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Phil
Phil, Mechanical Engineer
Category: HVAC
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Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I installed an 80% gas furnace in a house with foam insulation

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I installed an 80% gas furnace in a house with foam insulation on walls and under roof. So the envelope includes the attic. An engineer has suggested that I should have installed a 90% furnace with a sealed combustion system ( I have a 4" duct feeding fresh outside air to the combustion chamber and 4" double wall flue pipe). He says the attic should be totally sealed from infiltration.
To me, this is a tiny amount of infiltration to cause me to have to replace the furnace.

Phil :

Hi,This is risky business, not responsible for damages on your end. We *keep going until *you are happy. I come and go. be patient

Phil :

You are correct, IF the flue goes out through the roof... insulation is irrelevant in that case.

Phil :

tell me what you think he saw to have reason to give you that advice. maybe i am missing something.

Phil :

we can go from there, Phil

Customer:

The end user (homeowner) is having difficulties maintaining comfort level in the upstairs area where this furnace is installed. He is looking for things that would cause excessive heat to get into the conditioned area.

Phil :

The furnace wont contribute to that.

Phil :

but with an insulated roof, the attic and the second floor should run about the same temperature as the first floor with the same size cooling system installed, and equal duct sizes, and no kinked ducts, and no air diffusers shut off.

Phil :

Tell me the square footage of the home, and the type of construction, and city..and btu or tonnage rating of the outside condensing unit.

Phil :

we can go from there. Phil

Customer:

Its just the fact that foam is used to insulate a house because it is good at reducing infiltration of outside air that gets through fiberglass insulation in walls and ceilings, yet here i am, furnace installer, adding outside air to the attic. It bothers me to not have some fresh air in the attic space where the outgassing from the foam will build up over time/

Customer:

This house has 2700 s.f. of conditioned space upstairs with a 4 ton 15 SEER condenser and evaporator,and 5600 s f. downstairs on three 2.5 ton15 SEER systems downstairs. The house is outside the city next to a huge lake.

Customer:

The outside walls are rock with wallboard inside and composition roof.

Phil :

Thanks, XXXXX XXXXX of AC... my guess is that the units are over charged by a few ounces each...the suction pressures should run at 32F equivalent pressure off the chart for the the refrigerant used

Phil :

Lately I see techs not well trained in this, over charging systems

Phil :

Tell me what refrigerants are being used (its on the outside unit model number labels as an R number, such as R-22 or R 410a etc

Phil :

I will give you the proper suction pressures.

Phil :

Phil

Phil :

I might not b back until the AM.. its late here.

Phil :

Phil

Customer:

It's R410A.

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Customer: replied 6 years ago.
I still need help getting through this, but do not know haw to get back with you. Will try again tomorrow.
http://www51.honeywell.com/sm/genetron/common/documents/technical-documents/Genetron-AZ20-R22-Pressure-Temperature-Chart.pdf

charge to suction pressure of 100 psig to 102 psig max
98 psig will be most efficent though, but might result in a tendenct to ice the coils.

if this does it for you, plase click the big green accept button next to this question, thats how i get paid

Thanks!
Phil
Customer: replied 6 years ago.

That makes sense, you want the coil as cold as possible without freezing up. When it was checked, recently, the suction pressure was in the range you specified and the TD across the coil was 16. Currently, my primary issue with this engineer is that he told the homeowner that the systems should have sealed combustion furnaces in a foam insulated house, but I was originally advised against this due to condensation during the heating season mixed with a cold attic that could produce mold in the attic.

A forced draft furnace would not produce condensation, it would burn it off. The current opinion of others at this point, for the difficulty the system is having in keeping the area cool, is the envelope includes the attic with no insulation barrier between the living space and attic. The attic space is quite large. All the other foam houses I have worked on have had little or no attic space.

Hello again, as long as the furnace is vented to the outside.... and has its own air intake from the outside via plastic vent and intakes as with true sealed systems there will be no condensation in the attic.

a forced draft furnace still *must be vented to the outside, and still must have outside air intake vent from the outside

condensation is not an issue in any of these cases.

if the furnace exhaust were vented illegally to the attic.. then you would have condensation and a very very dangerous installation for carbon monoxide poisoning of the occupants.




If the furnace has only an exhaust vent, as with most high efficiency furnaces, its not a true sealed system and air intake from outside into the attic must be provided.

Leaving the top floor ceiling exposed to cooler air in the *winter, and only slightly warmer air in the summer due to the relatively small make up air opening required by law in that case will put a bit more load into the top floor rooms... in any event this would not be your problem.. its an engineering issue.

the engineer, not you, might owe the home owner 2 or 3 inches of insulation over the bottom of the attic space.

the Ruud unit specified by the way is not a sealed furnace by any stretch, so it must have outside air, and it *must be vented through the roof as you most likely have done.
refer to the furnace installation instructions.

http://www.acdirect.com/xcart/product.php?productid=366
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