How critical to IAQ is HRV/ERV ducting?
Pretty much every bit of information I can find says that for IAQ an ERV/HRV should have exhaust ducts from bathrooms, kitchens and other ‘dirtiest air’ sources and that fresh air supply s/b directly to bedrooms.
Our HVAC contractor wants to do ours with both in the return duct. E.G., pull exhaust air from the return duct about 10′ from the furnace and introduce fresh air in to the duct about 7′ from the furnace.
My understanding is that drawbacks to this include; CO2 levels in occupied bedrooms can then be 1200-1800 ppm, furnace blower must also run which wastes energy.
Questions:
1) How important is this ducting choice? Is this something that should really be pushed with the HVAC folks or not a big deal so let them do it their way?
2) How conditioned is the fresh air from an HRV? E.G., if it is -20°f outside then what temp is the fresh air likely to be and how critical for it to be further warmed/cooled before being introduced to rooms? These will be Braun 7.1 (http://www.broan.ca/products/product/35312fd6-61d8-4fc8-9e7a-c2cc54599220) which indicate an “Apparent Sensible Effectiveness (% -25ᴼC) = 78%”.
Is this an issue worth making a stink over with an HVAC contractor who doesn’t want to do it?
Thank you,
| Apparent Sensible Effectiveness (% -25ᴼC) | 78% |
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
This BSC article explains the advantages and disadvantages of the different ventilation systems:
https://www.buildingscience.com/documents/information-sheets/info-611-balanced-ventilation-systems
With a good HRV, you should expect that the fresh air delivered from the HRV to be equal to or greater than 15C or 60F. At the low flow rate of a typical HRV, you are unlikely to notice it. The Broan unit you list is a pretty low efficiency HRV. I guess it's what you might call "contractor grade", in other words the cheapest thing they can buy that still fulfills the minimum specs required.
Thanks Trevor. Can you tell me what a better HRV option would be?
Giving specific recommendations is hard, because it depends on various factors. Take a look at the HVI product directory and get a feel for what are good, average, and poor efficiency numbers.
https://hvi.org/proddirectory/CPD_Reports/section_3/index.cfm
Ignore Apparent Sensible Effectiveness, Sensible Recovery Efficiency is the real efficiency number. For the Broan 7.1, at 64CFM it's only 61%, which is why I said it's on the lower end. The efficiency numbers at the below-freezing numbers, while not exactly irrelevant, are highly deceptive because most HRVs recirculate air as a defrost strategy. So some of that heat being "recovered" is heat that would otherwise not left the house. Put another way, the ventilation rate just goes down, so you're only getting say 30CFM fresh air average when it's set to be delivering 60CFM.
Find out what your design ventilation rate is. Pick a unit that is capable of at least 50% more than that, double if you live in a cold climate (see above issue of reduced effective rate). One with ECM fans will be more expensive, but worth it IMO.
There are lots of articles and discussions on GBA about picking an HRV, I would suggest perusing those.
I'm not especially endorsing a unit; I'm just a DIYer and don't have the expertise to do that. But I thought I'd mention that if your contractor is stuck using Broan for whatever reason, even Broan makes higher efficiency units. The HRV200TE has ECM motors with better CFM/watt than the Broan you listed and sensible recovery efficiency from 70-81%, depending on flow rate. I have no idea on price comparison or payback.
Trevor: I'm curious what you think of the efficiency of this unit.
https://www.broan.com/BroanPublicWebSite/media/BroanUS_Canada/Documents/spec_sheets/HRV200TE_SPEC_SHEET_eng_HRV200ECMd170111.pdf?ext=.pdf
Ramsay,
If it is a new build, I would get the proper ducts put in. If you want to save a bit on ducting, I prefer the hybrid exhaust ducted option shown here:
https://www.finehomebuilding.com/2014/11/05/ducting-hrvs-and-ervs
This way you don't need to interlock with your furnace which is an energy pig. You can run one of the stale air returns to your bedroom to help air circulation.
This has the side benefit of the furnace tampering the colder air in the winter time.
