I am looking at designing a house (who knows if it will ever be built....trying to figure out if building a home I want with my specs is about the same price as buying a similar one without my specs, should I have to move) and have stumbled across the ERV/HRV questions. I understand what their purpose is and what they do (the main idea is that they bring in fresh air into a tight house) and I understand how they vary from one another (one uses the heat recovery, so the intake air is close to the same temp as the outgoing air while the other helps control humidity), but the idea of them seems to be a little contradicting with various other systems in the house, particularly various heating and cooling options that people tend to cut out in order to either save monthly energy costs or to build a little "greener". It should be noted that so far I am almost decided on ICF for my plans. So here are my concerns, I hope you can help.

1) Use of ERV/HRV with radiant heating system: Reason 1 for radiant heating system is so you don't have moving air, yet by adding the ERV/HRV, you create moving air. Seems somewhat contradictory. Reason 2 for radiant heating system (hydronic) is to use less electricity, but then you turn around and use electricity for the ERV/HRV. Reason 3 is to become more "green", to use less resources, or at least waste less, which is commendable, however, I don't know that this combination does that, which is why I'm having the questions. Assumption is that the ERV/HRV uses the same ducts that is needed for the air conditioning, is this correct?

2) Use of ERV/HRV with geothermal based electric heat pump: Reason 1 for Geothermal is because the heating of the water to the required temp will cost less on a monthly basis, because it requires less energy to heat from 70 degrees (I believe that to be the output temp of the geothermal portion) to 100 degrees (heat exchanger requirement) than what it does from 50 degrees (assumed cold water temp for fresh water entering the house) to 100 degrees (heat exchanger requirement). The "Green" portion is that it requires the homeowner to use less energy on a monthly basis. However, you still need a water pump for the geothermal water to get into the house and the ERV/HRV still requires electricity, as does the heat pump (to heat the water and to blow the hot air through the ducts). Does the ERV/HRV work into the same ducts used for the heating/cooling? If so, do the two systems run at the same time, possibly forcing more air than needed into the ducts? The end result of the mixing of the two air cycles is that the hot air from the heat pump will lose some heat from the not so hot air from the ERV/HRV (remember, the air from these systems will be below room temperature) before it even gets to the room, meaning the electric pump will have to run a bit longer to get that final temperature to the thermostat setting in the air return.

3) Use of ERV/HRV with high efficiency gas furnace: Reason 1 for high efficiency gas furnace is that it burns more efficiently than other gas furnaces. Reason 2 is that it is generally a cheap option. The condensation can be used to add humidity to the house, if needed, or discarded if not needed. Again would the ERV/HRV, use the same ducts as the furnace? If so, the same question as above applies. If not, options can be added to these furnaces to help monitor air quality, which I think would make the separate ERV/HRV system redundant and therefore a waste of electricity.

4) Should ERV/HRV require their own ducts? Out of all the various things that would need exhaust in a house, I can't think of any that would be good to link with the ERV/HRV except maybe the bathroom ventilation. Any natural gas/propane or wood burning exhaust, I would expect, would require it's own exhaust so the various chemicals would not leave any residue/creosote build up in that exhaust duct which could be a fire hazard. The same for a stove exhaust, as grease would build up and the lint from dryers could cause a fire hazard. And indeed, any of these things would shorten the life of the ERV/HRV fans.

5) A little off topic, but can geothermal heating produce hot enough water to use in radiant flooring? If so, that would be a great way to recover a lot of costs....not having to pay to heat the water.

I understand that decent indoor air quality is a must, but I wonder if perhaps what we initially believe as lowering our bills is really just allowing us to pay for an additional system. What seems to be the payback of the HVAC and Air Quality systems combined? Is anyone actually saving money running both of these systems combined compared with their bills before? How much of that savings is due to the savings from the rest of the construction (air tight ICF's, SIP's, a lot of extra insulation, better windows, etc.)? Yes, you can call me a critic of new technologies, but I really just want to do what's best for my pocketbook and if a new technology saves the gas or electric company a few bucks (less production needed), it should save me a few bucks as well, as opposed to just shifting cost from monthly to upfront. Which is why I'm asking, because I don't know if it's really needed, which cases it's needed, which cases it isn't; or if it is just something people want for their own comfort level. That way, when I deal with a contractor (with HVAC needs in any house, bought or built, the contractors try to sell ERV/HRV systems) I can better understand whether I need it, or whether I will want it (distinct difference which will be needed to address financial concerns).



