Furnace ventilation

[JDgreen227]

Homeowners are constantly reminded about the importance to caulk, weatherstrip, insulate, etc. to lower their energy consumption. During the last two winters I have caulked our foundation sills, heavily insulated the rim joist area with R 39 fiberglass and built stud walls around the entire basement. The walls are insulated with R 13 fiberglass.

While I was doing all this work, I kept looking at the ventilation ductwork our heating contractor installed for the new furnace. The furnace is a Lennox condensing model with a 94% efficiency rating that has a PVC pipe that leads outdoors to supply combustion fresh air. The contractor also installed a gas water heater next to the furnace. In the process they installed two 4-inch flexible air ducts from outdoors, the ducts run down the wall to about two feet off the floor right behind the appliances.

I understand that they need to have fresh air, but having what is basically a 25 square inch opening feeding cold air into the house seems rather inane to me. The same contractor installed an identical furnace in the older part of the house back in 1993, and they only used the PVC pipe for a supply of combustion air then.

When I asked the contractor, he said he only installed the two additional vents because the mechanical code requires them. I'm wondering if I am wasting money by having those ducts feeding cold air into the basement. Any ideas? Thanks.

 

[Highbeam]

When the appliances are not running, put your hand by those 4" vents. Are they flowing air? They should only flow air if there is a pressure differential. The chimney for the HWH might be enough to get a good suck going in the room, but a suction could also be caused by bathroom fans, microwaves, woodstoves, etc. You need make up air for these vents to work.

You're right, it's a big hole. I am also irritated by the little vents in the double pane windows or the bathroom fan on a set timer. Seems the airtight house is not desirable, an insulated one is. So caulk and seal to prevent water from getting in and to control the amount of air leakage.

 

[thcri]

That vent is for products of combustion only. Your Lennox furnace has a two pipe system. One pipe is exhaust and the other is air for combustion. Your older furnace may have been that way also. The 4" one he installed is for your water heater which probably has a natural draft vent to it or a single pipe pvc vent attached to it. Now if the water heater has a two pipe system then he would not have needed the extra vent. As far as ventilation for your house each state looks at it somewhat different. California and Minnesota have some of the strickest laws in regards to new homes. Infiltration is tyipically enough for most homes to cover exhaust fans. In Minnesota we don't have to have the extra vent in the mechanical room for exhaust fans.

As for your original question, that vent in the basement is for your water heater and if it can't get air for combustion your just asking for trouble. I would not disconnect it under any circumstances especially if you have tightened your house up otherwise we could be reading about you in the papers.


murph

 

[kenmac]

Exhaust fans are ok here, but we have to supply fresh air for loss of air due to combustion / dilution of gas equip. If the FAF hav it's own pipe for combustion you should be ok ., but you still need the additional air for the W.H. just like Murph said. Unless you install an elec. W.H.

 

[inveresk]

Room sealed furnaces are now pretty much the norm in europe. They are supplied with combustion air either via a balanced flue or by a separate vent that connects directly with the combustion chamber. I don't know what type of furnace you have but maybe it can be adapted and remain in compliance with the relevant codes. No doubt you can get advice on this from the furnace manufacturer.

If you don't have a furnace that can be adapted, I'm afraid you must continue with the existing vents to provide combustion air to the furnace via the room. As you rightly identify, this will impact negatively on the thermal efficiency of your home. You could construct an insulated housing (essentially a cupboard) around the furnace provided the vents still supply fresh air to it. Consult your local building inspector on compliance if this would work for you to make sure you're not inadvertently falling foul of some other regulation.

 

[JDgreen227]

Thanks to everyone who answered my post. Highbeam, there is an airflow from both 4 inch vent pipes even when the HWH and furnaces are not running. It's strong enough to blow out a lit match. I should have made it clear that both of my Lennox furnaces use the 2 pipe PVC setup for intake and venting purposes. There are no vent fans in the house that would cause a pressure differential when the fuel fired appliances are not operating. There is a gas dryer in the first floor laundry room upstairs. We were not required to install a fresh air vent for that. /forums/images/graemlins/confused.gif

 

[techman]

You could put the furnace and HWH in a room (frame them a room with a door) and insulate the room. The mentioned pipes will provide the vent requirements, but the room can be thermally isolated from the rest of the house.

paul

 

[inveresk]

Sorry, just re-read your initial posting and you made it clear that one furnace at least is room sealed.

I've not come across room sealed furnaces which also demand separate room vents. I suspect you've received a belt and braces installation that is in excess of the codes. I'd ask your installer to clarify which part of the code requires room vents and also check the accuracy of his answer with the building inspector.

Nice choice of furnace by the way.

 

[thcri]

</font><font color="blue" class="small">( there is an airflow from both 4 inch vent pipes even when the HWH and furnaces are not running. It's strong enough to blow out a lit match. I should have made it clear that both of my Lennox furnaces use the 2 pipe PVC setup for intake and venting purposes. There are no vent fans in the house that would cause a pressure differential when the fuel fired appliances are not operating. There is a gas dryer in the first floor laundry room upstairs. We were not required to install a fresh air vent for that. /forums/images/graemlins/confused.gif )</font>


Your water heater if it is a natural draft vent it air will flow through it all the time, running or not. If it is a PVC vent single pipe you could still have some air going through it.

However, your house being in a negative pressure is what will cause air to flow through your fresh air. A water heater will cause a house or the room it is in to go into a negative pressure. Exhaust fans, kitchen fans and dryers will cause your house to go into a negative pressure. Wind shear can also cause your house to go into a negative pressure.

Again don't cap it off unless you want to take the chance of carbon monoxide poisoning.

Another person posted to seal your mech room off. May be fine in his state, in Minnesota and according to the Uniform Mechanical Code book you can not do that.

murph

 

[graybeard451]

<<snip>><<There is a gas dryer in the first floor laundry room upstairs. We were not required to install a fresh air vent for that. >>
And I hope for your sake you have at least a CO detector in that room. Yes a dryer can bring that room into a negative pressure also. Everyday I see more combustion problems caused by lack of fresh or make-up air in these new tighter homes. Gas, wood, coal, pellet or oil if you're venting something outside it is very easy to pull a tight house into the negative. Be safe, Richard