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Shamus 10-03-2008 01:37 PM

Heating question, negative pressure 2nd floor
I'm not even close to an HVAC novice but was asked by a customer for my thoughts so he could understand the problem.

Typical 2 story home in this northern climate. Open stairwell to second floor, forced air furnace. This one has updated windows, insulation and a 4 yr old furnace.

The issue is not much heat to the second floor bedrooms. Family of 4, one to a bedroom. Each keeps their door shut. Some gap under doors for air movement but no second floor return air vent.

In my home, very similar with a central hall to an open stairwell to first floor they added a return air vent/duct back to the basement and into the furnace. I have a more even heat distribution, I believe.

Homeowner is not a DIY kind of guy and I suggested he call in someone that is able to check his entire system and better advise. He pressed for my thoughts so I explained that his second floor did not have enough negative pressure (closed doors) to allow the air to flow easily back to the furnace. The first floor return air was either working ok for the furnace requirements or the furnace was starving for return air.

Was I close?

SD515 10-03-2008 04:42 PM

Without being there and doing my own investigation on the house...I'd say you're right in the ball return air would be one of the first things I'd investigate. My explanation to customers is pretty simple can't get new air in unless you can get the old air out.

hvaclover 10-03-2008 07:10 PM

The simple answer is transfer grills communicating the upstairs bed rooms with the hall way and down stair case.
This is a remedy I have used in many a bungalow and many times with out additional cold air opening.

Stair case is a natural cold air return.

biggles 10-03-2008 07:10 PM

he is balloning the 2nd floor rooms and counting on bottom of doors as the return.have him test the house by leaving all the 2nd floor room doors open,run the heat and let it satisfy then check upstairs in a general area hallway to see if it is close on the usually go with the one return on the installs 2nd floor attic furnace...1st floor baesment furnace

Shamus 10-03-2008 07:23 PM

Ok, thanks. I sorta figured I was close with my thought process on this.

I'll suggest he try some doors open now that temps have dropped he should be able to tell when the heat comes on. At least now he will feel more comfortable talking with a real HVAC guy once he understands whats going on.

The idea of transfer grills might be a good option for him as well. I'll hold off mentioning this until he has things checked out. He's one of those guys that never makes a decision until he knows he's making the right decision. Can be a pain sometimes but at least he knows what he's getting into before he says "do it". I've spent the last 16 months doing projects on his home, at this rate I'll retire before he's done. :yes:

4just1don 10-03-2008 08:11 PM

Just a story of old days here. I lived in a huge open 2 story years ago,,it had seperate heat etc. for each end of this house,,,baseboard on the other end. Due to sheer size I heated this primary with wood,,but that isnt the story I am revealing.

When the furnace in the two story portion would run their was a distinct whistle. never could figure out what was going on. Cold air grill of large porportion was behind the stereo we had parked against the wall. the cold air grill was like 1 foot by 2 foot so thought gee thats plenty big of return. One bored day I took my screw driver to see what was whistling inide,,,found they ONLY cut a 2-3 inch by 6 inch HOLE in the bottom cause it must have been lunch time or sumttin. AFTER I cut all the space available for air to return,,,the whistling stopped)

so moral of the story is take the covers OFF the cold air,,it MAY not be as big as one would think looking at the grill. OR maybe enlarge the existing opening as much as possible,,,even buying a BIGGER grill!! MOST cold air returns are TOO small!! generally shows up in ac before heating,but effects the same!!

also the LAST house I lived in had the cold air return in the bedroom,when the furnace kicked in it would 'open' the door all on its own,cause the door didnt latch,,now thats what I call negative air pressure,,,wasnt my house or I wouldve cut another cold air on living room side of THAT wall.

Marvin Gardens 10-04-2008 09:48 AM

First question I would ask is did he ever have heat in the upstairs??? If the answer is no then my guess is that he doesn't have large enough ducts to the rooms.

If they are small ducts running up an outside wall (common) then they warm air is cooling on the way up making the problem worse.

But first off I need to know if he ever had heat up there and now he doesn't. Also the age of the house is important so I can tell what kind of construction period it was.

In older home it was all convection. Heat would just rise to the top. If there were ducts they were just wall ducts with lath and plaster for the cover. There were often only one per room and this would only allow about 200 cfm per room. Not enough for large rooms with high ceilings and very cold weather.

Shamus 10-04-2008 11:18 AM

Marvin, all good questions.
The home I'm working on is similar to this photo. Built around 1890's and had gravity heat from a coal furnace. Duct feed runs are on interior walls and appear from my quick inspection that they are newer metal lined runs probably installed in the 40's or 50's.

They do have heat comming from the registers, the cfm will have to be determined by the HVAC guys as I have no equipment to measure that.

Marvin Gardens 10-04-2008 11:24 AM

If you can get the dimensions of the duct and cu ft of each room I can tell you if they are large enough.

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