the plumbers are just at the process of installing the propane boiler and propane furnace.  I will have a relatively decent sized range hood vent guessing around 800cfm, plus dryer, plus fireplace. The furnace and boiler are both modulating condensing high efficient units that are set up to use 2" air intake and 2" vent pipes.

Here is the question, the plumbers feel that because the house will be very air tight they are concerned about make up air and have suggested that instead of using the 2" air intake pipes for the boiler and furnace, that we use a 5" insulated pipe and bring it from the outside into the mechanical room and put a large loop or J in it so that the cold air will stay in the pipe until makeup air  is required.  Best way I can explain it is the intake pipe comes into the mechanical room then drops down near the floor then curves up 5 or 6 feet to form a J thereby the principal is that the cold air will stay in the bottom of the loop and not move into the building until makeup air is required.  This would be instead of having each of the boiler and furnace having their own 2" intake drawing from outside, instead they would simply draw from the mechanical room, and when air was required it would be made up from the 5" intake pipe.  What are your thoughts on this practice.

additional info.

Live in Saskatchewan, Canada i.e. cold winters -30C warm summers +30C
2300sq ft house main floor forced air heat
2300sq ft basement in floor heat
Full ICF to the rafters
R60 roof insulation
High efficient windows.

Opinions Thanks

Bad idea, non code compliant and the installer should read the installation instructions which come with each appliance.

I have read the installation instructions and if the space is supplied with air and well isolated from the living space then it is sufficient from the installation manuals perspective.

What are the btu input ratings and the size of the mechanical room

You have lost me there somewhere, jojo12. The systems were going to be set up to draw air from the outside, and then direct the exhaust gases back outside. Having a house that is tight has NO bearing on the furnace being able to draw air from the outside, since the combustion system is completely sealed. It should not matter if the inside of the house is at a higher or lower pressure than the sealed furnace, since it is only communicating with the outside air pressure. Having a 5" line that is open to the furnace room does introduce the problem that the furnace and boiler draw air in from the inside air pressure, but exhaust air to the outside air pressure. With a large line to the outside, there would probably not be a problem with pressure imbalance, but there is no reason to expose the house to the extra leakage associated with the 5" pipe going to the outside.

jojo - While I can't comment on the 5" line your plumber suggests, he is correct being concerned about makeup air. An 800 cfm range hood has to get its air from somewhere and you certainly don't want that air coming down the chimney or worse yet backdrafting the furnace combustion gases. Here is a good link that will help to educate you on this topic: http://www.greenbuildingadvisor.com/blogs/dept/musings/makeup-air-range-hoods

Posted By Lee Dodge on 15 Mar 2013 12:28 AM
You have lost me there somewhere, jojo12. The systems were going to be set up to draw air from the outside, and then direct the exhaust gases back outside. Having a house that is tight has NO bearing on the furnace being able to draw air from the outside, since the combustion system is completely sealed. It should not matter if the inside of the house is at a higher or lower pressure than the sealed furnace, since it is only communicating with the outside air pressure. Having a 5" line that is open to the furnace room does introduce the problem that the furnace and boiler draw air in from the inside air pressure, but exhaust air to the outside air pressure. With a large line to the outside, there would probably not be a problem with pressure imbalance, but there is no reason to expose the house to the extra leakage associated with the 5" pipe going to the outside.

the reason for the 5" line open to the mechanical room is to provide for make up air for other items such as dryer, range hood, etc.  Instead of having the furnace and boiler sealed with combustion air and venting the venting would still be to the outside but combustion air would be draw from the general mechanical room, which is fed by the 5" line then when the range hood, dryer, etc was turned on the 5" line could also provide make up air, vs having a separate line to provide the makeup air.

From what I can tell if I direct vent combustion air from outside with separate lines to each the furnace and boiler then I would still require a makeup air line in addition to the other lines, whereas the 5" line would supply the furnace and boiler, and makeup air.  Just wondering what the downside is to this method vs the other of three intake lines.

Posted By acwizard on 14 Mar 2013 11:42 PM
What are the btu input ratings and the size of the mechanical room

the mechanical room is roughly 150sq ft and furnace is 60000btu, not sure what the boiler is, I need to go look.

