Our home was finished in 2007. We acted as the general contractor and did a fair amount of work ourselves. Paid a lot of attention to potential air infiltration and insulation issues. Recently had a blower door test done and the house checked out at 0.4 air changes per hour (50 Pascals pressure). From the Building Science web site, I understand that 1-2 ACH is more desirable. There is outside air being brought into the geothermal return air plenum via a Honeywell Y8150 control and damper, but there is concern that the tight house won’t allow this setup to bring in enough to meet the ASHRAE 62.2 guidelines. (There has not been a flow hood placed over the outside air inlet to actually measure the flow we have.)
I feel that more ‘leakage’ needs to be introduced into the house. A simple way might be be to slave a second motorized damper (as a house exhaust) to the Honeywell outside air controller. Then check the flow in/out with a duct hood and program the flow rate into the Honeywell control unit.
Others have said that a heat recovery ventilator is needed; that is the only way to know what is coming in and going out. I don’t buy this; this looks to me more like ‘when the only tool you have is a hammer, everything starts to look like a nail’ syndrome. Besides, the guys at the Building Science web site don’t push HRV’s, so I figure there must be a good reason, like very poor payback numbers.
What do you think we should do? Any comments welcome.
Rick
Replies
Any comments welcome.
Call in the cable TV guy. Tell him you need to have a couple "jacks" put in.
Well, you need an air exchanger. An HRV is just an air exchanger with heat recovery features.
im with calvin, call the cable guy lol
I'm a "open a window" sort of
I'm a "open a window" sort of guy but a HRV will add tremendously to your quality of life. Some things don't have to pay for themselves in dollars and cents.
Another vote for open a window.
However, you can DIY a HRV easily for a 50 Pa differential (50 Pa = 0.2"water)
Was going to design it for you quick, but you did not say how many cuft your house is --- I'll say 20,000 cu ft.
So, jsut doing mental numbers (so might now be precise) ya need say 15,000 cu ft hour = 300 cuftmin for a total of 1 air change/hr/
At 50Pa, for a 40 ft long HRV, ya need the equivalent of about a 8" duct (round trip, in and out).
Hey, yas got lots of those - close off a section of the space between the basement joists, after you put about 4 ea aluminum 4" dryer vent pipes. I'm assuming ya use the whole house width of say 30 feet?
Juncture the 4" pipes at the outside end. Ya want to puch holes in the wall at the joist ends so that ya pull in cold air thru the joist space around the 4" vent pipe, and let hot house air go out thru the vent pipes. Best to have a small invert facing up on the ducts at the outside.
There ya go -- thermal transfer ratio not as easy in head (know 2 to 8" duct air drop table by heart, but not LTD ratios)
at 300 cuft/min = about 5 BTU/min per dT, or say 15,000 BTU/hr at 20F outside.
Ya gonna lose at least 10F due to film effects. so, 30 ft of 4" duct has a surface area of 50 sq inches per inch, or 50*12*30 = 125 sq ft.
A rule of thumb is about 2 BTU/hr ftsq per deg FLMTD. We got say dT=25 F to work with here, without trying to go into LMTD calcs.
125 sq ft provides close to 7,000 BTU/hr, so this simple setup gets you 50% f you losses back.
You can install 2 of these with only 2 Al ducts in each joist bay and probabl7y get close to 10,000 BTU/hr recovery - since the LMTD will be greater and the flow in the joist annulus slower, means operation nearer the 3 BTU/min vs. the 2 BTU/min
Have fun..
Art
You left out the part: "or in English"................
>>> "or in
>>> "or in English"................
Hahahah....no kidding.
Art, sometimes I get a kick out of 'your world'.
On the other hand, to the OP, it seems to me that you've taken a whole lot of care, time, and probably money, to get the house this far. Many people would be envious of the energy efficiency, especially when you look at the probable trajectory of future energy costs.
If you've gone this far, I think you owe it to yourself to install an HRV. I think this would complete the picture nicely.
thanks Junkhound, I've been searching for a simple DIY HRV solution for a while - even your calcs fit my situation pretty closely. My only worry is condesation. I'm up in Canada, so in January I will be pulling pretty cold air into that joist space. I have been tempted to fork out the $$$ for a real one, as I assume they must handle the condensation problem.
Have you actually done this yourself?
Hopefully my wife won't need to do the math......
Well, with JH's design, most condensation would occur inside the AL ducts. If they're sealed tight and tilted slightly towards the outside that shouldn't be a serious problem (though you'd want to make sure there is some allowance for the drainage at the exit). But you might also want to tightly caulk the joist cavity so no house air is drawn into it, causing condensation in the cavity proper.
thanks Dan,
I get it now - the warm moist air from the house, going through the AL ducts, will condense against the walls due to the cold air on the other side. Instead of using a joist cavity, my only option is to buid a square duct with the round 4" pipes inside. I have no basement and have cathedral ceilings except for a 20' section upstairs. I'm planning to route my dryer exhaust and kitchen exhaust through the same system - do you see this as being worth it or just causing problems?
I also assume I need to put a fan on the inlet side of this "DIY HRV" which pulls cold air in. I plan to wire this fan so that it turns on whenever the bathroom/kitchen fans are activated, and put a manual switch next to the dryer.
