Run furnace blower or HRV?
Carfar96
| Posted in Green Building Techniques on
We are building a home is southeastern Michigan. It will have 3 110cfm bath fans (one in basement bathroom) and one 80 cfm bath fan (toilet room) and a 400 cfm hood range. I am concerned about air quality, as we a building a very tight house. Our HVAC contractor has said that he can simply program the furnace to run the blower every 1/2 hour to bring in more air which would be filtered. I believe he will also have a scuttle installed on the furnace. What are your thoughts? Most of the HVAC professionals I have talked to say that HRV systems don’t work that well anyways. Thank you!
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Unless it's a highly efficient air handler with an ECM drive motor that can run at very low speed, duty-cycling the furnace blower is going to use more power and create noise & wind-chill discomfort issues. Using a hot air furnace as the ventilation system has all sorts of down sides. It tends to over-ventilate due to the higher duty in winter, making the air overly dry, and it adds to the heat load.
HVAC pros who say "...HRV systems don't work very well..." aren't exactly credible witnesses. Done right they work GREAT, but don't presume that a pro who tells you otherwise is capable of getting it right.
Carolyn,
Every tight house needs a ventilation system. It sounds like your contractor is describing a central-fan-integrated supply ventilation system -- but be careful, since some ignorant contractors forget to install a motorized damper and a Fan Cycler control. (If the contractor leaves out these essential components, and fails to commission the system by measuring the airflow through the outdoor air duct, this kind of system can exact a huge energy penalty.)
A properly designed and commissioned central-fan-integrated supply ventilation system can work, as long as the contractor knows what he or she is doing, and as long as the furnace has an energy-efficient ECM blower.
For more information on your options, see Designing a Good Ventilation System.
What is a scuttle? Merriam Webster says:
1 : a shallow open basket for carrying something (as grain or garden produce)
2 : a metal pail that usually has a bail and a sloped lip and is used especially for carrying coal.
If that describes something your HVAC contractor is installing on your furnace, he may be a little behind the times.
It is spelled Skuttle I believe. Sorry.
Thank you Martin and Dana for your expertise!
Combining Dana's and Martin's answers, I would guess that the contractors you are talking to would be unlikely to implement the ventilation as well as Martin recommends. One of the advantages of an HRV system that is completely separate from the heating and air conditioning system is that there are fewer ways to do the installation wrong. I'd be curious whether the HRV systems the contractors are talking about having problems with were systems that were integrated poorly with the heating and air conditioning, using shared ductwork.
With a ductless HRV system like Lunos (https://foursevenfive.com/product-category/ventilation/) there might be even fewer ways to mess up, although I'm sure there are plenty of people who can still find ways to do it wrong.
I did look at the Lunos, but unfortunately our house is well over 3000 square feet including the finished basement and cost would be prohibitive. Our contractor has already started the work in the home. He was recommended as being the best HVAC installer in the area and was selected by our builder.
The furnace is 96% efficiency with a ECM fan motor. What about doing a ventilation only system? How would this be accomplished?
For what it's worth I just figured out where the "skuttle vent" term is coming from: A company called Skuttle Indoor Air Quality products makes a few different types of outdoor air inlet products--one for adding outside air to the return ducts and one for providing combustion air to the furnace. Both are on this page:
http://skuttle.com/wp1/?page_id=278
The installation instructions do no talk about doing any of what Martin recommends, and instead imply that it will magically provide the right amount of ventilation, which it simply can't do, unless the contractor also does what Martin recommends, at which point you don't really need the special damper that Skuttle sells.
And for Reid, the company's first product, invented by Mr. John Skuttle in 1917, was "was an automatic fill valve to monitor water flow into coal furnace humidifiers." So there's the link to coal heating. Their primary product appears to still be humidifiers, something that the GBA community tends to despise almost as much as we dislike coal furnaces.
Carolyn,
You wrote, "Most of the HVAC professionals I have talked to say that HRV systems don't work that well anyways."
I'm sorry to hear about your experience. These HVAC "professionals" don't know what they are talking about.
