Trying to alleviate temperature difference between floors
Hi! Recently I had a new Trane XL-16i heat pump installed. The installers seemed to be very good and came highly recommended so I believe that everything was installed correctly. One thing that seems to be really nice with this new unit is that it keeps the temperature right on what I have it set. With my old unit, my air would kick on when the temperature went a degree above what was set and would kick off when it got to the degree below. This new unit kicks the air on at 74 and turns it off at 74...I'm guessing that it probably kicks on just before it would hit 75 and kicks off just before it would go down to 73?
I have a 1.5 story house (approx. 2500 sq. feet). One problem that we've always had in this house (and I know that it's a problem with a lot of 1.5 and 2 story places with a single thermostat on the first floor) is that the upstairs is always considerably warmer (on the order of 10+ degrees on a hot summer day) than the downstairs. In the winter, the upstairs can be upwards of 5 degrees cooler. The installer recommended that we keep the fan on at all times to try to help keep the upstairs a bit cooler. This seems to be helping marginally but when the fan is on, the air output is fairly low. How much airflow should there be when just the fan is on? Right now, it's just that I can feel air coming out and have to listen very closely to even hear that any air is coming out. Is there any way to increase the output of the air from the fan? Does anyone have an idea how much electricity running the fan all of the time would take?
I know my installer said that one thing that may help the upstairs is to put b-board over the insulation in the knee walls since just having insulation directly exposed to the small attic space that is in a 1.5 story house makes the insulation much less effective. The space that can be insulated in the attic is well insulated and the airflow from the soffits to the ridge vent seems pretty good as long as there is some wind outside. Does anyone know if adding b-board will help out much? I have thought about trying an attic fan too but most places I've read seem to indicate that shouldn't be needed if you have a ridge vent for ventilation. Or would I be much better off adding a thermostat upstairs so that I have 2 different zones in my house?
Any help or advice would be greatly appreciated. Thanks!
without seeing the application, I would assume the upstairs ductwork is possibly undersized. ask your contractor if its possible to enlarge duct. If I were doing the job I would first do a heat load on the upstairs(hopefully already done by contractor) to see exactly how much air is required. If the contractor agrees, that may alleviate problem. I live in Florida, this is a frequent occurance in my area( people buy existing single level houses then build 2nd stories)
although keep in mind if problem is airflow, by enlarging ducts, temp should be more consistent. Winter time you may find it to hot though due to radiant heat as well as heat from system.
Adding some insulation and an air seal to knee walls will help,can't tell how much from here.
They could do a load calculation and tell.
How large is the second and first floor??
Do you have return air vents upstairs?
Do any of the supply ducts run through the attic? If yes, are they insulated.
Does the flow at the supply vents feel weak?
If the air handler has a variable speed blower, you could try closing a couple of vents on the first floor. (Proceed with great caution - if it has a standard PSC motor, do not try this)
I agree with gregg. I to am in the south florida area and always perform a heat load calc whether new initial install or a changeout. We find the duct work is always on the light side due to the homeowner/previous homeowners demanding a larger unit without any ductwork changes. You may also ask you contractor to "air balance" the air flow to each room in the house. Each room requires a set amount of cfm to accomplish cooling/heating. If your contractor cannot establish the cfm range for each room your duct may very well be undersized. Is your attic insulation R Value up to date? Florida is up to R-30. It helps. Good Luck.
The ducts are likely undersized or lack of return,but adding insulation to reduce the amount of air reqired may be less costly and saves operating cost.
My HVAC contractor seems to think that adding an air seal to the knee walls will make a big difference. I have also talked to an insulation guy and he agreed that adding some kind of air seal would help but he admitted that in a 1.5 story house it is very difficult to keep out the hot or cold air since there is a limited amount of space to add insulation between the dry wall and the roof area and still keep enough space to allow for the air flow from the soffits to the ridge vent.
If the air is running, even on 1st stage, instead of just the fan, the upstairs is within a couple of degrees or so of the downstairs. Aside from the bonus room over the garage, all of the other rooms upstairs have good airflow and have airflow similar to downstairs. The size of the downstairs is 1325 sq ft and upstairs is about 1100 sq ft. The downstairs has 1 return and the upstairs has 2 returns. The only supply duct that runs through the attic is to the bonus room and that duct is insulated.
