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misiek

join:2000-12-25
Round Lake, IL

External air supply for an 80% AFUE furnace?

Is it possible to bring an external air supply for combustion to an 80% furnace?

Our current unit is past its 15 year old mark and will need to be replaced with a new one. Hopefully, the old one will last through this winter.
The 90% + ones are gonna be a nightmare to install, according to a couple of HVAC contractors I spoke with. For ease of installation (read: $$$), both guys recommended 80% furnaces.
I am a complete layman when it comes to this stuff, so go easy on me guys.

Thanks in advance.



rockotman
...Blown On The Steel Breeze
Emerging Research
join:2000-08-06
DSotM
kudos:2

1 edit
Why do you think that a furnace that is 15 years old will need to be replaced? With proper maintenance, that furnace should last at least another 15 years.

In our last house, the original furnace (from 1971) was a contractor grade Janitrol (or the equivalent) that lasted until January 1998, when the heat exchanger developed a crack. We replaced the furnace with a Carrier, and that furnace is still going great after nearly 15 years (my son and his wife now own the house). Other than routine cleaning, the only thing that has failed on it over the years was an add-on low-voltage transformer for the humidifier and about every other year the flame sensor probe needs to be pulled, cleaned with steel wool, and reinstalled.

Secondly, why would you need an external air source for an 80% efficiency unit?
--
Shine on you crazy diamond...


cdru
Go Colts
Premium,MVM
join:2003-05-14
Fort Wayne, IN
kudos:7
said by rockotman:

Secondly, why would you need an external air source for an 80% efficiency unit?

To improve it's efficiency. Using conditioned air for combustion isn't as efficient as using outside, unconditioned air that's just going to go up the flue/exhaust pipe.

Some (most/all?) condensing furnaces allow for condensing air to be used for combustion, although it's not as efficient. I don't think I've ever seen a 80% furnace that uses outside air for combustion, and cobbling something together is going to be Rube Goldbergish if not against code flat out.


cdru
Go Colts
Premium,MVM
join:2003-05-14
Fort Wayne, IN
kudos:7
reply to misiek
said by misiek:

The 90% + ones are gonna be a nightmare to install, according to a couple of HVAC contractors I spoke with. For ease of installation (read: $$$), both guys recommended 80% furnaces.

Did they say WHY it was going to be a nightmare?


rockotman
...Blown On The Steel Breeze
Emerging Research
join:2000-08-06
DSotM
kudos:2
reply to cdru
said by cdru:

said by rockotman:

Secondly, why would you need an external air source for an 80% efficiency unit?

To improve it's efficiency. Using conditioned air for combustion isn't as efficient as using outside, unconditioned air that's just going to go up the flue/exhaust pipe.

I would be concerned that the extra humidity from the denser outside air would hasten corrosion in the heat exchanger and flue, especially with a furnace that is not designed to typically use outside air - but I am by no means an HVAC expert.
--
Shine on you crazy diamond...


cdru
Go Colts
Premium,MVM
join:2003-05-14
Fort Wayne, IN
kudos:7
said by rockotman:

I would be concerned that the extra humidity from the denser outside air would hasten corrosion in the heat exchanger and flue, especially with a furnace that is not designed to typically use outside air - but I am by no means an HVAC expert.

I'm no expert either, but the humidity is already lower during the periods where air is actively being drawn in when the furnace is being used. That's one of the reasons to use outside air as you aren't using conditioned air and pulling new air in lowering humidity levels.

The times when condensation would be an issue is when the outside air is warm and moist, and the heat exchanger is cold as if the AC was running. But that's not an issue as you aren't drawing air through the exchanger when the exchanger is cold enough to condense moisture. And when the burners have just turned on and the exchanger hasn't reached operating temperature, there's far more moisture in the air from the combustion process than what is in the combustion supply air.

