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Old 11-25-2006, 11:17 AM   #1
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DIY sealed intake/combustion


I did some rough calculations and determined that on a typical winter day my natural gas furnace (Bryant, nonsealed combustion, standing pilot, type B vent) draws about enough air to replace the entire volume of the house once. That's a lot of cold dry air to have to heat and humidify every day. I am not ready to replace the furnace with a nice new sealed combustion unit since it still has some life left.

So I was thinking that I could perhaps make a plenum to go over the existing burner compartment vented access door and plumb in a source of outside air. Then I could seal around the plenum with metal tape and otherwise seal the few wire penetrations and such. Then the burner would be directly drawing outside air for combustion instead of the interior conditioned air. That should make the unit significantly more effecient.

Has anyone tried this? Any advice?

Thanks,
Ray

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Old 11-25-2006, 11:30 AM   #2
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DIY sealed intake/combustion


How about your hot water, which runs 12 months rather than just your furnace that is very seasonal.

You might be better off with building a "utility room" with a fresh air supply.

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Old 11-25-2006, 11:36 AM   #3
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How about your hot water, which runs 12 months rather than just your furnace that is very seasonal.

You might be better off with building a "utility room" with a fresh air supply.
Not possible since I am in a townhouse and the utility room also contains a commode, sink, and washer and dryer. The hot water heater could certainly benefit from a similar treatment but one step at a time. Besides I have very low hot water use so the hot water heater burner doesn't run anywhere near as much as the furnace.
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Old 11-25-2006, 12:27 PM   #4
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DIY sealed intake/combustion


Sounds like a familiar situation. I am also in a townhouse.

It appears all your air is going into the "utility/commode" room.

Your furnace has a vent (seasonal?) . Your water heater has a vent (12 months). Your drier (gas I assume) has a loose vent (12 months). All are in the same room. If you have a commode, you should have a vent (12 months) to be legal.

Use the room as a utility room with a new fresh air supply and keep the door closed as much as possible.

I had a similar situation (without a commode), but with a fresh air supply. When I replaced my furnace, I went to 80+ efficiency since the 90+ venting was difficult/impossible. Considering our low heating cost in Minnesota, it was difficult to justify the 90+ anyway. We have yet to be over $100 any month (1400 sf) for heat, water heating and clothes drying.

We keep the door closed to control the air flow and minimize the use of interior air for combustion. Since the utility room gets warm, I have a "5 gallon pail" anti-siphon (fresh air into the bottom) to prevent the warm air from going out the fresh air return when the furnace, water heater or drier are not running. - My wife hangs clothes from everything she shouldn't before fluffing in the drier, which helps keep the temperature down.

You can't spit hairs on calculated air flows unless you have the ability to include everything, including the benefits. Use common sense and look at what you do on an annual basis and not a snapshot.
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Old 11-25-2006, 12:53 PM   #5
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[quote=concretemasonry;24728]Sounds like a familiar situation. I am also in a townhouse.

Concrete, I am aware of the tradeoffs and the relative magnitudes of the loads. Hot water heater is like a tree and the furnace is comparatively like a forest. Gas bills are 5x higher in winter than during no-heat seasons. So relative air infiltration is proportional. No gas drier. And I can't take my utility room outside the conditioned envelope or the pipes would freeze. That being the case the air the furnace draws must come from outside that envelope past windows, doors, siding, floors, whatever, and sealing the utilty room door would be irrelevant. Much simpler to plumb a direct source of air to the furnace like they do with the more efficient models available today.

I am just trying to see if anyone has done this mod to their furnace and if there are any lessons to take away from their experience in terms of materials and techniques. I'd like to keep this thread focused on that aspect if possible.
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Old 11-25-2006, 01:44 PM   #6
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DIY sealed intake/combustion


[quote=raylo32;24731]
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Originally Posted by concretemasonry View Post
Sounds like a familiar situation. I am also in a townhouse.

Concrete, I am aware of the tradeoffs and the relative magnitudes of the loads. Hot water heater is like a tree and the furnace is comparatively like a forest. Gas bills are 5x higher in winter than during no-heat seasons. So relative air infiltration is proportional. No gas drier. And I can't take my utility room outside the conditioned envelope or the pipes would freeze. That being the case the air the furnace draws must come from outside that envelope past windows, doors, siding, floors, whatever, and sealing the utilty room door would be irrelevant. Much simpler to plumb a direct source of air to the furnace like they do with the more efficient models available today.

I am just trying to see if anyone has done this mod to their furnace and if there are any lessons to take away from their experience in terms of materials and techniques. I'd like to keep this thread focused on that aspect if possible.
Without more information on the big picture like pipes freezing, etc., it is impossible to make an intellegent comment on your system.

If you just want to concentrate on the furnace itself, just make sure the alterations do not void any warantees/guarantees and are in compliance with the codes and common sense relative to life safety. Modifying a furnace ventilation/supply can be a very tricky thing with many possible unknowns. I see them often when inspecting homes, when one appliance can affect the operation of others.

When you go to sell, it will be a red flag if the modifications made are still in place then. It may be proven correct, but it is still a red flag since it is a change in a safety item, irregarless of any code requirements. Pre-purchase inspections go beyond code to safety items. Same goes for the continued required ventilation for other items like driers, heaters and commodes.

