ERV or Central fan integrated ventilation best for hot humid climate?
I read an article by Martin Holladay who said ERV could actually make the situation worse in hot humid climate during those days when temp and humidity level are mild. That confused me. I would appreciate answers to questions about the best vent. system for a new build air tight house at approx. 2200 sq ft, in zone 3A below the red line. Heat pump HVAC system.
1. Why might an ERV make matters worse on mild days with low humidity?
2. Would a central fan integrated system be better or as good using the existing HVAC ducts to ventilate? Is that system a positive pressurized sys. with fresh air supply line cut into the HVAC ducts at the plenum next to air handler? Where is that system’s exhaust line cut in?
3. Since the HVAC ducts move around 1200 cfm of air and only about 120 cfm of moving air is needed to provide good ventilation, would ventilating through the larger HVAC ductwork pose problems or energy waste because of the disparity between the ducts capacity and the actual amount of capacity needed?
4. The HVAC contractor said that running HVAC ducts can be problematic when a house has a lot of pot lights in ceiling (which our plan calls for), so, wouldn’t it make it almost impossible for him to find room to run dedicated ventilation lines in addition to HVAC ducts? It seems that the only solution might be to use existing HVAC ducts to ventilate, if not the most efficient.
5. Can you use an ERV without dedicated ductwork? Having hard time picturing that system.
6. I guess either system will require a humidifier tied into the air handler with drain to outside, and a ECM with fan to run at low speed to handle moisture between the heating and cooling cycles of heat pump. Is that right?
7. I guess I also have to have a humidistat integrated with thermostat.. Does the humidistat control both the ECM fan and the humidifier?
8. Pros and cons about the two types of ventilation systems would be appreciated
9. Confused on whether ERV has any relevance regarding moisture removal in house if you also have a humidifier tied into system, and what is purpose of ECM fan running on slow speed between heat/cool cycles if you already have a dehumidifier working. I am definitely missing something. Please explain.
As you can tell, I am a lay person regarding HVAC and Ventilation, so answer accordingly. Thanks.
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Replies
Rob,
Lots of questions. I'll do my best to answer some of them.
Before I begin, however, I'd like to provide links to two relevant articles. You mention that you have read one of my articles -- I'm not sure which one, however. In case you haven't seen them yet, I advise you to read these two articles:
Designing a Good Ventilation System
HRV or ERV?
Q. "Why might an ERV make matters worse on mild days with low humidity?"
A. I'm not sure what you mean by "worse." Running an ERV always incurs an energy penalty. It takes energy to run the fans, and the air that enters the house has to be conditioned (heated in winter and cooled in winter), requiring more energy. When it comes to figuring out whether an ERV moves the indoor relative humidity in the correct direction, that's a complicated equation. It depends on many factors, including how humid the indoor air is, how humid the outdoor air is, and the homeowner's preferences. In general, running an ERV during the winter generally lowers the indoor humidity level, which can be either good or bad, depending on what you want. Running an ERV during the summer usually (but not always) raises the indoor humidity level (especially if you live east of the Rocky Mountains), which is usually undesirable.
Q. "Would a central fan integrated system be better or as good using the existing HVAC ducts to ventilate?"
A. Either system can work. In general, an ERV or HRV with dedicated ductwork is the preferred system; however, it is more expensive to install.
Q. "Is that system a positive pressurized system with fresh air supply line cut into the HVAC ducts at the plenum next to air handler?"
A. Yes.
Q. "Where is that system's exhaust line cut in?"
A. A central-fan-integrated supply ventilation system is a supply-only system. If you want balanced ventilation, you wouldn't choose this type of system. However, independent bathroom exhaust fans (not connected to the ventilation system) can always be installed and usually are. They are operated by flipping a switch in the bathroom, whenever the homeowner wants.
Q. "Since the HVAC ducts move around 1200 cfm of air and only about 120 cfm of moving air is needed to provide good ventilation, would ventilating through the larger HVAC ductwork pose problems or energy waste because of the disparity between the ducts capacity and the actual amount of capacity needed?"
A. The short answer is yes. The long answer is that energy waste that accompanies a central-fan-integrated supply ventilation system can be minimized if the furnace has an ECM blower, if the ventilation system includes a FanCycler control and a motorized damper on the outdoor air duct, and if the system is carefully commissioned. However, this type of system will always require more energy than an ERV or HRV with dedicated ventilation ductwork.