For efficient HRV, look for one with a hexagonal counter-flow core like:
http://www.vanee.ca/DATA/DOCUMENT/169_en~v~technical-sheet-g2400h-ecm.pdf
Interesting. The specs on that Vanee are so similar to the Broan HRV200TE that the units must be at least the same core and motor, if not the same unit. Who is making the units?
Keith,
Vanee is owned by Venmar, a manufacturer of ventilation equipment with headquarters in Quebec. Broan's HRVs are re-labeled Venmar units. So both of these units are actually manufactured by Venmar.
Not that it matters much, but Broan owns Venmar (per Venmar's website). I can't find any evidence that Venmar owns vanEE. I think that vanEE is the manufacturer, and both Broan and Venmar are re-labelled units. vanEE started producing HRVs in 1981, whereas Venmar didn't start until 1990. No matter what the exact relationship, the fact remains that today, all three brands are all the same machines.
edit: I just discovered that vanEE and Venmar have the same address. So at some point, they merged brands. They each list different histories, so at some point in the past they were separate companies.
I think a part of this is how to quantify the pros/cons of the various approaches. Our HVAC guy insists that a fully ducted system is not worth the cost and that his approach (and which I think is standard for all HVAC folks in Minnesota) is just about as good for much less cost.
IAQ is likely much more important to me than his other customers who largely have no idea about it. But what cost for what improvement? Is $4k in extra cost for fully ducted worth it for 989 ppm CO2 instead of 1010 ppm in bedrooms at night? Likely not. But if we're talking 800 ppm for fully ducted rather than 1400 ppm for his approach then it is.
Thanks,
Central air does a decent job of mixing the house air when it is running. The issue is shoulder season and night time setbacks. In this case the IAQ in your bedrooms would be significantly worse if there is no dedicated air exchange.
Anecdotally, after taco night, it is good to have dedicated air exchange in the bedroom.
Martin referenced a study a bit ago that looked at CO2 in bedrooms at night and from what I remember inferred that current ASHRAE standards could result in 1200-1800 ppm in bedrooms at night when bedroom doors are closed. Opening the bedroom doors made a dramatic difference.
This could have been an issue of night time setback as you mentioned. Would having a CO2 monitor in the bedrooms that forces the HRV to run anytime it rises above 1000 (or 800? or 600?) be a solution?
With a fully ducted, balanced ventilation system you can have almost-atmospheric levels of CO2 (like 500-600ppm). There is some debate whether 1000 is the right limit. Most agree that above 1000 is too high, but lots also say even 1000 is too high.
If you close your bedroom doors at night, you are almost certainly going to end up with high CO2 levels at least some of the time when combining the HRV with the heating/cooling ducts. If your house is very well insulated, there will be plenty of times when there is no heat demand, but it's still too cold outside to open windows. The HRV will not have the ability to effectively or predictably push air down those big ducts by itself.
Whether this matters is up to you. There are surely lots of homes with poorly functioning ventilation systems, and very few people seem bothered by it. But if IAQ is a real concern for you, fully ducted and balanced ventilation is the way to go.
"Would having a CO2 monitor in the bedrooms that forces the HRV to run anytime it rises above 1000 (or 800? or 600?) be a solution?"
Yes, but only a very select few models come equipped to do this. They are expensive HRVs, and the monitors can be expensive (you can find good deals if you go "off label, but if you go with the units offered by the HRV manufacturers you're going to get fleeced). I have one monitor in the main living area, and one in the master bedroom. While it's nice peace of mind, I used them mostly just to determine appropriate flow rates, which you could do with a portable meter. Very occasionally they've caused the HRV to ramp up when I've manually set it to a lower rate and forgot to change it back.
Thanks.
"There is some debate whether 1000 is the right limit. Most agree that above 1000 is too high, but lots also say even 1000 is too high."
I lean towards the latter but I don't think there's enough info yet to say for sure. There is some info that, at least temporally, cognitive ability begins to decline and sleep problems develop with CO2 levels above about 500. Anecdotally I do sleep much better w/ windows open though I've not yet fully correlated it to CO2 or PM2.5 levels.
The other bit is any longer term health problems. We've no idea, I believe, to what extent exposure to higher levels of CO2 (and other VOC's) is cumulative. How many people who have developed various diseases would not have if they'd lived in a house with better IAQ?
