Looking for ERV recommendations for a new two level ICF residence located at 6800 ft elevation with 6001 HDD base 65 and 0 CDD base 50. Radiant floor heating only. Conditioned living space is 20,000 cf per level plus small amount of conditioned attic due to foaming the roof deck. Lower level is walkout on one side and buried on three. Upper level has garage attached on one side only.

Using total volume * 0.35 ach / 60 min/hour = ~120 cfm flow per level. There are two baths and laundry per level and two bedrooms and great area on each level. Plan on drawing from baths and laundry and supply bedrooms and great area.

Looking for easy to install and maintain system with auto dehumidification boost capability for the bathrooms. Will install in storage room on lower level and conditioned attic space on upper. Prefer side pipes due to limited height in attic space. House as gas fireplaces on each level that vent to the exterior, though currently drawing from the house. Also, large range hood over island stove top with no makeup air. Considering making the house slightly positive pressure. 

What do you find that works in the dry southwest at altitude? Remote area with limited HVAC technicians to rely on! Any design considerations we should keep in mind?

Thank you in advance for sharing your knowledge.

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I generally recommend against using an ERV as primary ventilation for bathrooms & laundry since the enthalpy core acts to minimize moisture transfer from "high to low" (referring to partial vapor pressure of outdoor and indoor air). That means that when dew point at exhaust location is higher than outdoor dew point, a large percentage of that moisture will be recycled back into the fresh air stream via the enthalpy core. Perhaps not what you were expecting?

On the other hand, since you live in a dry climate, it may be advantageous to retain that moisture. I live in SE Arizona and we don't use exhaust fans (except for odor control) since indoor RH is well below the 40%-60% comfort range most of the year. We can use all the moisture we can get! During summer monsoon when outdoor dew point routinely climbs into the 60's, I anticipated using the fan when showering. But as it turns out, even then my indoor RH never rises above low 50's without using fans, and localized moisture (from shower and laundry) naturally equalizes throughout the floor where it's generated. At your location in northern NM, the summer 'monsoon season' is much weaker than here, so I doubt seriously you'll ever see 50% indoor RH.

BTW, in my opinion, 240 CFM ventilation is overkill for your house, and will unnecessarily increase heat loads. The 0.35 ACH(n) target is way outdated. ASHRAE 62.2 calls for (7.5 CFM per bedroom +1) + (0.03 x conditioned floor area) - verified envelope leakage (i.e., CFM50 converted to CFM-natural). You can either dial down the ventilation rate with a cycle timer or go with smaller units. Also, you can save some money by going with one ERV instead of two, assuming there's a way to route small ducts between floors.

I typically specify RenewAir or Venmar recovery ventilators. I'll leave it to others to recommend a particular model.

You mentioned gas fireplaces and a big range hood that draw from the house. Those are red flags, indeed. How do you intend to make the house positive? Have you had a blower door test done?

Thank you for the insight David. House only has the first level walls poured, second level floor installed and they are stacking the second level ICF now. Blower door test is some time away. Need to design ventilation based on assumptions and tune when more data becomes available.

Based on the ASHRAE 62.2 formula the whole house would require 139 cfm - tested CFM natural. Ceiling heights vary throughout the building from 9'-9" to 12' to 14'. ASHRAE 62.2 does not address  height in their formula directly. Not sure the effective difference.

Hoping the ERV can be set to provide the slight positive pressure. Most have two fans that can be independently adjusted during the balancing process. Difficult to install single unit.

Lower level bathrooms open to an interior space or underground with no windows between a bedroom and a walk in closet full of clothes. Thinking of installing a humidistat on both bathroom exhaust to activate a booster fan speed on the ERV. Otherwise continuous low speed operation. Aldes has an interesting ERV system. The closet on this level has a stackable washer/dryer with venting to the exterior. Concern with stagnate air flow in the closet since this is a spare bedroom and minimal W/D action in the closet. House is all radiant floor heating with no A/C ducting. Clothes will absorb the moisture and could develop a musty smell if circulation is not maintained. Considering installing a low flow supply to the closet and bedroom to balance this one bathroom exhaust since all could be behind closed doors to the Great Area with Bar/Game room. The one bathroom is open to this Great Area. Thinking to run supply to Great Area to balance the second bathroom exhaust.

