HRV Proposal
Hi. I live in NY (climate zone 4) and have a side split house measuring 3,000 sq. ft. that’s on its way to airtightness. There are just 3 people in the house currently with hopefully one in the future.
The HVAC contractor is installing a hydo-air system with two air handlers. The largest air handler covers 4 dampered zones (zone #1- 3 bedrooms, 2 baths, laundry room. Zone#2_ finished attic, Zone#3 – living room / dining/ kitchen area -open floor plan, Zone#4 – Family room). I have a finished basement which has a bathroom and is on its own air handler, but would be very hard to get ducting to.
The HVAC contractor has proposed a Honeywell HRV (model HR200B) to be connected to the large air handler. I believe this HRV is 200 cfm, but don’t know the power usage.
One, I don’t know if that is the proper size unit? Two, he intends to connect it to the supply and return on the new HVAC system which is a heat/AC shared trunk.
From what I’ve been reading here it doesn’t sound like this the proper way to do it. Any input would be great. Also, are there any brand HRVs that are recommended or is the Honeywell just fine, but in a different configuration?
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Replies
Eric,
A house your size with 3 occupants requires 53 cfm of continuous ventilation, according to the ASHRAE residential ventilation standard (ASHRAE 62.2). If your house has 4 occupants, the ventilation rate should be increased to 60 cfm. A rate of 200 cfm would be excessive and would carry a heavy energy penalty.
To learn more about calculating ventilation rates and designing ventilation systems, see Designing a Good Ventilation System.
It is possible to use your home's heating and cooling ducts as ventilation ducts, but such a system often uses more electricity than a ventilation system with dedicated ductwork. (There are several reasons: heating ducts are much larger than necessary for ventilation; air handler fans move too many cfms; and air handler fans are often inefficient.)
During the swing seasons (spring and fall), when you don't need space heat or space cooling, a ventilation system that uses heat ducts will require much more electricity to operate than a ventilation system with dedicated ventilation ductwork.
That said, many homes have HRVs that have been installed as your contractor proposes. They just aren't particularly efficient. However, they work.
Be sure that your contractor understands ASHRAE 62.2 requirements. A 200-cfm HRV must be commissioned properly. It may be necessary to include a timer so that the HRV doesn't overventilate your house.
Martin, your comment about operating cost for a ventilation system that makes use of heating system ductwork implies use of the heating system blower to move the ventilation air. I would think that the fans in the HRV itself would do the air distribution when the heating system is not in use. Can you explain your reasoning?
Dick,
Unfortunately, most heating contractors who connect HRVs to heating and cooling ductwork wire the systems to use the air handler fan to distribute ventilation air during the swing seasons.
As far as I know, there really isn't any other way to do it. Typical residential ventilation air flows range from 40 cfm to 70 cfm. That's why dedicated ventilation ductwork is generally 4-inch diameter rigid galvanized ductwork with sealed seams. Such low air flows require very tight ductwork and careful commissioning and balancing, or remote bedrooms are unlikely to get the 5 cfm or 10 cfm that the system hopes to deliver.
If you try to distribute 40 cfm through big honking HVAC ducts -- 8x16 trunks, and so on -- it's easy for the fresh air to get lost, to escape from small seam leaks, and never reach remote bedrooms. So HVAC contractors crank up the 600 cfm or 800 cfm fans on the air handler to deliver air through the heating ductwork.
It's not a pretty picture.
I seem to be in a silmilar situation with suppliers recommending, what I believe, oversized systems.
We have a 2500sqft airtight home, plus 1400sqft basement area, with two occupants.
I'm being offered systems with 150-200cfm ratings. Going by the 62.2 standard (which is belived to be excessive to begin with) wouldn't a 60-80cfm system be enough in my case?
Raff (is your partner Riff?),
Yes, 60-80 CFM continuous is plenty, but if you don't also have bathroom exhaust fans, then you'll need a boost speed to get decent moisture evacuation when you're showering (and that 200 CFM is distributed to all exhaust ports).
that would be something Robert...lol
Below is one of these guy's reasoning when I asked to back up the 200cfm requirment:
"HRVs are typically sized to ventilate the whole house at a minimum of 0.35 air changes per
hour. To calculate, simply take the square footage of the house (including basement) and multiply
by the height of the ceiling to get cubic volume. Then, divide by 60 and multiply by 0.35."
