen fan running continuously for most of the day . Sometimes you can get so bogged down with information you just end up doing the simplest thing out of frustration……should I try to get to the bottom of this? The plumber recommends a Renewair 130 ERV, and when I asked if he meant HRV he agreed with me and then sent me specs on the ERV. But if we use an ERV for our moldy bathrooms won’t we make things worse in there by conserving moisture? Also, he is proposing a duct connecting the 2 bathrooms via the attic along our 20 foot hallway, expelling the new air into the far end of the hallway. Won’t it just cycle straight back to the fan? We have electric baseboard heat so no ducts anywhere.
So the main question is, how much money does an HRV save over a fan? Should I just get the fan & forget about the HRV/ERV dilemma?
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Replies
Jude,
An ERV or an HRV will have the lowest operating cost, but they are more expensive to purchase and install than an exhaust-only system.
An ERV or an HRV, if properly installed, will also give you better fresh air distribution in all rooms than an exhaust system.
The decision depends on the layout of your house and your expectations. The payback period for the energy savings for a $2,000 HRV will probably be long -- faster in cold climates than in hot climates, though, and faster in regions with high energy costs than in regions with low energy costs.
For a simple house, most people are satisfied with an exhaust-only system. But if you want a Cadillac system, get an HRV or ERV.
Your existing bath fans for me anyway would work if you added timer switches that you can buy at Lowe's. I buy the electronic ones and set them to run for 30 minutes per shower use. If you want to go a bit more money, then there are switches that can sense humidity and the Panasonic fans are wonderful. I use Panasonic's now and wire them so they come on whenever the bathroom is used.
There is a thread here with all the info you could ever read somewhere.
I did a quick search typing in "Panasonic" Too many hits! Here is a good one on switches... titled "Bath Fan Controls."
https://www.greenbuildingadvisor.com/community/forum/mechanicals/17777/bath-fan-controls
Jude,
Let's back up a bit. Since your "auditor" seemed to have been poorly informed about ventilation options, why are you accepting the validity of their judgement that you need precisely 57 cfm of additional ventilation?
What were the results of the blower door test in ACH50 and CFM50? Where were existing air leaks found? Do the two existing bath fans vent outdoors and are they good quality quiet fans? Were the fans tested for flow rate and, if so, how much air are they actually moving?
Panasonic fans are the best on the market, but you may be able to get away with simply adding programmable timers to your existing fans so they run longer and more often.
More information please.
Just as an example I modeled a superinsulated ~1870 sq/ft colonial located in PHPP. I ran the model in Buffalo NY which has about 6800HDD, which should be roughly similar to NH. According to PHPP this 3BR, 2.5BA house has an average ventilation air flow rate of 64 cfm (close to 57cfm) . When this house is spec'd with an ultimate air recouperator (~84% heat recovery efficiency) it has an annual heat demand of 10.7k/btu/sf/yr or roughly 5862kw per year. When I changed the ERV to an exhaust only system with 0% heat recovery efficiency I got an annual heat demand of 15.83k/btu/sf/yr or around 8656kw per year. Thats a difference of 2794kw per year with just changing to an ERV. Like Martin said one of the biggest parts of the equation is how much you pay for energy. If you figure oil is about $.09kw (~$3.20/gallon, 95% efficient furnace) you'll save $251 a year by using an ERV in this example. If you pay less per KW,live in a milder climate, or have a smaller house (less ventilation cfm) the annual benefit wont be as good. If you live in a colder climate, larger house (more ventilation cfm), or pay more per KW, and like the idea that your fresh air is pre filtered before entering the house, then go with the ERV.
You could also consider a supply ventilation strategy and install a fresh air duct and tie it into your return air trunk line using a fan cycler. I installed one by Aprilaire when we had a new furnace installed. Not as good as an ERV or HRV, but I like how the air intake is controlled and then conditioned vs. the exhaust strategy.
"We have electric baseboard heat so no ducts anywhere.
I missed the electric baseboard as well... In that case I would go with a simple extract fan and invest money into low temp mini splits and possibly more insulation.
Our current 70CFM fans are Panasonic, very quiet, and run on 30 minute timers. On average I'd say the 2 bathroom fans are running about 2 hours a day (bigger b/r) and 1 hour a day (smaller b/r). The fans were not assessed during the blower door test. The venting needs to be revised because the ducts currently just lie under the eves, spewing moist air onto the timbers which are starting to go black...(!)
Blower door test/audit results: CFM50=1200; ACH50=4.2; ACHn=0.28.
Leakage: The house is insulated "better than average: HHI between 4 and 8"). There were some areas of leakage recommended for sealing: we already took care of the fireplace leaks when we installed a new pellet stove insert (external air supply). Also leakages up through the ceiling to the attic space (not fixed yet). Leakage was 1197 CFM @50Pa = 0.16ACH. Another blower test is recommended after we fix the leaks, so I expect we will need more than 57CFM of continuous venting after that.
