HRV, Humidity and Radon
I've been lurking around here for some time. I thought it was time to post my "big questions" since I've gleaned lots of great info but I still don't have a comprehensive picture of what I need to to. Here goes...
I have a 3 year old, very tightly insulated three level home in Minnesota. We have high Radon in the area although tests here have been clean.
The majority of the home is heated via a 2-zone forced air furnace air conditioner unit. The basement uses in-floor heat and is ducted to the forced air but most of the ducts are kept closed (yes, I know the basement is where the Radon builds up, more on that later).
There is a ducted Carrier HRV installed but it was installed into the existing HVAC ductwork and does not have it's own ducting beyond that. The installer did not put in an HVAC / HRV fan interlock. The controller is a Carrier comfort controller with humidity control and interval timer.
There is also a pad based humidifier attached to the HVAC output venting that is controlled by a mechanical humidistat on the HVAC intake venting.
There are at least 6 exhaust-only vents in the house including a high-flow indoor grill/griddle vent on the stove.
When I think about the discussions that have gone on here and look at what the HVAC contractor installed in the house, I see at least 5 things wrong with the setup. This house was built before we purchased so I did not have any input.
My biggest initial problem is humidity control. The A/C system does well keeping the summer time humidity down. In the early fall and late spring I do get some increased humidity when the A/C doesn't need to run. My *big* problem is humidity control in the winter time. We can get massive temperature swings over a period of a few days. Even though we have pretty energy efficient windows they will still freeze up and the window frames are painted enamel on wood. The moisture pops the paint right off. We also have fairly soft wood floors that shrink considerably if they get too dry for too long.
Every day I have to run downstairs and try to guess where I should set the humidistat based on where the temp will be over the next few days. Then I have to adjust the HRV to make sure that it won't try to fight against the humidifier. If I do it just right I can keep the windows pretty dry, our sinuses from hurting, and our floors happy. If I don't do it for a few days then everything goes to heck.
On to the questions:
1) Isn't there a controller out there that has an external temp sensor and can run the HRV *AND* humidifier as a single system? A smart controller should be able to do a way better job than me (and it won't forget when its busy).
2) Since the HRV doesn't have its own ducts, wouldn't it need to be interlocked to the HVAC fan? The shortest path between inside fresh and inside exhaust seems to me to be straight through the furnace unless the HVAC fan is running, which doesn't accomplish anything for the house.
3) Would it make sense to commandeer one or two of the basement ceiling HVAC vents and hook them directly to the HRV ducts to enable basement air recirculation? The vents are currently closed because the basement gets too hot/cold when the HVAC is sent in. There is no HVAC thermostat in the basement, only a water circulation controller.
Thanks for your time
The answers to your questions:
1- yes Honeywell makes a outdoor temperature compensated humidistat-dehumid controller. Or change T stat to an 8000 pro with humidification feature & outdoor sensor. It also has a fan cycle feature for ventilation applications. This would be your best solution in my opinion. I'm sure Carrier has a thermostat that will perform the same funtions as the Honeywell
2- Yes it should be interlocked to the air handler, your HRV should have a controller available as an option.
3- Yes that would help with your radon remediation, but you will have to hook up some exhaust registers also in the basement.
Thanks for the reply. I can't change out the T-stats because they are communicating stats to a central controller. Unfortunately the company does not support humidity control. Do you have a model # for the Honeywell, I'm coming up dry for one that does not also act as a T-stat.
Originally Posted by jackpiner
what's your problem
actually, I read your post two times.I haven't understood what's your problem for my a little bad English.
I really want to help you.
If you want to control humidity ,you should pay more money than only control temprture. I think.
and Hrv just give you more fresh air. at winter keep humidity ,Erv is better more than hrv.and Erv can save more Engery.
I found the smart Honeywell humidity controller (H1008D1002). It is installed and seems to be working properly. It is pretty darn nice with some internal smarts to try to determine the outdoor temp by watching the heat call cycles and humidity. It comes with an outdoor temp sensor if it is possible to get it run for better accuracy. For my region they recommend the sensor as the temps can swing so much in a short period. It also connects to the HRV override to dehumidify.
I'm having trouble locating the fan interlock relay board mentioned in the Carrier tech diagram. They mention the connection point but not the part#. A call to my local HVAC supplier resulted in a "Huh?" - you need what?
Does anyone know a part# or supplier?
Originally Posted by JOrt
Regarding the HRV install, it actually doesn't need to be interlocked in a return/supply simplified installation if certain conditions are met. However, if it's a return/return volume install and there is no interlock, you're just continuously ventilating the air in the six foot section of the return plenum between the HRV's two rigid taps, LOL.
So which type of an install do you have?
It is a return/supply installation. I did a bit of informal testing when I installed the humidistat on the return duct. When the HRV is running most of the air flow appears to be through the furnace (I think). I put a piece of paper into the hole cut into the duct and when the HRV was running the paper flapped, indicating flow below the HRV insert point, just above the furnace filter. It seems to me that the flow would be simply from the supply air to the return air ducts via the furnace, which is a closed loop not involving the house air. I'm sure a bit circulates through the house but the distances involved are many times greater when the ducting is considered.
How would a return/supply work without HVAC fan assist? Would it just be because of the resistance from the filter/fan/heat exchanger?
Thanks for taking the time.
