HRV’s, Bath fans, stove exhaust fans, gas dryers, and ballance?
I’m building a super insulated, very tight home. I have infloor heat using a sealed high efficiency condensing hot water heater. I am installing a HRV capable of venting a 1200 sq foot house. I am placing the fresh air into the bedrooms and the living room and the exhaust vents in the kitchen (above the sink/dishwasher) and in the bath/laundryroom. I will ballance the system and have a booster control for the bathroom. My question is if everything is installed properly and I turn on the exhaust fan above the stove and run the dryer, where does the makup air come from? I havn’t been able to find ANY information regarding proper HRV system design and how it is affected by kitchen/bath/dryer vents. Seems to me that if the HRV is ballanced and you turn on the dryer it will simply pull more air in through the HRV inlet reducing it’s efficiency only for the time in which the fan is running. Would it also create a negative pressure in the house? Any thoughts from experienced HVAC guys out there? Much appreciated!
Replies
Wow, I haven't logged in to this site in a long time. Ever since "the big change" in fact.
Good question for which I do not have an answer. In new construction, you would use the "fully ducted" installation method so your bathroom (and possible kitchen) exhausts would actually be connected to your HRV. As for the kitchen vent hood and clothes dryer, those would create negative pressure unless you have makeup air of some kind. You could have both of these connected to an electric damper which opens when the fans come on.
You might want to ask your questions on http://hvac-talk.com/vbb/. They only deal with HVAC issues. They are not a DIY site though and they will not answer if it looks like a DIY question.
Good questions. Generally, you are right, it will create a 'more negative' condition and attempt to pull more air from the fresh air intakes. Their ability to accomodate a higher pressure depends on the specifics of the ducting layout. If your system has balancing dampers in the ventilation side to achieve balance, you could tweak it open just a little more to accomodate the range hood and dryer use.
As the other poster said ... you could also provide dedicated dampers for make-up air for the two uses as well. I used to be much more familiar w/ these systems, but you don't often hear of their use anymore (or at least I don't). I forget what the specific strategy is for these two devices ... maybe you balance the system assuming their use and if they aren't used, then the system simply moves a little less ventilation air. The differences in efficiency (i.e. energy recovery) I'm guessing to be pretty small.
The answer is
the makeup air comes from everywhere. The very best sealed houses leak at a rate of 0.1 air changes per hour (ACH). Which means that your house, with 8 ft ceilings will leak about 16 cubic feet per minute. This is not accounting for opening doors and windows, which happens in most houses fairly frequently. In reality, most individuals and builders mean something more like 1 ACH vs 2 when refering to "very tight".
If you havent been able to find any information on proper HRV system design, how is it sized to be "capable of venting a 1200 sq foot house"?
All the "transients" will affect how much the low pressure vents and fans pass. With all the air users running, the house will be negative, sometimes.
I have a barometric vent (a "Skuttle") on the return on my larger furnace that allows fresh air into the house via the continuosly ventilating forced air system. Since it works on pressure differential, it passes next to no air when the house is near positive and as much as I have it balanced to pass at the greatest negative inside pressure.
An excellent source of real, detailed, technical information on this subject can be found in ASHRAE Haanbook, Fundamentals, Chapter 16, Ventilation and Infiltration .
hey Tim
Thanks for the responce. I believe I said " I havn't been able to find ANY information regarding proper HRV system design and how it is affected by kitchen/bath/dryer vents" I think you missed the "and" part. I have read everything I could find on properly sizing an HRV to achieve a desired ACH yet none of the literature addresses requirements due to additional need from Kitchen exhaust or dryer vents. Easy enough I know the flow rates from these fans, 300 cfm from the kitchen exhaust and about 200 cfm from the dryer. Your estimate of even 1 ACH would mean I would get about 160 CFM which would not be enough for either the dryer or exhaust fan and certainly not both. When I say very tight I mean it although I will know when I do a blower door test. As for the barometric vent (a "Skuttle") I will check into it. I apprecitate the info. I was hoping to find information regarding HRV's that are self ballancing and can accomodate increased flow rates based on pressure drop changes with the home when it goes negative. I would like an all inclusive design if possible. I have written several HRV manufacturers and hope to get some info to illuminate the best solution.
Self balancing
would entail a differential pressure transducer and a way to modulate the supply and/or exhaust fans/flows. Some sort of a programmable logic controller would provide for almost infinite possibilities.
More simply, you could use the pressure signal to open or close a partial flow MOAD or stage a two speed motor. IMPO, all more work and expense than beneficial. I like KISS.
Complications would include the wind blowing and/or changing.
Tim,
I checked out the Skuttle. Did you install it yourself and if so were there any advice you'd give? Are you happy with it?
Yes
I did install it myself. Advice: put a volume damper in the duct.
I "fiddled" with it for quite a while to get it set the way I wanted it, that is such that my house is sightly positive in the winter with all exhausts on and the furnce on 1st stage of heat. I am happy with it.