It's pretty cold here in NH in the winter. Also, the ERV guy says he usually programs it to run for a proportion of every hour. I suggested could we run it once , continuously in the day when external temp is highest, and we are out at work so on/off noise wouldn't be a problem at night.
Mini splits? I'll have to look that one up, havent heard of that.....
Does $2000 for an HRV just refer to the cost of the unit?
Jude,
If you decide to go with an exhaust-only ventilation system, your existing bath fans can be used, although their exhaust ductwork may have to be re-routed.
You may be able to get an HRV installed for $2,000 -- or maybe not. It could easily cost more to install a fully ducted system in an existing house.
If your ventilation system runs intermittently instead of continuously, that fact lengthens your payback period for an HRV.
A ductless minisplit unit is a type of heating and cooling system. It has nothing to do with ventilation.
Thanks for clarifying the minisplit.
Is there a cut off point below which exhaust-only fans shouldn't be used because they can't suck enough air into the tight house? ( Or maybe I mean a level of leakiness/tightness at which air inlets have to be put in the walls to avoid that negative pressure?)
The plumber proposing the ERV suggested one duct opening the fresh air into the hallway about 15 or 20 ft from the bathrooms. That wouldn't do much for downstairs air quality, presumably. Is one duct/opening enough? Could/should it be routed to downstairs?
This forum is a real eye opener, thanks. No idea what to choose yet but at least I'm getting an idea of the issues!
Jude,
There is at least one vocal proponent of passive fresh-air vents (holes in the wall) who posts regularly on this forum, but there is plenty of evidence that they are almost always unnecessary and often counterproductive (because they often act as air outlets, not air inlets).
In the vast majority of houses, ordinary cracks in your building envelope easily supply 50 or 60 cfm makeup air when your exhaust fans are in operation.
However, the fresh air will not be distributed equally to all the rooms of your house. For that, you need an HRV or ERV with dedicated ventilation ductwork.
If you haven't read it yet, you should probably check out Designing a Good Ventilation System.
Jude, My last home has Panasonic bath fans installed in all bathrooms and they come on with the light switch every time the bathroom is used. To me that is a lot of venting for an older home.
You mentioned moisture related issues in your bathrooms. That doesn't make sense to me with how much you say the fans are running. I would think if you have timer switches and use fans every time the bathrooms are occupied that you would be all set. Meaning no more has to be done.
And if you do put in an HRV since you may have been convinced you need one (needlessly I might add) install an HRV not an ERV. ERVs generally are more complicated and they are made to transfer moisture. You want to get rid of moisture. HRVs get rid of moisture (in cold climate, closed up house, conditions.)
But still, setting up your existing fans to run more is a good solution if you are having moisture issues.
Why do you think the air quality is poor? Because you paid someone and they said so? Get your money back. Crack a window, breath in that nice air, take a walk with the pup, get air and exercise. So many ways to freshen up your life. Every time you pop outside you are letting fresh air in your house and not even having to buy a $5,000 HRV which is what a good HRV costs installed up my way.
Think about all this... Enjoy your time hear and good luck with what you choose to do.
Jude,
I'm that "vocal proponent of passive fresh-air vents", as I've been designing and building affordable superinsulated homes for 30 years and have had very good results with simple, low-cost exhaust-only ventilation systems in cold climates (up to 8500 HDD) using 24-hour programmable timers to run Panasonic bath fans along with Airlet 100 passive make-up air inlets.
I disagree with Martin that "there is plenty of evidence that they are almost always unnecessary and often counterproductive." Martin has offered a single study of poorly-built homes that constitutes this purported "evidence".
However, I will agree with him that, unless your house is exceptionally tight (< 3ACH50), there may be enough cracks to allow sufficient make-up air. When a house is under negative pressure, there is not likely to be any exfiltration of moist air, which is what causes condensation and moisture problems. And the fact that air inlets in a 2-storey house can function as a minimal passive ventilation system when the fan is not running or the power is out (lower vents as inlets and upper vents as outlets) is an asset more than a liability. In fact, a "balanced" HRV system does change the house pressure balance and so does not reduce the natural stack effect leakage and can still allow dangerous exfiltration.
I will also disagree with Martin that it is only with an HRV/ERV that it's possible to get well-distributed ventilation. In fact, the system that was proposed for your house - with one inlet in a hallway - is completely inadequate for whole-house ventilation and distribution. You would need extensive ductwork throughout your house, which will be very expensive. By relying either on existing envelope leaks or strategically-located passive air inlets, you will have much better distribution of fresh air than with a minimal or inadequate ducted HRV/ERV.