Originally Posted by goldenear
Jon, a return/supply can work without the furnace fan if you've got a couple of things working for you. First, it's best to have the home's supplies and returns at different levels. So if your supplies are at the floor, you need your returns to be high. If you don't have this, then you'll experience stratification as fresh air is recirculated either at the floor or the ceiling without ever circulating the stale air through the HRV/ERV.
Second, it helps tremendously to have a thick media filter pre-furnace because it's a source of incredible resistance when dealing with low flow devices like HRV/ERV's. So when the little 100-200CFM motor tries to pull from the return plenum and it "hits" that filter, it's going to say I'd rather pull from the home's room returns because the resistance is so much less.
Third, on the fresh air supply side, you need to stick a 90* elle actually inside the supply plenum so it will direct the air in the direction that the furnace fan will push it when it engages. This will be away from the furnace fan obviously. Doing this also gives directionality to the air flow toward the room supplies when the ff is not running. Again, it's easier for the air to exit the supply registers along that trunk than it is for it to effectively fight itself and reverse course toward the furnace fan. Just sticking a collar or takeoff on that supply plenum is not going to work. That elle has to physically sit inside the trunk pointed in the correct direction.
Fourth, you can't close up the rooms where the fresh air is dumping into because that will positively pressurize those rooms while the remainder of the home is depressurized, resulting in a lack of fresh air distribution. So either everything has to be open, or you need to undercut your doors if they're ever closed.
Now, the layout of your home and your equipment will determine where to tap into the return and supply plenums. This is easiest to do with a single story home because all you have to worry about is the airflow on one floor. The general idea is to supply fresh air on one side of the home and then pull that fresh air across the home by tapping the return at the other end of the home. If your return and supply taps are right next to each other along the plenums, then you'll just change over the air in the area nearest those supplies assuming you have returns in those rooms. And the only time you'll get good distribution of fresh air is when your furnace fan runs. Pushing and pulling at opposite ends of the home solves this problem. And then setting your furnace fan to circulate makes everything work even better.
This is how I have modified my ERV install (from a return/return interlocked install) and it works exactly as described. Since my furnace is centrally located with both return and supply plenums running from one side of the house to the other (think a big "T" layout with the plenums ~62 ft in length forming the top of the "T" and the furnace is the | - the layout also makes this much easier to do), I figured I would get fresh air out of every supply register on the side where I tapped the supply plenum. After firing it up and removing the interlock, I went around and sure enough, that's exactly what happened, even on low speed (~100CFM). Fresh air is distributed out of every supply west of the furnace (& 6" media filter) while not a single drop of air is exhuasted out of the supply registers to the east of the furnace (post filter). But that's still ok because I'm pulling that air across the home by tapping the return plenum directly beneath the last return grill on the east side of the furnace. And this is how that mass of fresh air is pulled across the home.
Good explanation. I have a MERV8 AirBear on the HVAC. I'll have to pull the HRV connections off the ducts and see if it is just a collar or an actual ele.
The supplies are generally floor and the returns are near ceiling. The only things I don't have going for me is that the house is 2-level and the ducts are tapped for the HRV right at the top of the furnace.
Originally Posted by goldenear
Lets start at the begining. Is your HRV an HRV or an ERV. I have seen allot of ERV's installed and if humidity control is a problem you do not want the latent transfer of an ERV. Is the intake clean? Was the system balanced? Do you now how may CFM the unit is bringing in to your house and how much ventilation you need? Are there any unusually large sources of moisture? Aquariums or allot of plants, running the bath fans for at least 20 minute after showers, dryer vent intact and to the outside, dirt floor in crawl space?
I do not like the supply side discharge. You can't just "stick" a 6" 90 into the supply plenum with out consequences unless your system was designed for the large restriction and adding a restrictive air filter will only make it worse.
Extra restriction will decrease airflow and increase temp rise through furnace that can lead to early failure. Also you lose on filtering the supply air from the HRV.
To clear up some misconceptions... return grills do not "pull" air into them to eliminate stratification, they provide a low restriction path back to the furnace. The location of the return (hi or low) makes little difference. Air mixture in a room is determined by the supply registers location,face velocity, throw, spread and drop. Undercutting doors for a passive return is usually not effective due to the small area and extra resistance of carpet under the door (unless you are willing to cut 4 to 12 inches off the bottom!).
From what you describe my advise would be to verify the HRV CFM and sytem balance. Run the HRV continuously on the proper fan setting. There should be a setting for high fan when humidity rises above set point, set to 50%. Set the furnace to circulate the air regularly along with the normal heat run time. Verify the humidistat for the humidifier is accurate (they loose accuracy after time) if you plan to use it but it seems that your problem is moisture so I would turn it off. My better guess is that you need a pro that can look at the entire system and set it up properly to get the control you desire. High humidity will lead to many IAQ and building problems and should be a high priority to solve, especially in a tight home.
Correct. The supply plenum needs to be designed to accommodate that restriction. I assumed his was since it was professionally installed. However, from the sound of the problem, it seems that the installer simply fastened a collar or a take off to the supply plenum.
Originally Posted by mbarson
That's fine when dealing with 1500CFM, but with 100-200CFM, the location of the returns relative to the supplies makes a lot of difference in the degree of ventilation achieved using this install method without an interlock. Don't believe me? Call Jean at VanEE and talk to him about it. The guy knows more about mechanical ventilation than anyone I've ever spoken with.
The location of the return (hi or low) makes little difference. Air mixture in a room is determined by the supply registers location,face velocity, throw, spread and drop.