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wml52 10-13-2012 06:02 PM

Furnace Condensation problem
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I purchased a new home about 4 years ago. The furnace is an Air Ease oil fired furnace LUF series with an 80% efficiency rating. It is vented thru the foundation wall using a Field Control power vent Mdl: SWD-4HD. I live in the North East and the first winter the furnace was used I noticed that there was a lot of condensation from the flue pipe (see attached photos) I have had several local HVAC companies come out to check the unit and make sure it was set up properly. I have also called Field Control to see if they could offer any explanation or solution, to date I still have the problem and no one seems to have any answers as to what might be causing the problem nor does anyone seem to be able to offer a solution. My concern is that if this condensation problem persists it may lead to an early death of my furnace.

I was wondering if the problem may be due to the fact that the furnace is in the basement which is an unheated space and therefore causing condensation to form in the flue pipe since the pipe would be hot when the furnace and power vent is running but then cools off quickly once the furnace has turned off?

If that were the case would the use of a double wall flue pipe such a Dura Vent be a solution?

Any help would be appreciated.


beenthere 10-13-2012 06:21 PM

Looks like its burning too clean, and possibly under fired.

techpappy 10-13-2012 07:33 PM

before I finished reading your whole statement I first though you had a combustion air intake from outside that was causing too much cold air for cold does your basement get? you could possiblty increase oil input which would increase combustion/heat exchanger temps maybe enoiuggh to prevent the condensation..if increased it still has to be checked to makle sure it's not producing smoke/carbon monoxide..if not dealt with the sulphur dioxide in the oil will turn into sulphuric acid thereby causing severe corrosion in the vent pipes....double wall vent would certainly help and likely prevent the condensation...have the exhaust temperature checked at the flue outlet to make sure it is above dew point to ensure no development of sulphuric acid in heat exchanger

joecaption 10-13-2012 07:34 PM

Should those pipes been sealed with high temp. mastic?
Is there at least a dehumifier running in the basement?

techpappy 10-13-2012 07:58 PM

good thing they weren't sealed, only signs of problems would have been too late,,have to get rid of cause...install double wall venting from furnace to termination top of chimney..otherwise could be deadly..i.e., perforation of venting then, condensation in chimney causing it to leak thereby blocking venting and causing back up of fumes into house..Carbon Monoxide Poisoning!

wml52 10-14-2012 05:42 PM

The basement averages about 43 deg F during the winter months with a humidity level that ranges between 55% - 64% with the use of an industrial dehumidifier

techpappy 10-14-2012 05:54 PM

hey guys just re read the thread again and noticed something strange to me///there is a vent from bottom front of furnace that joins in to the main vent at's an oil fired 80% furnace so I can't imagine that it is a secondary heat exchanger..just wondering if someone decided to hook that up as a combustion air intake ..if so, it certainly shouldn't be from the common flue. OR what is it..explanations please:huh:

wml52 10-14-2012 06:08 PM

The 4" pipe you are referring to is in fact a combustion air intake duct. As I mentioned in my write up I'm using a Field Controls power vent. The power vent is a double wall vent that uses the 4" center chamber to exhaust and the 6" chamber is used to draw outside air in so as not to use the air in the living space for combustion.


techpappy 10-14-2012 06:21 PM

Thanks that case I now understand your problem..the combustion air intake is causing the flue gases to cool down below dew point thereby causing excess condensation. It would be better if you could pull your combustion air from another said the space is not heated ..if it is a fairly large space with some ingress of air i.e., not completely sealed from the outside then you shouldn't need a direct vent from outside ..the air in the space should be sufficient for combustion..otherwise any 4 inch opening from outside to near the furnace would be adequate..this would prevent overcooling of the exhaust flue and excess deterioration of same..of course you would have to disconnect the existing combustion air piping and leave the inlet opening, open to the burner area

wml52 10-14-2012 08:10 PM

I would have to agree with you had it not been for one fact I did not mention in my write up, but let me explain now... I added the air intake duct after the first season I experienced the condensation problem and after speaking to Field Control who suggested it, but it made no measurable difference, in fact I bypassed the intake duct several times thinking along the same line as you, but it didn't make any difference.

However you did get me thinking and if I may think out loud here I'd like to get some opinions on my theory. The power vent has a pre and post purge cycle that is to say prior to firing the burner, the power vent will turn on and start to draw exhaust air from the combustion chamber, once the burner fires the vent continues to draw exhaust gases until the burner shuts off. Now the power vent has a timed post purge cycle that can range anywhere between 0 and 5 minutes. it is currently set to post purge for 4 minutes. What I was thinking was if the post purge cycle was too long it may be cooling down the flue to quickly and allowing the gases to condense. I was wondering if I shorten the post purge cycle it may allow the flue to remain hotter longer and thereby lessen the chance of any condensation forming, what I'm not sure about is what the post purge cycle should be. I'm sure it would not be advisable to have the power vent shut down when the burner first shuts down as I'm sure it does draw off any remaining gases in the combustion chamber?


techpappy 10-14-2012 10:32 PM

4 mins way too long..In my opinion 1 minute would be adequate..I have a power vented oil furnace and will check the post purge..if any..tomorrow morning when I anticipate the next requirement for heat...running manually right now as only need on about 1/2 hr per day.. house is well built and very well insulated and the basement is heated..the wife and I acted as the prime contractor when we had our house built 20 yrs ago

wml52 10-15-2012 02:40 AM

Knowing how long your post purge cycle is would be a great help in determining a starting point for me. Appreciate the help and look forward to hearing from you.



techpappy 10-15-2012 07:38 AM

I haven't tried my furnace yet this morning..waiting for wifey to get up and assist with TT operation while I observe.

However I just realized that your furnace id equipped with a draft regulator. Mine is not. If it is truly a power vented furnace draft reg not required. Draft reg only required for natural draft furnaces in order to maintain negative overfire draft in combustion chamber. AND to keep chimney warm by allowing warm air from room to maintain draft in chimney to assist in venting of products of combustion. In your application where you have vent directly to out doors draft regulator not required. In fact may be allowing cold air to down flow into furnace room. My furnace is also direct vent via 4 inch corrugated stainless steel vent straight out through basement wall. I will run my furnace through cycle asap and get back to you. You should verify with furnace manual/manufacturer that the furnace is, in fact, power vented. The burner blower should be all that is required to provide positive pressure for venting. My furnace has a Riello burner. Best in the world in my opinion. BTW I am a retired oil burner tech AND took the Riello course at the manufacturing facility when they first introduced their products to Canada about 25 yrs ago.

techpappy 10-15-2012 08:09 AM

OK ..only about 10 second pre purge before ignition and two minute post purge.

Hope that helps.

wml52 10-15-2012 02:55 PM

Thanks for the info.

BTW, the draft regulator is required as per Field Controls installation and set up manual. One of the things I'm wondering is why they ran 6" pipe when it terminates to 4" at the power vent, why not just run 4" all the way? I would think a smaller pipe 4" would get hotter and stay warmer longer then a 6" pipe? Just wondering...


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