Raff,
That formula has been superseded by the formula in ASHRAE 62.2, namely 7.5 cfm per occupant plus 1 cfm for every 100 square feet of occupiable floor area.
that's what I thought...seems a lot of suppliers are still using the old formula ( I wonder why?...lol)
I found a 96CFM unit made by Direct Air, which has the boost speed option Robert mentioned and has pretty decent power usage:
http://www.powrmatic.ca/brochures/english/directair/Manual%20DirectAir%20PHRV96%20Eng.pdf
I would like to vent the bathrooms via this unit, figure running it at max speed for 20min should do the trick?
That depends on how many exhaust ports have to share that 96 cfm (or less with duct losses). A decent bath exhaust fan is 80-110 cfm, and they can take 20 minutes to clear a steamy room. If you're exhausting from 2 bathrooms, kitchen and laundry, that's less than 25 cfm per port, and code requires a minimum of 50 cfm flow per bathroom.
Furnaces are changing rapidly. ECM air handlers with 40-100% CFM range, options to run continuously at low speeds, much more. Some companies are integrating HRVs. They may be too new yet to buy and or to even be proven technology.
What I am trying to say is lower CFM rates via variable speed motors exist today, though they cost much more.
Just sayin, not calling any of this stuff natural green worthy. Hawaii... no heat system need be. That's green living.
OK, so I've been doing some reading and this as other things is not as easy as it seems, though I understand the recommended way to set up the HRV. I read the install manual of the Honeywell unit and found there are three ways to install the HRV. One was the dedicated way with separate supply and stail air ducts which is recommended here by Martin, the second was return/exhaust air from the baths, laundry, kitchen ( min 10 ft from stove area) with the supply into the return of air handler of a forced air system (one option for a Fantech unit was to supply straight to the supply side of the air handler). Last, as was discussed above with both supply and exhaust being hooked up to the air handler. The latter is the easiest but less efficient, but can this set up be less inefficient somehow? The HRV has two speeds high/low. If it was connected completely to the AH how would it operate w/ the diff speeds? I have a variable speed AH, would that make a diiference? I aleady have the central ac/heat ducts and exhaust fans in the bathrooms. Wouldnt it be redundant to have the HRV exhaust in the baths with existing fans or should I replace them and duct the HRV exhaust to the baths instead? My wife has alergies and the AH has filtration. If I had separate ducting I would have to somehow filter the HRV fresh incoming air, correct? If the HRV was connected to the return of the AH it would then be filtered. What kind of control is the best for the HRV? I see timers that boost up the out put in the baths during use. What about a Humidistat? Do these controls for the the HRV have programmed air exchanges at low speeds and monitor humidity too? Does the unit then kick in between the programmed air exchanges when humidity is too high to a high speed, or is it running all the time more or less to vent humidity and only kick in to high when called for by hitting a button or only for the amount of air changes necessary and what would that number be? I read about something called Interlock between the AH and HRV is that when the HRV turns on it signals the AH to turn on too? Also, is a HRV with a recerculator duct a must, especially if connected to a AH? Fantech has units with built in damper, so that if you are connected to a AH and the HRV is off, no air can be pulled through the HRV. It would seem that it would be a good idea. I'm sorry I'm raising so many questions, but I'm understanding more as think through this. I already called a number of companies, but got mixed info. It would take alot of effort and coordination to run the ducting throughout the house. Some locations would be hard to get to unless I installed another HRV. That part of the house is the basement area w/ extra bedroom and guest bath. Would that be a must for an HRV? Sorry I'm a little slow grasping this stuff.
Yes, you're overthinking this. Keep it simple, keep your existing exhaust fans and interconnect the HRV to the air handler as in one of these options:
Thanks Robert, your right. If I use method 4 and not require the AH to run is that more efficient or will the lower cfm get lost in the trunk of the hvac without the AH fan running and the rooms not see the proper cfm? If I go this route what controls would be the best? Also, where did the pics you listed come from? Thanks.
Option 4 cannot work without the AH fan running - it requires an interlink switch. Use option 3, and discuss this with your HVAC contractor.
Robert, looks like Option 3 would be more efficient, but I worry whether the cfm of the HRV is strong enough to make it through the hvac trunk on its own. Question again, was this an install manual from a Fantech model HRV? Do you have any particular HRV's you recommend?
It was from a Broan HRV, but the installation principles are the same.
I have no recommendations since I've never spec'ed or installed an HRV. I use exhaust-only ventilation systems. And I never spec or install warm air systems - only hydronic radiant.