I'm not an expert in this field, so please make sure you check with one. But the research that I've done for my own case (ICF house 1200 CFM hood fan) leads me to believe that a 5" duct might not be big enough for all the make up air you would need in your case with an 800 CFM hood fan and dryer running at the same time. I believe that some codes require any hood fans above 300 CFM to provide an equal amount of makeup air. I would have someone do the calculations for you and size your intake vent correctly instead of winging it only to have your inspector tell you the bad news.

the reason for the 5" line open to the mechanical room is to provide for make up air for other items such as dryer, range hood, etc

Are those other items located in the mechanical room? If you put your make up air in closer proximity to the items needing it, you won't depressurize your house as much.

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I am curious about the rationale for using propane there. Was that the most efficient solution for an ICF home?

jojo12 - you need to meet the requirements of 9.32.3.8 (Protection Against Depressurization) of the building code. I am referencing the Alberta Code but your's will be similair as both are adaptations of the National Code. You also need to meet the requirements of Mechanical ventilation as set out in the different paragraphs of Section 9.32. Probably your furnace installer has looked after this with the "Principal Exhaust Fan. 9.32.3.3" The usual way (but not the best) is for an exhaust fan of the required size to be located in the main living area close to the kitchen. A similair sized intake fan will blow into the return air duct of the furnace (upstream of the exchanger such that it will mix prior to the exchanger). All three units must be interconnected.

There are many ways to meet the requirements, and a better way is to use some type of heat recovery unit in the mix. Just dumping a 5" combustion air into the furnace room will not cut it. But the main point is that the air out must be balance with the air in. Even if the tinsmith has balanced the system and then you add 800 cfm of exhaust to the mix for the kitchen fan, you will ruin the balance. 800 cfm is a lot of air! Also that amount of air will require more tempering ability than the f.a. furnace is allowed to provide.

Since you have a boiler a small make up air fan coil might be the answer.

the rationale for using propane was simply operating cost.  In my area the cost of propane has to go up by 60% from current levels just to get to even with the cost of using electric, and that takes into account efficiency differences, etc.  Also the cost differential I was quoted for electric vs propane within my total quote for plumbing was not significant at all.  likely make up the difference in one or two years on operating costs unless propane goes up massively and electric stays the same.  Also went to a propane fireplace, and water heater will regenerate a little quicker.

The other appliances dryer, and range hood are not in the mechanical room but the central vac will be.  I dont like the idea of the pipe open in the mechanical room, but from everything I have read it will be better than most other options other than very energy inefficient makeup air systems, so it seems as though it might be the best way to go at this point.  I am however, surprised at the limited amount of good information available about makeup air thoughts and systems for residential applications.  Thanks for all your replies.

I'm not an expert on mechanicals. That said, it appears that your mech contractors answer to solve the "problems" you're creating by inserting high air volume appliances (range hood & FP) into a tight envelope is to build a large hole in the envelope. At first reading, I thought the mech room would be tightly sealed from the house, but if it were, none of that "extra" air would get to the "problem" areas, hence the need to let that air migrate naturally.

I don't like the concept. Each appliance that needs those large volumes of air should have it's own dedicated air supply. Easy to do with a FP, harder with a range hood and necessary/required with the propane burners. My feeling on the range hood is to crack a window open when using it & close it when done. Why increase your heating bill 24/7 when you'll probably only use the hood a fraction of that time?

By the way, just finished a deep energy retrofit on a large home where we installed a chimney top damper on the fieldstone FP. Judging from the final blower door results - substantially lower than PH requirements - they work pretty well, so install one on your chimney.

Bob - for sure, the idea of just dumping air kind of ruins the point of building with ICF. 800 cfm would require more than a cracked open window to prevent depressurization and at minus 30ºC would probably freeze the kitchen taps before the bacon is fried! In any case, Canadian Code require heating season ventilation to be mechanical. By properly interconnecting the mua an exhaust, the heat loss would be restricted to operating time only. Wonder why jojo needs that much exhaust?

With regard to the stone fp, did you have to use any safety devise to prevent the damper from being closed prior to the fire being cold out (no longer using any oxygen?)

"800 cfm would require more than a cracked open window to prevent depressurization and at minus 30ºC would probably freeze the kitchen taps before the bacon is fried!" So maybe that would be an excellent reason not to use it in the middle of winter. Others have discussed increasing the HRV size or speed when using the hood. Whatever the solution, it would seem to necessarily require some type of dedicated supply. Funny thing is, we sealed the exhaust vent of a similar hood (to be removed) in the DER.

No, we did not "have to use", nor did anyone think of a safety device of that type. Is there such a device? In that case the owner is unlikely to close it when there are any coals in the house, but not everyone is so conscious. Fireplaces are installed even in Passive Houses, but they are sealed.

