Integrated ERV for multi-floor new ICF construction
New three story ICF construction / Andersen Series A windows, blown insulated attic in coastal area of Delaware. (No basement)
Geothermal HVAC – Two independent ducted zones
Zone 1: Ground Floor + 1st Floor (total square foot = 2,400)
Zone 2: 2nd (top) Floor (total square foot = 3,000)
Floors are connected by 4 foot stairwell “tower” with landings on each floor.
This is a very tight house and needs ventilation.
Builder forgot to install ERV (in spec) until after home was 99% complete, drywall, trim, paint, etc.
Builder/HVAC contractor who installed the Water Furnaces need to retrofit ERV into the home and are recommending that a single ERV be installed and tied into the Zone 1 only. Room Air from ERV would be tied to Return of furnace. Fresh Air from ERV would tied to Supply of furnace. They say the ERV blower may be operated separate from the Furnace Blower.
1) Will one ERV be sufficient to provide adequate air exchange to 2nd floor? They tell me there will be sufficient “mixing” of fresh air via the single stairwell.
2) I’ve read that it’s best to have completely separate ducted ERV system from HVAC, but since this would not be costly, the builder/contractor are recommending to use existing duct. Is this a valid design?
The design recommended is documented n the RenewAire Installation and Operation Manual for Models EV70, 130, 200, 300 in schematic “D” – title: “Furnace Return Air Into Furnace Supply Air”.
Thanks in advance for any input on this issue.
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Replies
Daniel,
There are two issues here. Once concerns ventilation performance; the other concerns contract law.
An ERV system with dedicated ventilation ductwork will perform better, and use less energy, than an ERV connected to a forced-air heating system. For more information on this topic, see Ducting HRVs and ERVs.
Concerning contract law: I don't know the details of your construction contract, but if you contracted for a house with certain features, and the contractor forgot to include those features, then in most cases the contractor is responsible for the cost of remedying the error. (Needless to say, only a lawyer who can review your documents can confirm this hunch.)
Thanks, Martin. I appreciate the reply. As an update, I spoke with the folks at RenewAire and they were super helpful. Here is what they said: Here is what they said:
"Ideally, the ERV has a dedicated duct system to the entire house so they operate independently.
But, they recommend that when the ERV is tied into existing HVAC ductwork, that there should be one ERV per zone.
So, we have two zones – two ERVs – they recommend EV130 for Ground/1st Floor and EV200 for top floor installed in the attic each with a Proportional Timer."
This makes sense to me – I think the single ERV design presented by my builder is a 50% solution and not a good one or acceptable to us.
We don’t believe we would get sufficient coverage / air exchange on the top floor – especially given that is where we will be spending all our time, and after talking to the manufacturer’s engineers.
The attic unit can be an independent duct system given the easy access to living areas and hung from the rafters (sound isolated) with exhaust/fresh air exiting through the soffits (so no drilling in concrete there) as we did the bathroom fans – separated 10ft apart.
The Fresh Air supply from the ERV on the 2nd floor needs to be introduced carefully so it doesn’t blow directly on people.
BTW, my builder has been great about this "miss" and is willing to pay for the retrofit since it is in spec.
Martin's article discusses the disadvantages of design "C". Design D also has disadvantages, that are a little different:
1) Unless you add a backdraft damper to the heat pump air handler, when the air handler is off, some of the ERV supply air will go backwards through the furnace, so you won't get as much fresh air into the house as you are "paying for" (paying by supplying the fan power).
2) When the air handler is on, the ERV will be working against the pressure created by the much higher airflow of the air handler, and again, the flow will be reduced, so you aren't getting what you paid for.
Another disadvantage of shared ducts is that you lose at leas some of the opportunity to target exhaust in high-humidity spaces like bathrooms and the kitchen.
A possible mitigation for the "getting what you pay for" disadvantages would be to get an ERV unit with higher efficiency fans--a unit with "ECM" fans will use something like half the fan power for the same flow. You could insist that the building pay for that upgrade, and you might end up with the same effective ventilation rate per unit fan power, or probably even better, as you would get with dedicated ducts and a crappy-efficiency fan motor.
Another option is the go with the Lunos ductless HRV/ERV system. http://foursevenfive.com/product-category/ventilation/ Some set of e2 and eGO units could provide the ventilation you want with no ducts and very low fan power.