I'm building as tight as I can(sprayfoam walls/ceiling, and caulking like crazy), and want to eliminate the negative pressure associated with fans blowing air out, with no allowance for compensation air coming in. I guess I could eliminate this inbound fan if I just put a damper on the inlet. This would be a more passive approach, as the negative pressure of the fans/dryer running would suck air through the inlet naturally. This would simplify things greatly for me - what do you guys think? I guess it depends on how tight things actually end up......
Positive pressure is good for comfort -- reduces drafts in leaky structures -- but in a leaky structure can also drive moist air into the walls and lead to condensation there. Negative pressure can create problems for open combustion appliances (furnaces and water heaters), if you have any of those.
For homes with central HVAC, an approach that is sometimes used is to add a small (and restricted) outdoor air feed to the return plenum. This creates a slight positive pressure (and also routes the outside air through the air filter). If you have central HVAC you could possibly extend that a bit and dump your air intake into the return plenum with a damper valve to adjust the flow.
You could use a duct within a duct.
Do some more reading on HRV/ERV
They are common in tight houses. There may be some good info on greenbuildingadvisor.com. If your ventilation system opnly works when the heating system is running.... what happens when it's not?
Did they do a combustion safety check with the BLD test?
Thanks to all for the replies, and the good-natured abuse! Guess I left myself open for that.
David M. - No need for a combustion safety check: geothermal heat/cool, electric water heater (w/geo desuperheater), direct vent fireplace. Only other gas appliance is kitchen stove, we run the kitchen exhaust hood when that is on. The Honeywell Y8150 outside air control can be wired to run the blower even when heat or cool system is idle. I do need to make that wiring change, and then keep heat setting very low or cool setting very high in the spring and fall ("off" on the thermostat shuts everything off).
Junkhound - Interesting; I just related your design to my wife and her comment to me was "be careful what you ask for!" Really surprised, though, that you didn't add some turbulators to break up those surface films.
Calvin - I am the cable guy! I wired the house - electrical, cable, computer, phone. I caulk everything. Sorry!
I realize that an HRV/ERV is an easy solution, but definitely not at minimal installed cost. If an installed HRV is $1K, what is going to be the yearly savings and payback time? Our home is 4100 sq. ft. (ranch w/walkout basement) and the highest electric bill has been about $250/month (in the winter, summers a lot less). Could we even save $100/year with an HRV? I kinda doubt it. And, I consider a 10 year payback pretty lousy.
Upping the house air leakage (via a slaved exhaust damper ) to 1 or 2 ACH is barely noticeable in utility bills, according to the Building Science guys. I may just go that route, and, OK, maybe add to it a DIY HRV a la junkhound.
At risk of alienating the numerically challenged
You may save 10,000 BTU /day when it is 20F or colder, say 60 days for calc purposes. = 15 million BTUs
Say the COP of your HP is good, say 4.0, so you lose maybe 4 million BTUs worth of energy.
so, 4MBTU = rounded about 11,000 kW-hrs.
That is $110/year saved for the heat exchanger. Thus if you DIY for say just $50 of materials, it is only a few months payback time.....
Have your wife do the math.....
20 years ago I added a very small HRV from Mistubishi. This looks like a small window air conditioner and mounts through the wall. I have a very tight super insulated house. The little wonder is still going and is silent on low speed. We have it switched to run when the furnace fan runs or run it manually when the smokers show up. Don't know if they still make them though.
Just trying to keep this site from being 'dumbed down' below all recognition of it's former self.
Maybe not worth the bother??
>>>Maybe not worth the
>>>Maybe not worth the bother??
Nope; keep it coming. We need all the integrity we can get.
And thanks.
I'll add my 2 cents. You
I'll add my 2 cents. I think there is no such thing as too tight. Tight means you have more control on air leakage ... the key is that you do have to design a 'system' to control it.
You tended to be on the right track w/ your comment about the payback on HRV. Generally HRV equipment is expensive. Even more so when you aren't ventilating too much. You only need like 15cfm/person during occupied periods and those ventilators often run at hundreds of cfm.
A very tight house is OK. Don't worry ... HOWEVER, you do need to then provide some controlled ventilation ... both on the intake side and the exhaust side as you implied.
Some do this with an HRV to ventilate ... probably a lot and then recover the wasted conditioned air through the heat recovery process. I'm not advocating for or against the HRV systems ... just you approaching it w/ your eyes wide open.
How much ventilation you need depends a lot on your lifestyle ... showers, cooking, etc. If you have a modest lifestyle, HRV will save you less, because there is less to be done. You can save a lot IF you overventilate, but overventilating wastes a lot in the first place.
Those who advocate that you HAVE to have e.g. HRV if the house is tight ... I think are mistaken. You don't HAVE to have anything ... but balanced ventilation (again ... intake AND exhaust); it has to be a full system regardless of the way you approach it. You can do it with exhaust fans and motorized intake dampers. You can accomplish it in many ways. To say or imply that HRV is the best or only way to go, IMO is incorrect.
BTW ... should have posted this in the Energy forum for optimum exposure.