I'm becoming more used to being in tight, well ventlated, non fossill fuel-ed houses and its a very different experience and very difficult to describe. I do think we have to be clearer to potential owners that there are downsides to living in such a building and they just may include having to give up on parts of their life long "wish lists". Life is full of compromises, and they might well have to deep six the restaurant sized stove & hood or the huge, open roaring fireplace. Worth the trade IMO.

Posted By MSG79 on 15 Mar 2013 02:46 AM
jojo - While I can't comment on the 5" line your plumber suggests, he is correct being concerned about makeup air. An 800 cfm range hood has to get its air from somewhere and you certainly don't want that air coming down the chimney or worse yet backdrafting the furnace combustion gases. Here is a good link that will help to educate you on this topic: http://www.greenbuildingadvisor.com/blogs/dept/musings/makeup-air-range-hoods

The reason to maintain the sealed combustion system for the furnace is to avoid backdrafting.  If the combustion system is sealed, it only communicates with inlet and exhaust piping to the outside air pressure, and is insensitive to the interior air pressure.  The same approach should be used for a propane-fired fireplace, a sealed combustion system that draws fresh air from the outside and exhausts air to the outside.  The idea of an open fireplace in a tightly sealed house is counterintuitive.  My house uses a sealed, natural-gas fueled furnace and fireplace, both drawing combustion air from the outside, and they work fine on keeping any combustion gases out of the house, with no issue from backdrafting under any circumstances.  In my case, an HRV helps with depressurization issues associated with the dryer and the kitchen stove vent fan.   

If makeup air is desired for the dryer and the kitchen vent fan, then dampered, outside air supplies to those areas would be more desirable than simply dumping air into the mechanical room, especially if it involves opening up the combustion devices so that they are no longer sealed, and thus subject to backdrafting.  I think the HRV is another approach to the same problem. 

My understanding is that ERV/HRVs are no good for supplying make up air, because they always remove the same amount of air that they bring in, so you still end up needing make up air for that being exausted by a hood fan, dryer, or central vac. Wolf makes a automatic damper kit that works when the included pressure switch tells it to open. The pressure switch goes in the hood vent and controls the damper in the make up air vent. I'm still not sold on the whole idea, but I'lm pondering the though of putting some sort of decorative vents on each side of the cook top against the wall. The idea would be that the unconditioned make up air would not travel throughout the house, but have a short path from the sides of the cook top and up and out the hood.

Many people say why have such a big hood fan? One reason is that I believe code requires appliances to be installed per the manufacturer's instructions. So if the manufacturer says their cook top (which includes a grill) requires 1200 CFM, then you have to put in a 1200 CFM hood fan. Now, that thing won't get much use in my house, but when I want it, I want it! The majority of the time the hood will be sitting there with the damper shut. Or when "regular" cooking is taking place the damper will open with the hood (variable speed) pulling much less air, therefore much less makeup air will be needed most of the time and not destroy the integrity of my tight home. At least that's what I'm thinking. (Fortuneately, it's all still on paper, so if you guys convince me that this idea is off base it won't cost me anything to change plans.) :-)

okay I think I will tell the plumbers to direct vent and provide direct air intake both the appliances individually.  As for the range hood I have not picked one yet, and it is not a big commercial stove.  It will be a 30" induction cooktop.  I just did not want it to not have enough power because it will have a long exhaust duct down behind the wall into the floor space and then a long run to the outside.  To be clear the fireplace is propane and does have its own air intake and exhaust, it is a sealed unit so that so not have any bearing on my make up air.  I guess from the discussion I will probably be able to down size my hood substantially and may not need to worry to much about makeup air.  If I find I am running in a regular depressurization state I will then need to address it some way.

my understanding of the ERV/HRV system is also that it intakes the same amount of air as it exhausts so that it is not going to provide make up air.

Thanks for the comments

I've been hearing "old fashioned wood burning fireplace" so often that it's become the default thought. I wouldn't worry too much about a sealed gas fireplace unit, but check the specs. Why not install a recycling non exhausting stove vent? You can (again) open a window or turn the HRV up to high if you burn something.

I have a tight house and a fantech erv. When I run the (400 cfm on high) hood and or the dryer, I can definitely feel air coming in the erv vent when it is not running. It does a ok job of preasure equalization in my house. I can still backdraft my wood stove when I crank on the range vent right after I lite the woodstove. Instead, I open the front door partialy to let in enough air. If the woodstove is cranking and hot, it has enough draft to overcome any preasure differntial. I have filled the house with smoke a few times figuring it all out. cheers, Eric