Replies

1. GBA Editor
   Martin Holladay | Oct 07, 2015 02:31pm | #1

   Peter,
   I'm not sure what type of makeup air you are talking about.

   There are two types of propane boilers: sealed-combustion boilers and atmospherically vented boilers.

   If you are building a tight house insulated with spray polyurethane foam, then the type of boiler you want is a sealed-combustion boiler. This type of boiler has a dedicated duct to deliver outdoor air to the combustion chamber of the boiler. It doesn't need any other source of combustion air.

   If your contractor installed an atmospherically vented boiler, that's not ideal. This type of boiler would need some provision for bringing outdoor combustion air to the room where the boiler is located. Most building codes specify the size of this type of outdoor air duct.

   An HRV cannot be used to provide combustion air to a boiler or furnace. It is a balanced ventilation device, not a makeup air device.

2. Expert Member
   Dana Dorsett | Oct 07, 2015 03:15pm | #2

   In any tight new construction (even at a code-max 3ACH/50 air leakage) it's better to go with direct-vented sealed combustion equipment, which needs no make-up air, and can't backdraft into the house.

   With low-temp radiant systems it's worth going with modulating condensing propane boilers, all of which are designed as sealed-combustion equipement that needs no makeup air. But if cast-iron, there are still some units out there that fill the bill (eg. Burnham's ESC mid-efficiency series).

   Closed cell foam is about the least-green method of insulating a house. It uses ~2x the polymer per R of open cell foam, uses severely climate damaging blowing agents (HFC245fa, at ~1000x CO2) and a large fraction of that higher-R is undercut by the framing when installed between studs or rafters. There are cheaper & greener ways of hitting the same or better performance points in most assemblies, and methods that don't insert undesirably low vapor permenace layers into the stackup.

3. 
   user-2890856 | Oct 07, 2015 03:30pm | #3

   Peter , What is the make and model # of the boiler / appliance being used to heat the home and did the plumber penetrate the outside wall with one or two pipes?

4. Expert Member
   MALCOLM TAYLOR | Oct 07, 2015 03:32pm | #4

   Martin and dana,
   I wonder if we could expand this question to include other combustion sources. Woodheat.org advocates against directly supplying air to wood stoves. I'd be interested in your views on this:
   http://woodheat.org/the-outdoor-air-myth-exposed.html

5. GBA Editor
   Martin Holladay | Oct 07, 2015 04:07pm | #5

   Malcolm,
   The debate has been pretty thoroughly hashed out here on GBA, in several articles and in the comments elicited by those articles. Here are four of the articles that come to mind:

   How to Provide Makeup Air for a Wood Stove

   Vermont House Uses Only Half a Cord of Firewood

   All About Wood Stoves

   Providing Outdoor Combustion Air for a Wood Stove

6. Expert Member
   MALCOLM TAYLOR | Oct 07, 2015 08:02pm | #6

   Thanks Martin, I had missed all that.

7. 
   Dana1 | Oct 07, 2015 08:24pm | #7

   My take on it is that outdoor air supply direct to the firebox of the woodstove isn't perfect, but it's important to not let the perfect be the enemy of the good. Having the outdoor air supply somewhat isolates pressure differences between the firebox and the conditioned space generated by wind or by exhaust venting.

   While this makes for less steady combustion air supply than it would have if pulling air directly from conditioned space, and has the potential for backdrafting out the air supply vent under high wind conditions, the conditioned space air is not part of the path of that backdraft. No wood stove is perfectly air tight, and leakage can & does occur, there is still a lot less combustion products entering the house with that sort of configuration- it's semi-isolated, if not perfectly isolated.

   If the supply air vent is installed in a code-compliant manner, observing the material & clearance requirements there is no real hazard that comes from backdrafting into the supply vent. The code prescriptives fully acknowledge the statement in the woodheat.org piece:

   "If it is decided to supply combustion air directly to a firebox, it should be done with full awareness that spillage is still likely if the room becomes seriously depressurized and, for safety reasons, steps should be taken to control temperatures on combustibles adjacent to the air supply duct in case wind effects lead to a flow reversal."

   There's a real net benefit in reducing spillage volumes into the room air, but you have to assume the air vent can get hot under optimally bad circumstances. Ignoring the building code on the air vent materials & clearances is indeed a hazard, but a completely avoidable hazard- just stick with the code!

8. Expert Member
   MALCOLM TAYLOR | Oct 07, 2015 08:53pm | #8

   Thanks Dana,
   I've always used a directly ducted air supply for stoves and was a bit taken aback reading the woodheat advice yesterday.