Rob,
Q. "The HVAC contractor said that running HVAC ducts can be problematic when a house has a lot of pot lights in ceiling (which our plan calls for), so, wouldn't it make it almost impossible for him to find room to run dedicated ventilation lines in addition to HVAC ducts?"
A. I don't know whether your recessed can lights are in an insulated ceiling (on the top floor of your house) or in an uninsulated ceiling (between the first floor and the second floor). If they are in an insulated ceiling, I strongly urge you not to install them, but to choose a different type of light fixture. I they are in an uninsulated ceiling, you are correct that you need to have a plan about where to locate your ducts before beginning construction. You may want to have a higher ceiling, or you may want to plan to install a few soffits. Design takes planning. For more information on this topic, see Keeping Ducts Indoors.
Rob,
Q. "Can you use an ERV without dedicated ductwork?"
A. Yes, but the systems are sub-optimal compared to a system with dedicated ductwork. All of the major ERV manufacturers provide installation instructions that are posted online, and all of these instructions include diagrams that show you how to install an ERV that is hooked up to forced-air ductwork used for home heating and cooling. I don't recommend this approach, but you can read all about it on the website of your favorite ERV manufacturer.
Rob,
Q. "I guess either system will require a humidifier tied into the air handler with drain to outside, and a ECM with fan to run at low speed to handle moisture between the heating and cooling cycles of heat pump."
A. No. I don't think any house should ever include a humidifier. The use of humidifiers is associated with sheathing rot and moisture problems in walls. If your house is so dry during the winter that you think it needs a humidifier, the building envelope is probably too leaky. The solution is to perform air sealing work.
Concerning your second suggestion -- the installation of "an ECM with fan to run at low speed to handle moisture between the heating and cooling cycles of heat pump" -- I have no idea what you mean, but I think you are trying to describe certain components that make up a central-fan-integrated supply ventilation system. The equipment you describe is not required if you install an HRV or ERV with dedicated ventilation ductwork.
Rob,
Q. "I guess I also have to have a humidistat integrated with thermostat.. Does the humidistat control both the ECM fan and the humidifier?"
A. Most ventilation systems don't include a humidistat, but a few do. A central-fan-integrated supply ventilation system is controlled by a FanCycler (also known as an AirCycler), not a humidistat.
Most ERVs or HRVs are either controlled by a 24-hour timer, or run continuously.
Once again, I strongly urge you not to install a humidifier.
Rob,
Q. "Pros and cons about the two types of ventilation systems would be appreciated."
A. An ERV or HRV with dedicated ductwork will require less energy to operate than any other type of ventilation system. However, it will cost more to install than any other type of ventilation system.
A central-fan-integrated supply ventilation system will be (a) cheaper to install, but (b) won't be balanced and (c) will cost more to operate.
Rob,
Q. "Confused on whether ERV has any relevance regarding moisture removal in house if you also have a humidifier tied into system, and what is purpose of ECM fan running on slow speed between heat/cool cycles if you already have a dehumidifier working. I am definitely missing something."
A. Once again, a humidifier would be a mistake.
An ERV is a device that provides ventilation. Its main purpose is not to adjust indoor humidity levels; its main purpose is to provide fresh air to occupants. Inevitably, however, running an ERV can raise or lower indoor humidity levels.
If your main interest is lowering indoor humidity levels, then you should choose an HRV, not an ERV. To lower indoor humidity levels during the summer, you need to operate an air conditioner or operate a dehumidifier.
Martin,
I am really sorry. I said humidifier when I meant dehumidifier. Changes everything. What I was trying to get at is in the coastal humid climate in SC that a dehumidifier would probably be needed to move the moisture out that an ECM fan could not handle. I'm not even sure I understand how the ECM fan and the dehumidifier work together. From what I thought I understood, the ECM fan turns on at a slow speed when the heat pump cooling (or heating) is cycled Off, so humidity won't build up in house. But if the outside humidity is very high, then somehow the dehumidifier kicks on to take on the heavy moisture load.