Discussing an interlock fresh air fan on the kitchen island range hood or a commercial unit with dual walled hood and fans that brings in fresh air between their walls. This creates a positive air flow during hood operation. Unfortunately, money factors into all these issues.

I think you're overreacting to the need for humidity control in your location. If indeed there's a need to remove any moisture, an ERV is the least effective way to do that. Moreover, I advise against locating an ERV supply in a closet, as it will simply dump moist air in there during and after showers & laundry ops (see my previous reply). If you want to induce air exchange in the closet, install an ERV exhaust vent, which will act to draw air in from adjoining rooms. A small passive transfer grille can be used to control which room it draws from.

As for 62.2 ignoring ceiling height... that's intentional.

In your original post, you mentioned providing positive pressure in the context of having gas fireplaces and a large range hood without makeup air, as if you intended for the ERV to provide makeup air for both, by setting fans for positive pressure. No recovery ventilator is capable of that. True, the fans can be adjusted to provide a 'slight positive' pressure, but that gets lost in the noise compared to the the makeup air requirements of a fireplace or commercial style range hood, assuming that's what you intend to install.

To me, installing a combustion fireplace in an ICF home (presumably purpose-built to be high performance) is an oxymoron. But if you must, it should have a sealed combustion chamber and draw 100% combustion air from the outside.

Likewise, providing makeup air for a commercial style range hood, typically 600 to 1200 CFM, is fundamentally problematic in a high performance home, especially if money is an issue (when is it not?). Radiant heat has very poor capacity to handle pick-up loads. Consider that 1200 CFM of 0F outside air creates a 90,000 BTUH load. Introducing makeup air close to the burners mitigates the impact on house loads. Also keep in mind that the closer you introduce MUA to the hood intake, the lower the capture efficiency of the hood. This is one reason why commercial style hoods require such a high flow rate.

You can find lots of discussions on that topic in the Home Performance Forum archives. Here's an example. My advice would be to go with a hood with a maximum flow rate of 300 CFM. If the hood has relative high capture efficiency (easy to do with mushroom style island hood), 300 CFM is adequate for even a 48" range, and is a lot more manageable in terms of makeup air.

Unfortunately I don't have anything to add here but wanted to give this post a bump as I'm a BPI Building Analyst and I'm trying to learn more about what ERV/HRV can accomplish as well as their installation. For what it's worth I'm also at a similar elevation.

Many ERVs, particularly those with enthalpic wheels, use only ONE motor to run two fans. The In/Out are balanced with dampers.

The ASHRE standards are guidelines. The installed ventilation system should be capable of moving at least the amount of air recommended by ASHRE 62.2.Those standards are based on average conditions. They are not necessarily appropriate for say a 4,000 sf house with 2 occupants, or a 1,500 sf. house with 6 occupants, two dogs, three cats and 23 potted plants. Humidity can be used as a general guideline for indoor air quality, but as we make our houses tighter, other pollutants, such as CO2, PM 2.5s, and VOCs, will become more important guides to good ventilation.

"Always look at the whole picture"; "The more you look, the more you see" and "if you don't look, you don't see".

Hello David -- Do you have an expected air tightness? What type of blower door test results are you projecting?

David is right; you should be using a low flow range hood of 300 cfm with good capture rate. Without an account for how that air will be replaced, you cannot use a large range hood. Even an ERV with slight positive continuous pressure is not going to assist when you are pulling 1000+ cfm from the kitchen.

Shoot for 25 cfm continuous from the bathrooms and 40 cfm when boosted. Switch the dryers to heat pump options without an exhaust. Then pull 15 cfm from those laundry spaces continuously.