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Old 11-25-2006, 02:41 PM   #7
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ANY CHANGES NOT APROVED BY THE FURNACE MANUFACTURE WOULD BE AT THE VERY LEAST FOOLISH! The entire liabilaty would be dropped into your lap, and removed from the manufacrure in the event something went wrong.Highly UNadvisable in any situation.
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Old 11-25-2006, 04:20 PM   #8
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acefurnace fixer -

You certainly havea bigger hammer than I do. I was just trying to steer him away from a very bad idea. I did not want to bring up the illegal bathroom and was trying to look at the big picture. The monthly minimum charges for gas would even justify a gas water heater in most areas. The obvious solution is a 90+ furnace with a variable speed fan and proper intakes and vents, but that may be big $$s too soon.

It is hard to imagine an interior room with frozen pipes that contains a furnace that operates onlywhen it is cold. - We only have -30 here and I have never seen that problem is 30 years, but it can exist. Heat tape is cheap for these rare situations.
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Old 11-25-2006, 05:36 PM   #9
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Hi raylo32

I am not going to tell you this is a stupid idea, you probably gathered that from everybody else. If your furnace was properly installed it should be in a non combustible closet with a high and low combustion air. The combustion air is not supposed to be pulled from the living space period!. So there should be no need what so ever to consider this. Dont mean to sound like a jerk. Read some of my posts about inducer assemblies and you will understand where I am coming from.

Good luck
Rusty
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Old 11-25-2006, 07:18 PM   #10
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Hi raylo32

I am not going to tell you this is a stupid idea, you probably gathered that from everybody else. If your furnace was properly installed it should be in a non combustible closet with a high and low combustion air. The combustion air is not supposed to be pulled from the living space period!. So there should be no need what so ever to consider this. Dont mean to sound like a jerk. Read some of my posts about inducer assemblies and you will understand where I am coming from.

Good luck
Rusty
Guys,

I respect the knowledge you pros bring but maybe we live on different planets. All the houses I have seen here have furnaces and air handler units in open utility rooms or basements that are not sealed off from finished spaces. Mine is adjacent to the lower level living spaces, is lined with metallic backed insulation and drywall and is about 10'x9' and includes the main water connection, fire sprinkler header, hot water heater, washer and dryer, and utility sink and commode. This is connected to the finished living spaces by a light wooden door with slotted vents that is not sealed in any fashion. In other words since the furnace is a non-sealed combustion unit it pulls air from the conditioned air in the living spaces through the vents on its burner access panel into the burner. This is a modern house as built, not some kludged mess.

I realize that I attempt any mods at my own risk for equipment damage. And I don't recall asking about building codes for bathrooms or closets for furnaces. This is how they build houses here and I have no issues with the setup.

But whatever I do in regard to supplying a source of outside air to my furnace will not be anything radical, will be safe, and will be easily testable and reversible. It'll be similar to how you pipe in air for a real sealed combustion furnace. That was all I wanted information about. But I'll figure it out myself. After all I am an engineer. Geez, I'm sorry I asked! ;-)
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Old 11-25-2006, 07:54 PM   #11
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Hi raylo32

I have said my piece along with the others. Its nice that you have taken an interest in this. We did not know that there apparently is no mechanical codes or an inspector to enforce them. The only information I would recommend before your experiment, find out what the velocity of air is required for operation. The particular furnace you have requires 50% air than is required for combustion. That is what the big hole in the center of the burner is about. The other thing that you will have to consider is that the combustion process does not depend on convection, it is induced. The other thing is that the inducer only pulls air for the combustion process. The convection takes over 12" above the top of the furnace. If you have already considered or already had this knowledge than you can disreguard this. As far as the burner velocity, you are on your own, I don't know that and I am not sure Carrier is willing to give that out. So if you are dead set on doing this. Hope some of this information is usefull to you. I have to add I would also strongly advise against it.

Good luck
Rusty
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Old 11-25-2006, 08:22 PM   #12
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Quote:
Originally Posted by #CARRIERMAN View Post
Hi raylo32

I have said my piece along with the others. Its nice that you have taken an interest in this. We did not know that there apparently is no mechanical codes or an inspector to enforce them. The only information I would recommend before your experiment, find out what the velocity of air is required for operation. The particular furnace you have requires 50% air than is required for combustion. That is what the big hole in the center of the burner is about. The other thing that you will have to consider is that the combustion process does not depend on convection, it is induced. The other thing is that the inducer only pulls air for the combustion process. The convection takes over 12" above the top of the furnace. If you have already considered or already had this knowledge than you can disreguard this. As far as the burner velocity, you are on your own, I don't know that and I am not sure Carrier is willing to give that out. So if you are dead set on doing this. Hope some of this information is usefull to you. I have to add I would also strongly advise against it.

Good luck
Rusty
Thanks for the advice, Rusty. I have considered the velocity issue and that may be a problem. Any reasonable sized duct would certainly result in a higer velocity of air as compared to the two or so square feet of intake area of the existing configuration on this open combustion furnace. It might require some sort of diffuser or baffle that would make it more complicated to design the add-on intake plenum. I also wonder about possible effects on the hot water heater draft since the 2 appliances share the same chimney.

Don't worry, I am not rushing into anything. I am actually pretty good at this sort of thing and won't do anything crazy. I may even lose interest in it if I get busy with other projects or if we have a warm winter.

But I'm Not sure what you mean about no mechanical codes or inspections. If you mean that my modifcation would not be code, I acknowledge that would probably be the case. But as far as I know everything existing here is code or else I and a few other hundred thousand of my neighbors need to go after our builders.

Thanks for the input.

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