Since you already have Panasonic exhaust fans, I would leave the short-term timers in place and wire in parallel at least one programmable 24-hr timer, such as the Grasslin KM2 ST, and experiment with various run times. I would also have the existing fans tested for actual flow rate so you'll have a basis for determining run time programming. And move those exhaust ducts so the humid air is not venting into the eaves. It's best, if possible, to vent down at least 3' before exiting the building as this prevents backdrafting and allows condensate to drain outward.
Jude,
Your numbers aren't making sense. If your house is only 1200 SF, then with 8' ceilings it would have a volume of 9600 CF, so 1 ACH would be 9600 CF/Hr or 160 CFM and 4.2 ACH50 would be 672 CFM50, not the reported 1200 CFM50.
Is your house 1200SF on each floor? And what's the average ceiling height?
The house is about 2150sf with 8 ft ceilings. The report gives me the following:
Sq ft=2150; CFM50=1200; CFM/ft2=0.56; ACH50=4.2; leakage sq inches=120; leakage area sq ft=0.83. Do you think the 1200 be a typo/brain spasm? There is also a bit in the report overview that says "Air leakage tested at 1200CFM @-50Pa".
From these answers nobody seems very excited about the cost & cost effectiveness of an HRV/ERV in my particular circumstances. Our current fans are FV-07VQ2 - Panasonic Whisper ceiling - are they good to run continuously? (The auditor recommended the Panasonic WhisperGreen for continuous use). How do these sound as solutions:
1. Run the existing fan FV-07VQ2 continuously for x hours a day till they fail.
or
2. in the larger bathroom which probably needed a 100CFM fan in the first place, I could replace the 70CFM fan with a Panasonic Whispergreen 130CFM, program it to run continuously at some low level (40-50CFM) and boost to 130 during showers for 30-60 minutes. Hopefully lowering the general house humidity via this extra venting means I could continue with the existing 70CFM fan in the smaller bathroom but maybe get a new timer to increase venting from 30 minutes to 45 (or 60) during bathroom use.
And what do you think about the idea of running not 50 CFM continuously, but 100 CFM for half the time during the day when incoming air will be warmer? ie not at night.
(FV-13VKS2 WhisperGreen 130 CFM Premium Ceiling Mounted Continuous and Spot Ventilation Fan)
Jude, great ideas... save your money as I think you only have a moisture issue not an air quality problem with the existing set up.
One thing I have learned of late from reading zillions of threads, is a radon removal system is also fantastic at both increasing air quality and... get this, drum roll, moisture reduction!!!
I never have been a fan of mechanical systems, but now will install Radon systems forever more for the double benefit of moisture, and radon reduction.... Triple benefit... air quality enhancement... Quad benefit... maybe even could be made to take care of upcoming mandatory air exchange issues.
Jude,
All the Panasonic fans can run continuously.
You could either add a programmable timer to one of the existing fans (or upgrade to a 110 CFM Whisper Ceiling or Whisper Lite for better short-term moisture removal), or replace one with the Whisper Green and operate at low speed 24/7 with the intermittent boost.
Continuous operation provides the best indoor air quality (as well as best moisture protection), and the current ventilation standards require a slight upwards flow rate adjustment for fans that cycle (ASHRAE 62.2 Table 4.2 attached).
I would not recommend shutting the fan off at night, since that's when you most need fresh air - when you are in the house for long periods and your body is is recovering from the diurnal stresses. And that's also why the most important place to install a make-up air inlet is in the bedrooms.
What a great discussion :-)
This is an older thread, however with all the numbers thrown around here, not a single mention was made of CO2 levels, monitoring or managing ventilation using these numbers. "Adequate, sufficient, insufficient" are all terms that IMHO are meaningless without a bit more data.
We've been running tests logging CO2 levels in both a new 9000 sq/ft high efficiency envelope building (with new, engineered HVAC), as well as in an extensively energy retrofitted 100 yr old home. I monitor and correlate energy use as a function of degree days at both sites. Running exhaust fans has about zero effect on CO2 levels in the 100 yr old house, where we are setting a 600-700 ppm target. Levels rise above 1200 ppm. Opening an exterior door at -10C for about 45 min (3 floors, about 1800 sq/ft) will lower levels to around 800. That is a lot of BTUs.
I'm experimenting with the 40cfm Panasonic ERV at this site to see how the co2 levels respond with continuous operation. Btw, on a windy (20+ km/h) running any ventilation may be a waste of time. The testing I've done at both sites (the. 9000 sq/ft is designed as a net zero envelope) shows that as wind speed increases, at least at these two sites/occupant loads, ventilation can be turned off. Unless you monitor something like Co2, it's pretty hard to make these observations. One of the simple strategies implemented at the new site was to implement Co2 control of the HRV system, keeping levels at 700-800 ppm for my staff. At that site, runtime of the commercial Venmar unit is ~3 hrs/day, with about 12 persons working 9-5. I much prefer that control strategy. The Co2 sensors we're using are $200. The USB powered logging sensor was only $90.
Cheers, Dennis Wood (CEO - Cinevate)