My beginning recommendation would be to have 65 cfm per level continuously. 100-150 cfm during periods of showering. Duct size in this scenario would be 5" or 6" trunks with 4" branches to all the supply and exhaust diffuser locations. This should meet ASHREA 62.2 requirements for ventilation and give you a solid IAQ strategy.

I would use push button boost options rather than humidity sensors. Sensors go bad over time, and might activate increased ventilation accidentally when you might not require it.

You should have several options available. Choose an ERV with over 80% heat recovery, DC motors, and filtration over MERV10. Consider select high performance models from UltimateAir, Panasonic, RenewAire, Zehnder, or Venmar (Broan).

FYI - I work for UltimateAir, but I believe that is a pretty unbiased recommendation :)

@Luke, since you work for a well-known ERV manufacturer, I'm curious if you have access to latent performance data (other than the single point @ summer conditions typically published). A graph would be ideal. I think most practitioners fail to consider what happens when an ERV is used as primary exhaust for high moisture events, especially showers.

When it comes to independently ducted ERV systems operating in humid climates during the summer; the high moisture event (shower), on a single exhaust branch combines with the other exhaust streams before reaching the ERV core, this lowers the total amount of moisture there is to "recover" back to the residence. Also, the event of showering is only taking place during a small part of the day. If you have three bathrooms, all with showers operating continuously, then you would probably want to dump all the moisture outside without recovering any for the interior. However, that is a very small period for most applications -- when you consider that the outside condition has the higher level of moisture content for the majority of the daily operation. You want to block the maximum amount of moisture from coming inside during 99% of the day.

ERV systems are going to transfer a percentage of moisture from the higher air stream to the lower air stream. Generally during the summer, the higher moisture air stream is outside, and therefore you want moisture transfer to occur to prevent adding RH to the interior. During the winter, the higher moisture air stream is coming from the interior, and we are trying to not "dry" out the interior condition, so we again want moisture transfer to occur to keep some moisture inside.

If an ERV system was able to actively measure the outside RH and interior RH and deactivate the moisture transfer ability when its not wanted....(turn itself into an HRV during periods of summer showering maybe)...that would be beneficial. For our system, you would need two separate wheels, one with heat exchange material, and one with moisture exchange material. Adjusting the speed of rotation given the interior/exterior condition. It would be very cool to do this, but very cost prohibitive.

Thanks, Luke. I agree it wouldn't make sense to design an ERV to accommodate high interior moisture loads. I think the simplest strategy for that is to not rely on an ERV to exhaust excess interior moisture, as they're not designed for that. Whether or not recycled (recovered) water vapor might push indoor humidity to risky levels depends on several factors, but at least designers need to be better educated on this point. Dismissing this as a non-issue is not helpful to that end.

As you point out, having multiple exhaust inlets reduces the incidence of recovered moisture, although I would argue it doesn't eliminate the risk. In any case, I doubt many designers (if any?) consider this when prescribing the number of exhaust inlets. Worst case is a single exhaust inlet - located in master bath. This scenario is certainly not unheard of and isn't addressed in ERV installation manuals, WhisperComfort being the exception (possibly because it's designed to be single-port, as you observed in a similar discussion here last year: https://homeenergypros.org/xn/detail/6069565:Comment:238379).

Keep in mind, the problem I'm describing isn't really a summer issue. Highest risk is during swing seasons (when there's no cooling load), and in winter when moisture recovery is generally welcome. However, this can backfire in tight homes, which naturally have higher wintertime humidity than leaky homes (unless ventilated via HRV). I've seen been plenty of anecdotal reports of wintertime moisture issues in high performance homes.

It's easy to understand why we need to maintain significantly lower RH levels in winter: (a) lower indoor ambients, (b) natural temperature variations across the house (especially in unventilated closets with exterior exposure), (c) aggressive nighttime setback and/or shutting off heat to unused rooms, and (d) potential for air leakage into exterior cavities. I generally advise clients that anything above about 40% RH is risky business in winter.

