HVAC question

woodbutcher

Well-known Member
We bought a house in town last year. The furnace, the water heater, and the cooktop use natural gas. The AC is slightly undersized to allow it to remove more humidity while it cools, but this summer has been rough. We had quite a few days above 100 degrees, and the unit could not keep up. It's 10 years old, so it may be showing its age, too. When the remodel was done, we had can lights placed in the ceiling throughout the house. The previous owner had accumulated more than a foot of loose-fill insulation in the joist spaces in the fifty years he owned the house, but it looked like a sow's bed after the electricians were finished. So, I had spray-foam applied to the underside of the roof deck and asked for full encapsulation. They vacuumed out the loose-fill. I hope to have a new HVAC system and water heater in place before next summer, but I have a question about where to obtain combustion air for the burners. It's a pier-and-beam house, so is there anything wrong with getting it from the crawl space? There will still be flues going thru the roof for the exhaust.

Butch
 
That can get complicated, lots of rules apply, and they probably vary from state/city.

If you'r doing it yourself, check with the local building code.

If hiring it done the installer should know.

I believe it can be vented from below provided there is adequate air available to be brought in around the perimeter, and is above any possible flood level. But don't take my word for it!
 
Here in the Minnesota/Wisconsin area you do not want to seal the roof with the top deck of your construction unless you use the chutes between the rafters or it will sweat during the colder months. A lot of times the make up air is supplied through a air to air heat exchanger. Furnaces can have a dedicated air inlet and some boilers and water heaters also. Do not forget about range hoods and bath fans when sizing make up air.Most of this is in book form Manual J for one.
 
Hello woodbutcher,

There are houses that are built so tight that even an open fireplace needs outside air like mine. If I don't open the outside air while burning my fireplace, the smoke alarm goes off! I would definitely provide outside combustion air,

Guido.
 
I do not know your area, but in some areas radon is a problem and is handled by vapor barriers being installed in crawl spaces, ventilation fans of crawl spaces and cracks in basement floors and floor/wall junction being sealed with various calks. I would not want to pull air for my furnace from this area as possible blending of heat and combustion air is possible with some air handling units used for HVAC. It has been proven radon can contribute to lung cancer.
 
My two cents: Do not seal the attic, let it breathe. Blow in at least a minimum of 18? of insulation ( more is better, depending on location). Hi efficiency gas heaters can have their air piped in from outside. Remember, any air vented outside (bathroom fans, dryer vents, gas hotwater heater vents, etc) will need makeup air from inside the house. I really believe that houses are built so tight that they can become unhealthy.
 
Thanks for all replies. I'm in northeast Texas, and I don't know of any radon problems. The sheetrock ceilings are so perforated with can light fixtures that a solid layer of anything would be impossible. I haven't found any codes that apply...this is Texas. The house was built in 1968. It's well constructed, but not tight by any means. I'm familiar with Manual J. Maybe I can find somebody who can do a blower door test.
Butch
 
We are just about finished building our last and final new home. I had the walls and roof deck foamed. It is also on a full basement. I asked both hvac and insulation guys that same question about fresh air from an air exchanger, there reply was this.

How many kids do you have and I replied one. Then there is your air exchanger, every time he goes out the door. Nathan
 

It may be unlikely but it would still be possible to get enough snow to block air from getting into the crawl space. Code could prohibit drawing combustion air from there.
 
I have never believed the undersized AC reduces humidity.

Moisture is condensed when the discharge temperature is below dew point,if you do not get enough cooling across the coil, then it will not pull out the moisture.

Old wives tail.
 
Hello Mtf,

Really? All I can say is that two wrong does not Make it right.
I would get someone that can tell you about the air volume required to maintain a healthy invoirnment, and is able to measure and determine how to achieve it.
Money well spend.

Guido.
 
If it is going to be a new stand alone furnace and not a Dedicated Outside Air Unit.

The fresh air intake will come from the outside of the house and the exhaust will run next to it and exit about the same location using SCH-40 PVC or CPVC etc. But most here use SCH-40.

Some of the fresh air inducted is used to cool the exhaust to 120 degree F or less which allows for the use of PVC pipe.
 
For kitchen stove and hoods that vent to outside, I always put a vent in the floor under the stove. With the hood running, there will be no "negative presure" created which can can cause burner problems.

Most newer gas furnaces will have outside air line running from the outside directly to the burner.

HTH

John,PA
 
Hello woodbutcher,

Here is another thing you can install for you attic. They also have an optional humidistat to keep the attic dry. By the way, it's worth about 6.000 btu's of cooling. It gets pretty warm up there,

Guido.
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Your contractor did you no favor. NEVER insulate between rafters while eliminating ceiling insulation. I'm going on over 50 years of house building, so I know whereof I speak. You MUST have circulation above living areas contrary to the contractors that claim a new roof will save you money. NO house is so airtight that simply living in it will cause problems with Life. This caveat does NOT apply to mis-installed water heaters, fire places or attached garages.
 
(quoted from post at 08:56:13 08/31/18) Your contractor did you no favor. NEVER insulate between rafters while eliminating ceiling insulation. I'm going on over 50 years of house building, so I know whereof I speak. You MUST have circulation above living areas contrary to the contractors that claim a new roof will save you money. NO house is so airtight that simply living in it will cause problems with Life. This caveat does NOT apply to mis-installed water heaters, fire places or attached garages.

Waldo, you may want to get up to date with modern materials and procedures.
 

The new heat pumps and AC units are either “two speed” or variable speed . No longer does the HVAC unit need to be compromised with an undersized unit .
 

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