Is the attic conditioned or is it vented? Putting ducts in an unconditioned (vented) attic is not good practice.
I am curious about a house that has 600 ft^2 more area on the top floor than the combined area of the two floors below. Is that area on top of a garage or something?
Thanks, Charles. The article by Martin was very helpful, and it is helpful that you elaborated on this other design option, as there is no discussion "Design D" in his article. Your points are well taken.
First, my HVAC person did say that the backdraft damper would need to be installed. But it's confusing to me to see one manufacturer say this design is acceptable and then another (American Standard) say this is not acceptable and can actually "make the house a negative pressure volume". We have a LP fireplace within the envelope so this concerns me greatly. (See attached diagrams.)
Your point on the ECM is also a good one - I believe the RenewAire residential are not ECM. Surprisingly, there is no spec, that I can find, that specifies the fan type, but I suspect it is a PSC type for residential as I can see that the commercial versions offer ECM as an "upgrade".
On the attic: it is a 17'X12' wood framed, asphalt-shingled unconditioned space. Totally sealed with blown insulation and a small access (uninsulated) door from an adjacent room that has a vaulted ceiling. There are no soffits or venting of any kind consistent with ICF construction. Insulated HVAC supply and return ducts are run there as this is a centrally located space.
The one question I have that neither Martin or you addressed: 1) Will one ERV be sufficient to provide adequate air exchange to 2nd floor? The manufacturer recommends one ERV per Zone/Floor. Do you agree or could I use one ERV in a single Zone/Floor for the whole house even though the zones are totally separate/independent - one for each floor.
Also, I will check out the Lunos system - thank you for the lead.
Dan
Dan,
Q. "Will one ERV be sufficient to provide adequate air exchange to 2nd floor? The manufacturer recommends one ERV per Zone/Floor."
A. Different homeowners have different expectations for the performance of a ventilation system. Some homes have no ventilation system -- so any attempt to introduce a little fresh air and remove a little stale air is an improvement in that type of house.
Not every homeowner needs a Cadillac ventilation system, or cares whether fresh air is delivered to most rooms. Some do.
If you have specified a house with a fully ducted ERV system -- a Cadillac system -- then you have every reason to insist that your builder provide it. If you settle for less, the system will still work -- it just won't perform as well.
Thanks, Martin. Your article was helpful. I'm learning a lot here and I'm leaning to allow the retrofit to be tie into existing HVAC ductwork as the spec is not that specific and I don't want the disruption of tearing the house apart again, plus I don't need the Cadillac solution, but we do want a safe and functional design.
Do you have a view on the recommended design to tie into the HVAC system? I'm seeing conflicting info from two ERV suppliers - one says the design is fine, the other says it can cause issues. Which to believe? Is the RenewAire approach safe - especially with an LP fireplace within the envelope?
Dan
Daniel,
Your description is confusing, so I don't know what is being proposed. In all cases I'm familiar with, HRV or ERV ducts are connected to the main return duct of a furnace, not to a supply duct.
ERVs have 4 ducts.
2 of these ducts go from the ERV to exterior terminations: one is the fresh air intake duct and the other is the duct that removes stale air from the building.
2 of the ducts go from the ERV to interior diffusors or grilles. One is the fresh air duct that delivers fresh air to the house. The other is the exhaust duct that pulls stale air from your bathroom, laundry room, and kitchen.
You wrote, "Room Air from ERV would be tied to Return of furnace. Fresh Air from ERV would tied to Supply of furnace. They say the ERV blower may be operated separate from the Furnace Blower."
I think I know what you mean by "fresh air from ERV," but I don't know what you mean by "room air from ERV."
Thanks, Martin. Sorry for confusion. By "Room Air from the ERV, I mean my HVAC person is recommending a design so the stale air pulled from the HVAC return duct that feeds into the "Room Air" duct of the ERV.
Maybe it's best to just look at the attachments from two posts ago.
The design recommended by my HVAC installer AND RenewAire engineers is depicted in Option D in the attachment above.
Yet this exact design is a no-no as depicted in another ERV supplier (American Standard) installation guide. (Option IV in the attachment above).
In both cases, the Fresh Air from the ERV is dumped into the supply duct of the HVAC unit and the Room Air is pulled from the Return duct of the HVAC.