As I mentioned in last year's discussion, I first became aware of this problem when my brother spent more than $20k to remediate mold, proven to be caused by his ERV that had a single exhaust port in master bath, along with a boost switch, and no stand-alone exhaust fan. Neither his mechanical contractor nor the builder understood why that's a problem. We need to do better.

The obvious defensive strategy against excess interior moisture is to increase moisture removal at the source. Stand-alone bath fans are cheap, and very effective when installed properly.

 I second Luke's recommendation for Panasonic, RenewAire, Zehnder, or Venmar (Broan). All good units with humidity recovery. Hook through bathrooms to encourage humidity retention. I personally would choose a Panasonic or Zehnder. The core material is what your humidity recovery is based on. 

Also, how are you going to distribute the air in the house? Whole house distribution system? If its not being equally distributed through house it isn't much of a ventilation system.

I would focus on controls instead of the basic Ashrae calcs. ASHRAE is based on radon and off gassing of new construction materials. Very little to do with occupant load. How many people live in the house, how much cooking, and bathing takes place. So having a low and high function is helpful to deal with occupant load. Also, you should base the settings on your air infiltration rate, blower door test. If you have a house at 1ACh you obviously need more ventilation. If you are at 4ACH you need far less. 

Setting the balanced system for positive pressure will help with dust. But, setting to a slight depressurization will help keep wall caps shut and decrease loads on the house. 

Do you have any gas combustion appliances? any, naturally drafitn? Be careful of back drafting. A tight house with fireplaces and combustion appliances do not mix.

I would highly recommend efficient fireplaces with make up air from outside. Traditional fireplaces and new construction tight houses do not play nicely together. 

Bottom line is you need to know how tight the house is. 

Thank you for all your comments. House is ICF and local code is 2018 IECC listing maximum 3 ACH50. Depending on zone and shielding selected ACH averages out about 108 CFMnatural. ASHRAE 62.2 calculation indicates 32 CFM continuous required (3 beds, 3670 sf). House has NO conditioned air ducting. Only radiant floor and ceiling fans. Concern is bathrooms are next to closets on both levels and both are where there is no natural air circulation or operable windows. Lower level is underground and farthest from the walk out side. I live relatively nearby and am dealing with humidity in my closets in a 5 ACH50 house with better air circulation.

House will have enclosed gas fireplaces on each level, no wood, and highly encouraging them to connect to dedicated outside fresh air.

Current design is to shoot for 60 cfm per level on low and 120 cfm on high. Each level is roughly 20,000 cf. Bathrooms to have 20/40/60 minute push timers for manual local condition control and humidistat installed and set high (~70%+) to handle shower/bath moisture loads automatically. All returns vent to the Great Room on each level.

Considering Aldes ERV with CAR II inserts to maintain minimum air flow (20 cfm (2) bathrooms, 10 cfm closet and Great Room) and maximum air flow in closets & Great Room (10 CFM) and unlimited maximum in bathrooms on each level. Does anyone have experience with Aldes systems and using their constant air regulators?

Alternate is Venmar system, using inline damper controls for defining flow rates at registers. General Contractor has previous experience with this brand.

Location of equipment creates height limit of selected equipment and access issues to consider.

Concern is limited skill of technicians in this rural area. House is 45 minutes from nearest small town with two old school HVAC contractors. Next contractor is 1.5 hours away.

As a Rater and PH guy I have seen a number of units. You do get what you pay for. I also commission for Zehnder, about 180+ to date. Having the ability to dial  in every room to meet a design spec is really special. 2 ECM fans and there diffusers are very customizable. 

I have seen a few with enthalpy wheels fall apart overtime. I am not a fan of the wheel. Baths should have a min of 20 cfm, But  a little room for filters reducing the flow is advisable 24cfm. A MERV 13 is pretty good too. Having a boost ability on top off that is also important as well when needed intermittently.

I see many builders install a Zehnder systems themselves.They have some u-tube install advise too.

Good luck! 


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