Exchangers can rust out, but that happens when the combustion gasses condense, there isn't sufficient draft, the furnace short cycles, etc. But those all happen regardless of where the supply air comes from. Condensing furnaces are designed with corrosion in mind not because they are drawing cold outside air in, but because the exhaust gases become more corrosive when they cool and condense into liquids.


tschmidt
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join:2000-11-12
Milford, NH
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reply to misiek
I am by no means an HVAC expert and in our case we do not even use a furnace, rather a wood stove in the basement.

Our house is very tight so we have a problem with depressurization when exhaust fans are in use. Using the range hood will back-draft the chimney if the stove is not in operation.

I ran a 4" insulated duct out the rear basement wall and put a small fan to bring in air on the wall near the stove. I have no way to measure how effective this is but I have to assume it is better then nothing. I leave it running year round.

The hardest part was finding an inlet. When I first installed it modified a dryer vent. Took out the flapper and glued in some screen. That tended to get plugged up quickly. Recently I found a real inlet that has larger opening screen so it doesn't get plugged up. The louvers are removable for easy cleaning. I added a regular screen on the inside vent to keep out small bugs. That opening is much larger so clogging does not seem to be a problem.

/tom


misiek

join:2000-12-25
Round Lake, IL
reply to cdru
said by cdru:

Did they say WHY it was going to be a nightmare?

The spaces between the basement ceiling joists (not all of them) is packed with pipes. The ones that are free would allow to run PVC pipe, but the pipe would exit the building less than 3 feet away from windows / patio doors. I was told it's a code violation.


cdru
Go Colts
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join:2003-05-14
Fort Wayne, IN
kudos:7
said by misiek:

said by cdru:

Did they say WHY it was going to be a nightmare?

The spaces between the basement ceiling joists (not all of them) is packed with pipes. The ones that are free would allow to run PVC pipe, but the pipe would exit the building less than 3 feet away from windows / patio doors. I was told it's a code violation.

You might double check with the AHJ regarding that code. The install diagrams that I've seen (example, page 30) seem to indicate that it's only 12" from the edge of the opening. Maybe local code is more strict than national ones.

dmagerl
Premium
join:2007-08-06
Woodstock, IL
reply to misiek
Back in the 70s and 80s when energy began to get expensive, it was pretty common to read DIY articles about how to provide outside air to a furnace. Basically you just take a pipe that connects to the outside and place its other end somewhere near the furnace.

But the devil is in the details. You now have a hole directly into your house from the outside. You either let cold air blow in 100% of the time or you have to put some sort of damper on it to stop the draft when the furnace is off.

If you put it on the furnace end of the pipe, you now have cold outside air sitting in the pipe which is inside the house which sucks heat from the house. If you put it at the outside end of the pipe, you now need something that will tolerate working in exterior weather conditions.

It gets expensive real quick. There are reasons why no one does it anymore. Besides if you live in a house that is anywhere more than a few years old, the added energy loss caused by sending inside air up the flue is probably insignificant compared to the total amount of loss caused by air leaking though the house anyway.


ArgMeMatey

join:2001-08-09
Milwaukee, WI
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reply to misiek
Have you considered an HRV or ERV? They're not cheap, but they may increase your efficiency a bit without the downside of trying to simply pipe in unconditioned air from outside.

»en.wikipedia.org/wiki/Energy_rec···tilation


GadgetsRme
RIP lilhurricane
Premium
join:2002-01-30
Canon City, CO
reply to misiek
When I replaced our furnace in Denver the building code required two fresh air sources to the furnace room, one high and one low, to pass inspection. The furnace installer told me after everything was done and inspected I could shove a rag in the high and put a coffee can under the low to slow down the cold air. The low had to be within an inch or two from the floor so he put on a piece of pipe at the bottom that I could take off, put the coffee can in place and reinstall. When the furnace was off this would stop the low from drafting air and when the furnace was on would allow fresh air to enter through the low due to the turbulation caused by the furnace. I was kind of doubtful but it did work, never felt a draft out of the furnace room. The weight of the cold air in the coffee can was sufficient to stop any draft up into the warmer air above the top of the coffee can.
--
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misiek

join:2000-12-25
Round Lake, IL
reply to rockotman
said by rockotman:

Why do you think that a furnace that is 15 years old will need to be replaced? With proper maintenance, that furnace should last at least another 15 years.