Martin,
You said that an ERV might make the indoor moisture greater east of the Rockies. Since the ECM transfers the moisture from incoming humid air over to the exhaust line and out, I thought that an ECM was better because it would remove a certain amount of moisture before it actually got in the house. I'm guessing from your comment, however, that the amount of moisture removed (transferred) would not be anywhere nearly enough to keep a house at 55-60 degrees humidity in hot humid summer. When you said an HRV would be probably be preferable if humidity is the concern, that confused me, since it has no moisture removal. Trying to figure it out, I guess that an ERVs moisture removal capability is minimal, so the solution is the dehumidifier that turns on when needed to get humidity down to 55-60. I should not think of humidity control when thinking of an ERV.
As to running dedicated ventilation ducts, your comments made me realize that in order to keep can lights, we will have to design a larger ceiling cavity, which can be done I think. The roof will be spray foamed to enclose whole house, so if that means we don't need to insulate ceiling, then a larger ceiling space would probably create enough room for vent ducts along side HVAC ducts. Thanks for your guidance.
Rob,
The "ECM fan" your are talking about is presumably a furnace fan. I think you are talking about a central-fan-integrated supply ventilation system.
If that is what you are talking about, remember this key fact: a ventilation system is installed to bring fresh air into your house. During the summer and swing seasons (in your climate), it is not designed to lower your indoor humidity levels. You need an air conditioner or a dehumidifier for that.
If the outdoor air is warm and humid, then the operation of any type of ventilation system -- whether it is a supply ventilation system, or a balanced ventilation system with an HRV or an ERV -- will make things worse. During such weather, the usual recommendation is to keep ventilation to a minimum, and to operate an air conditioner or a dehumidifier if necessary.
During summer weather, an ERV will bring more moisture into your home. However, it will be "less bad" during the summer than an HRV -- assuming that your home is air conditioned (or made dry with a dehumidifier). On the other hand, during the winter, an HRV will do a better job of lowering your indoor humidity levels than an ERV.
Martin,
Thanks for clarifying some things. I guess the one thing I am still confused about is your saying that during the humid summer, the ERV would bring more moisture into your home. I thought that the main difference between ERV and HRV was that ERV removed some moisture by transferring it from supply line coming into house over to the exhaust line and out, before the moisture ever got into house.
As for the ECM, I thought that with a variable speed blower you need an ECM motor to run it, and that the ECM replaced the single-speed motor on an air handler. Anyway, thanks for your answers.
Rob,
Q. "I thought that the main difference between ERV and HRV was that ERV removed some moisture by transferring it from supply line coming into house over to the exhaust line and out, before the moisture ever got into house."
A. You've got it right -- "some moisture," but not all moisture. In a humid climate, the longer you ventilate with an ERV during the summer, the more moisture you introduce into the house. The less you use your ERV, the less moisture you will introduce into your house.
Q. "As for the ECM, I thought that with a variable speed blower you need an ECM motor to run it, and that the ECM replaced the single-speed motor on an air handler."
A. That's basically correct. An ECM is a type of motor. It is found on some high-end furnaces and air handlers. An ECM is not, in and of itself, a ventilation device. However, if you choose to install a central-fan-integrated supply ventilation system, it's best if your furnace includes an ECM blower.
Thanks, Martin, I think I get the idea now that even though the ERV will remove "some" moisture from entering house, in very humid weather, the amount will be insignificant, and that is where the dehumidifier is needed. I reread the article "ERV or HRV" and found the part where you say "Operating an ERV during the summer in Houston, Texas, doesn't lower the house indoor RH; rather, it makes the situation worse." I now understand that statement: the ERV supply fan is bringing more moisture into the house in humid days than the exchanger can eliminate by way of exhaust duct, So using ERV in very humid weather is therefore making RH level worse. Then, as you recommend, don't use ERV as much during those humid days. But how then do you adequately ventilate house if you have stopped or reduced ventilation in order to prevent excess moisture from entering home?
Rob,
Q. "How then do you adequately ventilate your house if you have stopped or reduced ventilation in order to prevent excess moisture from entering the home?"
A. You can ventilate your house at any rate you want. If you like lots of fresh air, run your ventilation system 24 hours a day. Just be aware of the energy penalty: the more you ventilate, the higher your energy bills. And during hot, humid weather, you'll also need to run an air conditioner or a dehumidifier.