How will you balance the system? If you have the insturments/knowhow to do it, then you should be able to determine the need for additional makeup air.
rdesigns
Read "hey Tim"
A simple water manometer. In fact all you need is two ports a piece of tubing and some water. Don't even need numbers on it just make the level the same, like a water level. You could get fancy with pitto tubes and all that but no real need. Balancing it and finding the necessary makeup are is not the problem its finding the best possible solution. I suppose you could just open up the window but not the best method when it is -20 out. I just havn't found any good info out there as to how to go about this... I suppose most people just don't bother and let it suck in from wherever the leaks are or even reduce their venting efficiency if it is too tight to accomodate the increased need for air flow.
I like your low-tech manometer. You'll never have to wonder if it's out of calibration.
I'm trying to think how it would work--say you have a pressure difference of 0.02"--would the difference in water levels be double that? Seems like it would. If the pressure side pushes down by 0.02, then the neg. side ought to raise by the same, making a level difference of 0.04. Is that right? If so, that makes it easier to see if there is a difference, because the difference is doubled.
Not that you would be looking for a number, just that it's a lot easier to see whether or not you're balanced when you're trying to see such small differences.
Back to your original Q: I think your assumption is correct that the dryer/kitchen exhausts will, temporarily, pull in their MUA partly thru the HRV, and partly thru infiltration. The temporary loss of efficiency would be so small as not to matter.
Uh ... not sure if that will work. It will basically tell you that you have balance, not necessarily where the balance is (could be sucking a lot from e.g. a crack in the door or down through the range vent.
Don't you need to balance using flow hoods? I would think that would be the way to do it. My balancing knowledge is a bit rusty, but for a simple residential system, I'd think balancing air volume is important and forget the manometer and pitot tubes for this kind of thing.
I don't think I would do a whole house pressure check for balancing a system like this. While the house may be tight, there are things that you have no control over (range hood and dryer exhaust).
Good question
and I think that residential design isn't quite to the point of addressing things like this clearly--you don't get a specific blueprint up front that analyzes all the sources of negative pressure and specifies makeup air for them. Right now the rush is on to "tighten" houses, and deal with the results ex post facto. What you might want to do is have a blower door test to see how tight yours really is. You can also measure the various pressures with fans running, the dryer running, etc. Until you know how tight it is, you don't know if you have a problem or not. My suspicion is that a person with a blower door and a pressure pan would find that the HRV is admitting make-up air when the dryer is running, and that the interior pressure is only very slightly negative, a few Pascals. If it's more than that you might need to add or adjust ventilation. If your HRV is ducted to the laundry room that should give you some flexibility. Another strategy is to simply allow a little more infiltration, and control where it happens, i.e. a slightly loose-fitting door shoe on the laundry room exterior door or something like that.
Any natural draft gas appliances in the house?
Good info davidmeiland
I spoke with an engineer at Fantech... he said that the HRV would accomodate the extra air but could not say how much. He thought it would probobly be enough if the dryer or kitchen hood were run seperatly but not at the same time. He then suggested using a mechanical damper then I have to place that cold air sucker somewhere in the house so you don't feel it and also consider possible condensation issues as it is drawn into the house. Makes sence but I was hoping that there was something like a pressure differential sensor for the HRV. Something that would sence that the pressure between inlet and outlet was not the same and self adjust to keep the house at neutral pressure by increasing the inflow and keeping the outflow the same. The HRV would then loose some effiencincy during this period but it would surely be better then having a cold draft damper open up. Added bonus is the HRV drains any condensate so no need to worry about moisture or frost developing under doors, mainly through locksets.
There are no natural draft gas appliances in the house but I know what your thinking there... there is a sealed gas fireplace that runs on it's own blower. It's got it's own inlet and outlet so not a concern. There is also a high efficincy gas hot water heater in the garage (used for potable water heat and heating the slab through a liquid to liquid heat exchanger) but this too is a sealed combustion appliance.
I do plan on doing a blower door test when it's finished so that will provide some insight as you suggest. I could always then add a mechanacal damper at that point. I think that is my only option as I see it. I used to do simulations of heat flow and mass transfer when I was an engineer. We worked on just about everything you will find in industry, but not much for residential homes. I would be curious to do some modeling regarding HVAC systems in modern homes. Maybe my parents house will become the experiment.
Thanks again everyone!
Practically speaking, you'll have difficulty measuring below about 0.1".
That's why I was hoping the difference would be doubled. You've made me think it thru more carefully, and I can see you're right. Oh, well.
That makes it seem to me that the OP will find that a water tube will be almost useless in trying to analyze room-to-room pressure differences because of the very small pressures created by his system. My guess, based on checking such differences in hundreds of homes, is that he will have PD's of 0.03" or less.
In theory you can run the tube at an angle to make the setup more sensitive. In practice it would probably be too hard to read the line, though.