Thanks,
Chris Gallaway

P.S. Before anyone starts calling me a critic of efficiency needs and "green" building, let me add that I am a critic of "green Building" because there is no real definition of "green" it is a buzzword for salesmen to use on the simple minded. And, all this would be unnecessary, if, we could make a geothermal steam generator for the residential market (using a heat exchanger and closed loop system with a low boiling point fluid in order to create electricity).

Chris,
Lets simplify things a little bit. If you build a super tight house, you must make some provision for getting rid of the stale air and bringing in fresh air. If you don't, you get sick and/or die. Now HRV/ERV's are not the only way to do this, but they are one of the best ways, because you have more control over the process. The 'stale' air should be removed from bathrooms, utility areas, and the kitchen (areas where the stalest air is located). The 'fresh' air can be introduced into the HVAC system (if you have one), or ducted to the living areas and bedrooms. The HRV/ERV ducts should NEVER be tied in with any other vents such as furnaces, hot water heaters, kitchen stoves. And remember, a well built and installed HVAC system does not introduce fresh air, it simply recirculates the air already in the house. (Unless you purposely provide a source of outside air).
BTW-my definition of 'green' is anything that saves resources, and my money, while paying for itself, in a reasonable length of time.

Thanks Wes. I've read a lot the past few days on this site and have noted that your responses are generally very useful and informative, without sales pitches. I really do appreciate that.

Chris
In most HRVs the only moving parts are two small fans so they aren't using a lot of electricity. Their efficiency in recovering heat varies from 60 - 90% so that with the most efficient you lose very little heat. Check out the website www.hvi.org - there you will find a list of brands and models with their electrical usage and their heat recovery efficiency.

To respond to your points in order.

1>HRV flow volumes are a TINY fraction of what you see with air conditioning, fossil-fired hot air or geo-air heating systems. It's also a miniscule amount of power compared to geo compressors & pumps/blowers.

2>I think you're misunderstanding the operational temps of geo. In heating mode the ground heat exchangers are running at or below typical deep well temps in your area (if that's 50F in your area, you'll start out at 50F, but it'll drop with sustained heating load, sometimes getting down to freezing levels or below late in the colder part of the heating season.) The water temp at the fan coils will be ~90-100F, resulting in somewhat tepid air at the output registers compared to typical fossil-fired hot air systems. The "green" part is that it delivers better than air-source heat pump efficienciencies, and on average ~3-4x+ the heat output of resistance heaters per watt of power in. At those efficiencies burning fossil fuel at the power plant to run the geo is at or above parity with burning fossil fuel in a condensing heating system on-site.

3>Gas exhaust condensate is never used for humidifying the air, as that would concentrate the acidity of the condensate to be disposed of. If the house is tight enough to need an HRV, it's usually necessary to de-humdify rather than add humidity to the air in winter. Between cooking, bathing, and breathing mammals, tight houses accumulate air moisture. Winter dryness inside a home is usually a symptom of excessive ventilation rates (controlled, in the HRV case, or uncontrolled, in an air-infiltration scenario. Reducing the duty cycle of the HRV is usually trivial compared to reducing uncontrolled infiltration.)

4>For highest heat recovery efficiency HRVs usually require their own balanced duct system, but can sometimes use the same ducting as heating & AC.

5>There are many very efficient geo-based radiant floor heating systems, but whether it can work in a particular situation depends on a lot of the other design issues. It's primarily an issue of having sufficient radiation that the necessary water temp low enough that geo can make it efficiently- it's typically a losing proposition if you need 110F+ water much of the (which would be on the very LOW end of domestic hot water temps.) Most radiant slabs never need more than 90F water, which is quite reasonable for geo. Pre-heating your domestic hot water with geo, in combination with another hot water heating system can work though. Geo can do the heavy lifting, getting the water up from 40-50F to 90-100F with great efficiency, but you don't want to store water at those temps though (legionella hazared etc). Any potable hot water storage needs to be 120F+ to halt active growth of pathogens, 130-140F to actively kill them in the tank. If part of the hydronic geo system's heating water buffering is a tank with an internal heat exchanger, very little domestic water is in the heat exchanger, which gets purged with nearly every hot water draw, eliminating the tepid-water storage. (ErgoMax, TurboMax, Everhot EA series tanks can be used for this.) Alternatively- preheating the water with a potable loop of PEX in a radiant slab is easy to do, albeit at lower pre-heat output temperature (~75F instead of 90F).

Thank you all for helping with my understanding of these systems. As I've read your responses and had done other research, I do understand the necessity for a "tight" house to have the ERV/HRV system in place and my design will include it. Dana, thanks especially for responding to #5....the growth of pathogens is not something that I even thought of. Still a lot more stuff to look at, but I think this is starting to come into view.

Thanks Again!