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HVAC HRV sized correctly for my home?

A HVAC contractor installed a Venmar HEPA 3100 HRV in my new construction home. I was wondering if it is sized correctly for my house and meets the international building code and international mechanical code requirements. HVAC Work began in 2011 but venmar was installed late 2013.

My house is:

4846 square feet with a 486 square foot attached 2 car garage. Total sq ft: 5332.
Bedrooms: 5, Bathrooms: 6..

I was ordering a new filter and noticed it was discontinued and said:

for homes up to 3100 square feet ***
*** for new construction refer to building codes.

We have experienced issues with condensation on window sils and eczema flair ups in the new home.


Asked by Sam Miller
Posted Jul 4, 2014 6:48 AM ET


4 Answers

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A few questions and maybe a few answers here. Was the HRV commissioned, meaning, did the HVAC contractor measure the airflows to and from each outlet or inlet? What if anything was done to determine ventilation requirements for the house? How is the unit controlled and how much is it running? How is it configured to serve areas where the condensation is occurring? Any chance that a blower door test was done?

Excess moisture in new construction is not uncommon, and although it may have taken a few years to finish your house, a lot of the late-stage work adds moisture, which takes a while to dissipate. You may have a problem that will at least partly go away on its own.

There is also the possibility that you should have an energy auditor/home performance contractor evaluate the house and maybe do some testing. A person with the right experience can often find issues that building contractors generally do not.

Answered by David Meiland
Posted Jul 4, 2014 12:02 PM ET
Edited Jul 4, 2014 12:03 PM ET.


I believe the HRV was commissioned in November 2013. But the duct work everything began install began in 2011. I don't think the HVAC contractor measured the outlet/inlet. I am not sure what was done to determine ventilation requirements. The unit is running 24/7. The condensation is happening on all window sills on upper 2 floors during very cold nights. The main culprit which happens more than others is the south facing bedroom window. No blower door test.

I was mostly wondering if the HRV met international building code and mechanical code requirements for my house. I read somewhere certain amount of outdoor area is required and determined by square feet of home and bedrooms. I forgot where that is. The HRV manufacture themselves says for homes up to 3100 square feet and refer to building codes. It could be undersized?

Answered by Sam Miller
Posted Jul 4, 2014 3:56 PM ET


I don't know the code your house was built under, or exactly how ventilation requirements were determined (or should have been)... but it's probably derived from the square footage of the building, number of bedrooms, number of occupants, etc. The ventilation rate is determined before the building is built, and the mechanical contractor is responsible for installing a system that will meet it. Without delving into that code, or the specifics of the HRV you have, I can't comment on whether or not the unit is simply too small, or is actually adequate.

"Commissioning" is the process of measuring performance and verifying that a system is doing what it's designed to do. For an HRV, the contractor should use a flow hood to measure the ventilation rate at each inlet and outlet. If they did not do that, there's no way of knowing if the overall ventilation rate for the house is being met. Long runs of ductwork, lots of elbows and bends, and other issues will reduce the flow rate significantly, so actual measurements need to be taken. There are sometimes checks that can be performed at the HRV itself using a pressure gauge.

Further questions: are there separate bath fans installed, and are they being used diligently (i.e. run 30-60 minutes after any shower)... or is the HRV supposed to provide bathroom ventilation? IF there are bath fans installed, and they are located near areas with humidity issues, have you tried running those more frequently to reduce humidity? It may be that the HRV is not doing quite enough, but that you could augment with some exhaust-only ventilation and improve the situation. Bedroom window condensation is fairly common, and if you can improve the ventilation in that specific area, it might help.

You may also have an issue with localized air leakage around your windows, either through the units themselves or around the units. Very cold air leaking in at windows can cause condensing conditions, and eliminating those leaks can improve or solve it. This is the domain of someone with a blower door and leak-finding experience. If that's what's happening, it needs to be addressed. There are lots of other possible contributors as well.

Answered by David Meiland
Posted Jul 4, 2014 5:02 PM ET


The standard formula for calculating residential ventilation rates is found in ASHRAE 62.2. You can read about the formula in this article: Designing a Good Ventilation System.

In the case of your house, here's how to do the math:
(6 * 7.5) + (3 * 48.5) = 45 + 145 = 190 cfm

It appears that the Venmar 3100 is rated at 105 cfm at high speed, but I may have that wrong. In any case, you need to have your system tested and commissioned.

You should know, moreover, that the ASHRAE 62.2 formula was recently changed; the older ASHRAE 62.2 formula would have resulted in a lower ventilation rate. The formula change was highly controversial. For more information on the controversy, see How Much Fresh Air Does Your Home Need?

Finally, the formula assumes that a 5-bedroom house like yours is occupied by 6 people. If you have fewer than 6 people living in your home, you won't need such a high ventilation rate.

Answered by Martin Holladay
Posted Jul 5, 2014 6:56 AM ET

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