The inducer (I think that's what it's called) motor is getting loud. A service technician said it would cost a few hundred $$ to have it replaced. Plus the heat exchanger may go soon too, so that's additional few hundred. This furnace had trouble keeping up with really cold temperatures. The techs said it's a bit too small for our house.


mix

join:2002-03-19
Utica, MI
Every service tech says this about every furnace...

Inducer motors are easy to change yourself. See if you can find the part online to at least figure out the cost.


tschmidt
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Milford, NH
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reply to misiek
said by misiek:

This furnace had trouble keeping up with really cold temperatures.

Before you invest in a larger furnace may want to check out improved insulation if you have not already done so. That should pay big dividends.

If what mix See Profile said is true you can buy some time by replacing the motor yourself and seeing what effect better insulation has on energy demand.

/tom


Lurch77
Premium
join:2001-11-22
Oconto, WI
kudos:4

1 recommendation

reply to mix
said by mix:

Every service tech says this about every furnace...

Every single one of us are evil humans. Not a single one of us will ever look at your safety, comfort, or budget. We'll never warn you of possible issues unless we can get maximum money from it. Each and every one of us are just looking for your wallet, nothing more. Every one of us does this every day with every customer and every heating system. It's how we roll.


nunya
LXI 483
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O Fallon, MO
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They learned all their evil conniving tactics from electricians and plumbers.

I am curious about the subject though. Would an ERV make any difference in an old drafty house. E.G., would the "least path of resistance" rule apply to air as well?

In my house I have the furnace, water heater, bath vents, and dryer. I'm sure they are pumping copious amounts of air out of the house, which means air is coming in from somewhere anyway.
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If someone refers to herself / himself as a "guru", they probably aren't.


ArgMeMatey

join:2001-08-09
Milwaukee, WI
kudos:2
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reply to misiek
said by misiek:

The inducer (I think that's what it's called) motor is getting loud. A service technician said it would cost a few hundred $$ to have it replaced. Plus the heat exchanger may go soon too, so that's additional few hundred. This furnace had trouble keeping up with really cold temperatures. The techs said it's a bit too small for our house.

I'm not saying service techs are dishonest. But whether he admits it to you or himself, he's got an interest in selling you a new furnace. He may just appreciate job security, like you and me, or he may be on commission. If you inform yourself and know what he's talking about, you'll feel better about your decisions regardless of his biases.

#1. Inducer, I agree with the others, it's typically not tough to replace yourself. However if you want to have it replaced preventively by a service tech, use that to your advantage by getting quotes from different dealers. They will want to sell you a seasonal package and free checkup but tell them you want a quote to just replace that one part and nothing else with the understanding that if they find more problems, you will have those fixed later. If you've ever had an auto shop tell you, "I shouldn't even let you drive that out of here," and left anyway, you should be well-prepared to deal with that eventuality. If there's a big problem they will have you talk to a supervisor or sign a statement that you've been informed of the problems and advised to fix them.

#2. Heat exchanger: Sure, it may go soon. Or it may not. Unless he has some verifiable figures on the actual life expectancy of your specific heat exchanger model, thank him for his observation and send him on his way.

#3. BTU capacity of furnace: Ask him for the Manual J calculations so you can see for yourself how far off the furnace output is, from what's optimal. Also try an online calculator to see what it thinks you need.

#4. Before jumping the gun on replacement, check out modulating furnaces. They cost more but if comfort is an issue, they can be a good solution.
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ArgMeMatey

join:2001-08-09
Milwaukee, WI
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Reviews:
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reply to nunya
said by nunya:

Would an ERV make any difference in an old drafty house. E.G., would the "least path of resistance" rule apply to air as well?