I plan to add an air seal to the knee walls and hope that helps. I know it can't hurt as the temperatures in the attic space get really hot even with the air flowing from the soffits to the ridge vent. I don't believe that much more insulation can be added to the knee walls since it's insulated pretty well already. In SW Ohio, does anyone know what the current standards are for insulation in the knee walls? I think the biggest problem is that when the fan is running by itself, it doesn't circulate enough air to compensate for the increase in the temperature upstairs. If the fan only would run at a speed similar to what it does when the air is on, I believe that it would help considerably to make the temperature upstairs much more consistent with the downstairs. What percentage of full output is the fan only supposed to run at? Is there any way to change this?
Insulation and air sealing are important, but don't expect miracles. The whole purpose of forced air HVAC is to carry heating and cooling BTUs to different rooms. If a whole floor is too hot in summer and too cool in winter, that is direct evidence that floor is Not Getting Enough Airflow.
That's my opinion and I am an amateur. A homeowner in S.Texas which obviously is a very different climate. But we are just talking about the most basic principle of forced air HVAC -- air exists to carry BTUs to where they are needed.
Hope this helps -- Pstu
Uneven temperatures between floors can be as problematic in Houston as anywhere else, Pstu. What just about all two story houses share in common is stack effect. The interior of the home is essentially a chimney, with open floor plans aggravating this effect.
Originally Posted by pstu
The other common characteristic is the natural bouyancy of warmer air. This is actually an essential component of stack effect. As air on the first floor warms it rises, which requires make-up air from wherever it can be drawn from...usually window and door leaks, and other penetrations, gaps, etc. in the building envelope.
Stack effect is more pronounced in winter, resulting in uneven heating between floors. Two story homes with just one system are the most difficult to heat evenly. Two story homes with one system per floor will heat more evenly, with the upstairs requiring less heat than downstairs.
In summertime, the upstairs areas again will tend to be too warm, but not as much from stack effect as from proximity to attic and roof exposures. You are correct in that if there is difficulty keeping an upstairs area sufficiently cool, airflow (the lack thereof) is one possible cause. For a single system house it is a never-ending struggle between the downstairs being overcooled and the upstairs not receiving enough cooling. Zoning is one solution but must be done properly to be effective. One system per floor is preferred.
In a cold climate I can see an advantage to having sleeping zones upstairs, as heat naturally rises and the sleepers will slumber less chilled. In warm climates, sleeping zones located upstairs will receive radiant heat from an overheated attic and any knee walls all night long, causing discomfort for sleepers unless either the temperature is forced lower or a breeze created by a fan is in place. I personally have found that a room's air can be cool but if ceiling surfaces are emitting radiant heat from the attic, I will not be comfortable without a fan. In HVAC engineering parlance this phenomena is known as mean radiant temperature, or MRT, which to the human body can make a room feel warmer and less comfortable than the actual air temperature indicates.
Building Physics Rule #1: Hot flows to cold.
Building Physics Rule #2: Higher air pressure moves toward lower air pressure
Building Physics Rule #3: Higher moisture concentration moves toward lower moisture concentration.
First be sure the bonus room or partial two story is properly insulated.
Many times we have found the the truses under the second floor are totally open to the first floor attic,sometimes insulated above the first floor ceiling ,but allowing hot attic air to circulate under the second floor.
Might be able to feel this at the floor ,on a hot afernoon,well worth checking to see if there's a problem.
Trusses open to attic
I recently noticed that the 2nd floor trusses in our house are open to the 1st floor attic with no insulation inside the trusses. In other words, the ceiling of our downstairs LR and floor of the upstairs LR is not insulated and open to the attic on one end.
When you mentioned this, I was reminded that I was wondering if stuffing some fiberglas batts into the ends of the open trusses would be a good thing or a bad thing.
Everett in N Texas
Originally Posted by aircooled6
I'd used R19 batts minimum ,with foil backing if possible and mastc all the seams or put high R sheating over the R19 and mastic the seams.
One end being exposed isn't as bad as 2 or more,but I'd think it's worth correcting.
He mentioned when the system is running, the temps are very close to the set point on the stat. This would indicate to me (IMHO) that the "fan only" setting is less than that which is called for during a comfort demand. Call your service tech and have him/her adjust the "fan only" speed. Add a return upstairs.. It can only help... but for a more immediate fix, if possible, have the "fan only" speed increased. Most factory settings on newer systems (non variable speed) are low.
"Surprised ?! If I woke up tomorrow with my head sewn to the carpet, I wouldn't be more surprised."