"The HRV will not have the ability to effectively or predictably push air down those big ducts by itself."
In our case the furnace blower would be interlocked w/ the HRV so that any time the HRV turns on the furnace blower does as well. If the fresh air supply went directly to the supply side of the furnace then this would certainly seem to be a problem.
"There are surely lots of homes with poorly functioning ventilation systems, and very few people seem bothered by it."
True. But how many are experiencing problems sleeping, headaches, or other symptoms and are trying anything and everything except... getting fresher air? Or have diminished cognitive ability and don't realize it? Or are, due to poor ventilation, being exposed to other VOC's that are causing them to become cancerous?
A lot of people aren't bothered by their being overweight until doc tells them that they've got diabetes or any number of other health problems resulting from it (or the lack of activity and poor eating habits that leads to being overweight).
Just found this: https://pdfs.semanticscholar.org/84f2/93c23b8d610f0944c106190799a804c78c0f.pdf
"Allen et al. [16] exposed 24 participants to CO2 levels of 550 ppm, 945 ppm or 1400 ppm for “full work days” (~8 h), and found that cognitive function scores were 15% and 50% lower, respectively, for the days with 945 ppm and 1400 ppm as compared with 550 ppm."
Combined with other studies we've discussed on here this would seem to further question the wisdom of allowing 1000ppm. If cognitive function is this impaired then what other bits of our bodies are harmed in what ways?
IMO this appears to be an issue of what is/is not familiar in a local market. Only you as the consumer can determine whether or not the cost of a fully ducted system is "worth it". Be forewarned however that if the local market doesn't normally install what you're asking, you will probably receive an outsized bid and are more likely to have install issues.
IIRC local codes in Minn do require ventilation and the method promoted by the HVAC company is possibly an acceptable option. You may want to double check that. You might also want to really investigate the CO2 issue. I mean don't take the word of some organization which performed a "study" commissioned by a HRV/ERV manufacturer that we're all on the verge of severe cognitive impairment because CO2 levels are too high when the bedroom door is closed.
Thanks. If the local market and 'an acceptable option' is antiquated and does not result in enough fresh air then?
As a consumer I can only make decisions based on available information. I'm finding reliable and adequate information extremely difficult to come by. Thus my questions above. This is made much more difficult when the supposed experts you rely on, the HVAC folks, insulation folks, builder, etc are in seeming disagreement with building science and what is supposed best practice. As a consumer I'm stuck in the middle without much to go on.
Your heating/cooling duct system is (or at least should be) designed to distribute air to rooms in proportion to their peak heating or cooling loads. If the need for fresh air in various rooms tracks heating and cooling loads fairly closely, then distribution of "freshness" should be fine. Having to run the central fan will use more energy, but it will likely take many years for the cost of that energy to add up to the cost of dedicated ventilation ductwork. A few things to consider:
- In the summer, running the central fan between cooling calls reduces the effectiveness of AC at removing humidity.
- If you have zoned ducting, that will impact the distribution of fresh air. Whenever some zones are calling for heat or cooling, other zones will not get any fresh air.
W. Ramsay,
My opinion on this matter has evolved over time. There is increasing evidence that tight homes need balanced ventilation systems with dedicated ventilation ductwork if we want adequate fresh air in bedrooms (where we spend about 8 hours a day).
Personally, I wouldn't compromise on this issue.
Here are links to two relevant articles:
"Ensuring Fresh Air in Bedrooms"
"Ducting HRVs and ERVs"
I agree - a balanced or slightly positively pressurized, ducted system offers the best air quality at the lowest energy consumption. Taking the exhaust from the return air duct will mean that some of the 'unused' conditioned fresh air will also be exhausted instead of recirculated again back to the occupied areas.
Also, if you follow correct fresh air calculation procedures, mixing the fresh air into the supply air will reduce the air distribution effectiveness of the fresh air system, especially with overhead warm air supply for heating. The required minimum fresh air volume (size of the ERV) is to be increased accordingly - depending on the ratio of overhead air supply, by as much as 25%.