In my house I have the furnace, water heater, bath vents, and dryer. I'm sure they are pumping copious amounts of air out of the house, which means air is coming in from somewhere anyway.

We have an old leaky house. I had an energy audit with a blower door test, and was given an ordinal list of improvements to reduce outside air infiltration. We did the top items on the list: Insulating all outside walls, weatherstripping doors better, sealing unused windows. Other items on the list were rebuilding windows to eliminate window weights, and then insulate those cavities, insulating basement walls. The ERV didn't even make the list. So in a loose house, your money is better spent on other stuff.

I also draw air with the bath and range vents, but I am not worried about backdrafting because I have a direct vent water heater and 90+ furnace drawing outside combustion air. Only the gas dryer draws combustion air from inside.

So anyway, after the insulation and weatherstripping, the auditor re-did the blower door test, showed me the reduced infiltration, and gave me my rebate certificate. With that, the state program paid for most of the audit and paid me for part of the improvement cost.
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Lurch77
Premium
join:2001-11-22
Oconto, WI
kudos:4
reply to nunya
said by nunya:

They learned all their evil conniving tactics from electricians and plumbers.

I am curious about the subject though. Would an ERV make any difference in an old drafty house. E.G., would the "least path of resistance" rule apply to air as well?

In my house I have the furnace, water heater, bath vents, and dryer. I'm sure they are pumping copious amounts of air out of the house, which means air is coming in from somewhere anyway.

An HRV/ERV takes in approximately the same amount of air as it exhausts. On the better units you can adjust the intake to exhaust ratio. But overall you are not bringing in a substantial amount of extra air for combustion. And the more you bring in compared to exhausting, you are lowering the ERVs efficiency anyway. It's bringing in more cold air compared to less warm air exhausting. So there is far less heat transfer to condition that incoming air.

For example, lets say your ERV brings in 100cfm. It is exchanging that air by also exhausting about 100cfm. This is what they do, create air changes per hour for better air quality. But your standard efficiency furnace is still consuming combustion air and pumping it out the stack. So where is that combustion air coming from? It's not from the ERV, as it is already pumping out what it brings in. Combustion air comes from same places it has always come from. Leaking windows, uninsulated walls, doors, vents, etc.

prairiesky

join:2008-12-08
canada
kudos:2
reply to misiek
HRv's are mainly used to exchange air. Their major benefit is that they can save a bunch of energy on what you're exchanging.

The 80% furnaces are required to have a combustion air opening, some places require 2 as previously mentioned. That being said, the older homes are usually considered to be quite leaky meaning a combustion air wasn't originally required. Newer, better insulated homes are tightly sealed which creates humidity problems, hence the need for HRV's. Chimney's on older homes help to circulate air in the home keeping that humidity down.

I have a furnace in my home that I swear must be original. It's a big brown 5 foot Tall Coleman that's still going strong. I've contemplated switching it out but it continues to run! When i do i think i'm going to find a gently used 80% efficiency furnace for 2 reasons, one they're WAY cheaper. I'm looking at about a 5 year old Heil SP80 for $50 bucks. A new condensing furnace will save me more, but it'll cost me at least $4000 to get installed which will take years to pay off. My worst gas bill in the winter is $250 bucks. I have 2 furnaces, 1 newer 80% and the older one, (maybe 60%). Say they're split evenly, at $125 each, upgrading the 60% to an 80% will save me $25 a month for say 4 months a year where as going with the 95% will save me. $50 a month for 4 months. The 80% will pay for itself in the first year, the 95% will probably never pay as there's bound to be repairs down the line.


misiek

join:2000-12-25
Round Lake, IL
reply to misiek
@all,

Thank you very much for responding, your replies were quite educational for me.


JustBurnt

@rr.com
reply to nunya
said by nunya:

They learned all their evil conniving tactics from electricians and plumbers.

Can't leave out Roofers