Properly installed, a ventilation system with an HRV is the best ventilation system out there. Of course, if the HVAC contractor hooks up the HRV to forced air ductwork connected to a furnace, instead of to dedicated ventilation ductwork, you can have poor system performance. But that's due to installer error -- it's not the fault of the HRV. (For more information on this issue, see Ducting HRVs and ERVs.)
Q. "What about doing a ventilation only system? How would this be accomplished?"
A. I'm not sure what you mean by a "ventilation only system." But no matter what type of ventilation system you choose -- a balanced system, a supply ventilation system, or an exhaust ventilation system -- it's best if you don't try to use heating and cooling ducts as ventilation ducts.
If you read the article I linked to, Designing a Good Ventilation System, you'll get a good overview of your options. Usually, residential builders choose either a balanced system (using an HRV or ERV), an exhaust-only system (using a high-quality bath exhaust fan), or a central-fan-integrated supply ventilation system (which we have already discussed).
If you have specific questions after reading that article, you can post your questions here.
I have an issue similar enough to Carolyn's that I will add it to this thread instead of posting a new question.
I just had a meeting with my HVAC contractor who is just starting to run ductwork. (SE Michigan like Carolyn) My plan was to use a Renewaire ERV drawing from all bathrooms, eliminating all bath fans. I planned to route the fresh air back to the return duct of the forced air furnace. He pushed back on that pretty hard, although he is willing to install it that way if I insist. He will be working on other stuff until Monday, but I need to give him firm directions by then. I am looking for guidance on whether I should stick to my guns or be a little flexible.
His recommendation was to use fans in each bathroom that can be ducted through the attic to a nearby gable wall and then use a fresh air intake for the furnace. I asked him if he meant a central-fan-integrated-supply system. He said no, the fresh air intake wouldn't need any controls. He said it would draw air whenever the house was depressurized by a bath fan, dryer, or range hood. Since he didn't seem knowledgeable about supply ventilation (although I didn't say that out loud), I rejected that recommendation.
His second recommendation was to use the ERV but not to replace bathroom fans. He suggested either having the ERV draw from the return duct and send to the supply duct or draw from one room and send to another room. I don't find either of those ideas completely satisfying, but there may be some middle ground. The guest bathroom is particularly far from the ERV location, so that would be the first place I would consider a conventional bath fan. The duct route to the master bathroom is also pretty circuitous, but that will be the most frequently used bathroom.
Reid,
There are plenty of articles on GBA to help you make your decision.
Briefly, an HRV or ERV with dedicated ventilation ductwork is the best way to go.
If you choose to use your forced-air ductwork for ventilation purposes, the performance of your system will be compromised. If you can't avoid using your forced-air ductwork for ventilation supply or exhaust, you can use it for either the supply or the exhaust, but not both.
Whether or not you will be satisfied with the HRV ventilating your bathrooms depends on your expectations. If you want a powerful fan to keep your mirror free of condensation at all times, they you should install separate bath exhaust fans.
Here are links to more articles on these issues:
Ducting HRVs and ERVs
Does a Home with an HRV Also Need Bath Fans?
Is Your Ventilation System Working?
Ried I will add that some people and especially when you have guests may be accustomed to having some background noise while occupying the bathroom. Even though most fans built now are pretty quiet, a bathroom without that background noise my seem uncomfortable to some. May seem kind of silly but something to think about.
Martin: Is there any air quality risk by dumping the fresh air into the return duct. Does that fresh air mix with the rest of the return air coming from inside the house, such that you get some fresh air and some not so fresh inside air? Maybe I'm missing something.
One thing I like about my dedicated HRV ducting system is that nothing is ever in the supply stream ducts but outside air, which runs through a MERV 13 filter. Does the typical hot air furnace permit that level of filtration?
Stephen,
The problem with the set-up you describe is mostly an energy efficiency issue, not an air quality issue. That said, if the supply ductwork is in an unconditioned attic, and the ductwork leaks, some of the fresh air introduced into the ductwork will never be delivered.
Of course, the air coming out of the supply registers won't be 100% outdoor air -- it will be mostly recirculated indoor air.
Yes, you can install a MERV 13 filter on a furnace. The filter will introduce static pressure; if the furnace is sophisticated, and has an ECM blower, the blower motor will ramp up in response to the increased static pressure, and deliver the expected air flow -- but the electricity required to operate the fan will go up.