(I believe this design is so I don't have to run the HVAC blower continuously.)
In both cases, the ERV unit is functionally identical (i.e. four ducts as you accurately described them).
So which do I believe, and to add to my concerns, American Standard says this design can create flue gas back drafting (I have an LP fireplace in the home so that would seem to create a danger).
Yes, there is potential for depressurization in the option D/Figure 10 arrangement. I think they are considering the scenario in which the supply ducts are long and skinny and extend to the furthest reaches of the house, whereas the return ducts are big wide short ducts. That results in higher impedance to airflow in the supply duct system than the return duct system, and a high pressure where the ERV connects to the supply duct. That reduces the fresh air supply rate of the ERV, while the exhaust rate is closer to normal. So with exhaust exceeding supply, the house is depressurized.
You can compensate for that by adjusting the fans or adjusting the dampers to slow the exhaust and get it better balanced, but then when it's better balanced for operation with the HVAC running, it's balanced wrong for when it's not running...
In theory, if your supply and return duct impedances were balanced, there would be no depressurization issue. If you have someone doing detailed engineering of your duct system, they might be able to figure out how much of an issue that is in your particular case, but I don't think that many duct systems get designed that carefully.
One solution would be automatic control that maintained neutral pressure. One of the ERV units on the market has an option to add a control that does that. http://www.ultimateair.com/ (They also have good ECM fans) I don't know what they say about the different connection options, but that would be worth a look.
It's interesting to see that the drawings from the two manufacturers look almost identical...and interesting that the one that does not recommend it still specifies minimum distances.
Having four ducts does not necessarily mean the ERVs are functionally identical. They may have different types of motors. Some motors tend to adjust to changes in the resistance to maintain a nearly constant airflow rate. Other motor operate at a pre-programmed power and the airflow changes as the resistance changes. When your furnace runs, there is a pressure drop in the return duct and a pressure increase in the supply duct. Some ERVs may adapt to this by changing motor power and keeping airflow rate nearly constant. In other ERVs, the airflow rates may change substantially.
Thanks, guys. I'm an IT trained techie and an HVAC wannabe techie/junkie now! So this stuff actually makes sense. I really appreciate all the input here.
I actually did check out the UltimateAir 200DX as it gets great reviews in testing here: http://www.northwestenergystar.com/sites/default/files/resources/HRV%20and%20ERV%20Best%20Practices.pdf
UltimateAire calls it: "Option 3 - Shared Forced Air Supply, Shared Return".
I've collected almost every installation manual for every ERV/HRV I can find online. The nomenclatures are not consistent and not all suppliers suggest the design we are discussing as an installation approach about half include it, the other half don't mention this as an option or in the case of American Standard, say it's not advised). Of the ones that do include this design, some say that the HVAC blower does not need to operate in sync, others (like Ultimate) suggest that the air handler run continuously and the ERV and HVAC be interlocked.
One tyhing I'm learning, is that I should require the HVAC installer to test for "balance"!
I'm really kicking myself for not catching this "miss" by my builder before rough in. All of this would be a moot point if we had done it right in the first place - as the best practice here seems to be new construction = dedicated ductwork (i.e. separate ERV and HVAC systems). But then again, maybe I would not have had this opportunity to do a deep dive and increase my knowledge of the subject thanks to y'all!
Daniel,
I'm a very strong advocate for dedicated ventilation ductwork. In my article, I explained two approaches to using heating ductwork for distributing ventilation air; both have major drawbacks. I'm not familiar with other approaches, although you're right -- there are more ways to do this.
Most ventilation systems that use heating ducts to distribute ventilation air have a huge drawback -- the need to operate the furnace blower for more hours a day than it would otherwise be used. That's a big problem, because many furnace blowers are big energy hogs. Do you know how many watts your furnace blower draws? Do you know whether your furnace has a ECM blower?
In any case, my advice will go back to first principles:
1. If you can possibly manage to do it, install dedicated ventilation ductwork.
2. If you can't follow Principle #1, follow the manufacturer's installation instructions.
3. If you end up using heating ductwork to distribute ventilation air, make sure that your furnace has an ECM blower.
The Ultimate Air manual says about the connection you are considering:
"If you use this method, we recommend that you wire your RecoupAerator® and your air handler to run simultaneously (i.e., they will be interlocked) to prevent recirculation shorting thru back past the furnace air handler."
As Martin says, that's bad from the point of view of fan energy use.
But it seems like a backdraft damper solves that problem, and the pressure-based control option solves the potential depresurization problem, with the net result that it's a pretty good plan. You just need to be sure the backdraft damper doesn't introduce a pressure drop that reduces airflow or makes the fan work harder in the HVAC system.
The RenewAire folks and my HVAC installer tell me I don't have to operate the HVAC blower for option D above. It is also noted on the diagram at the bottom. Yet, Martin, you are saying I have to operate the blower and so do some other manufacturer's installation instructions. So which to believe? Maybe with the backdraft damper, I can go with this option from RemewAire. Also I found a great document on best practices that shows this as one workable option, but not preferred to dedicated ducting. They als suggest that ERV supply duct be introduced into the HVAC supply duct by using a rigid elbow duct inside the the HVAC duct so the air is forced away from the blower.
Also, both Water Furnace HVAC air handlers have ECM blowers - but a moot point if I can use a design that does not require them to be interlocked.
Also here is a link from my dropbox to a "best practices" presentation that I found to be very helpful:
https://www.dropbox.com/s/w2ceb2mwkhymhk0/Best%20Practices.pdf?dl=0
A shout out to the Portland Metro Home Builders Association for this 2012 document on HRV, ERV Best Practices!
https://www.dropbox.com/s/w2ceb2mwkhymhk0/Best%20Practices.pdf?dl=0
The differences in installation instructions may be due to different philosophies but they may also be due to differences in the products.
You also need to ensure that the furnace will work properly when the ERV is running. In your proposed ducting arrangement, the ducts must carry the sum of the ERV flow and the furnace flow. Since the ducts were likely designed based on the furnace flow rates, it is worth looking into whether they were designed with enough margin that they can handle the additional flow from the ERV. You may actually want a reverse interlock that turns the ERV off when the furnace is running. If your furnace is as over-sized as most, that would still give you plenty of ventilation.
If you connect both ports of the ERV to the return air duct of the furnace, properly separated, the ductwork will only handle the furnace flow. That is great when the furnace is running. Of course, when the furnace is not running, the furnace flow is zero which is unacceptable for ventilation so an interlock is mandatory. One thing I like about this configuration is that the fresh air temperature is not as critical because it gets conditioned before being sent to the rooms.
I believe the RenewAire is designed to run for a controllable fraction of the time at a fixed flow rate. I believe the UltimateAir, on the other hand, is designed to run all of the time at a controllable flow rate. For an interlocked system, the RenewAire scheme may be preferable if the control logic is sufficiently sophisticated. I know it is capable of triggering the furnace blower to run, but I don't know if it is smart enough to schedule its own run time to coincide with the time the furnace needs to run anyway,
OK, thanks Reid - good to consider as I talk to these manufacturers.
Let's say for assumption purposes, we go with tying into the existing ductwork...
Do I
1) "settle" for one ERV for on e of the Zones (i.e. entire floor) and assume the stairwell will provide ventilation to the other floor? as suggested by my builder and his HVAC sub?
2) one ERV unit per zone as suggested by RenewAire?
The ERV manufacturers are in no position to tell you whether your ductwork is designed to accommodate the additional flow. For that, you need to talk to the person that specified the existing system. I don't have any particular insight regarding whether two ERVs would improve the ventilation by enough to justify it.
Reid,
Ventilation airflow (in cfm) is trivial compared to heating and cooling airflow. Most homes are adequately served by 60 cfm to 120 cfm of ventilation airflow. Furnace fans easily blow 900 to 1200 cfm.
With regard to most homes, Martin's comment above makes sense. I have been looking into Dettson's Right Size System lately which has lower air handler flow rates than typical furnaces and probably skewed my intuition about relative magnitudes.
Regarding Daniel's home, the ASHRAE formula for a 5400 ft^2 house with 4 bedrooms would indicate a ventilation flow rate of 200 cfm. (I recognize that not everyone buys into that formula.) If the ERV is designed to run part-time, then the flow rate when it is running may be considerably higher than that. If the flow is split between the two furnaces, say two RenewAire EV130s that each produce 130 cfm, then Martin's comment about it being very small relative to the furnace airflow applies. If all the ventilation is done through one zone, then the increase seems like enough that someone should do the calculations.