I think my AC/Furnace is generally undersized for the house.

We spend most of our time on the side of the house opposite the air return. And we wanted to have that side be a little warmer than it has been.

So, I went into the attic and adjusted the duct valve (roughly shaped door like flapper inside the duct with a t-handle outside the duct).

The air return duct going into the furnace is pretty large diameter. Coming out of the furnace systems are about 4 main ducts.

1) Goes to rooms on side of house with the air return duct. Medium duct. Has a duct valve and I think I have it set at about 40 degrees (90 full open, 0 full closed).

2) goes to family room and rooms (master, master bath, study) on the side we want to heat. large duct. no duct valve.

3) goes to living room on side of house we don't use. medium size duct. duct valve set to about 20-30% open.

4) goes to misc rooms (laundry room, shared bathroom, dining room. don't need to heat but it's on the side of house we want to be warm. smaller duct, no duct valve.

I didn't measure flow or temps before adjusting the duct valves; I should have.

Good news is that the house is warmer where we want. There is less flow in the rooms we don't care about.

However, it seems that I need to be careful to ensure I don't restrict output from the furnace too much. But have no idea how to tell if I've done so. I'm guessing one symptom might be if the output air is too hot. I'm measure about 105-110 degree F in the master, family room and study - where most of the flow seems to be.

Is this okay? Or is this a sign that I restricted the furnace output too much (and also AC for the summer - resulting in icing at the evaporator?).

Do I need to go back and open up the duct valves more?

Your concern with air flow is valid. There is several variables that effect temperature rise, air flow is a primary one.
Check the temperature rise at the indoor unit. Return air temp - supply air temp. typically the range is 30 to 70 deg. dependent on the unit. Max supply air temp for gas is around 170°. The data plate / sticker should have the temp rise range and max printed. A good service tech will check all the variables to confirm proper operations.

quote:

Originally posted by konata88:
I think my AC/Furnace is generally undersized for the house.

If this is the case, it wouldn't be able to maintain thermostat set temperature when it's at the coldest outside (design temperature).

quote:

Originally posted by konata88:
We spend most of our time on the side of the house opposite the air return.

Single returns are like installing a water faucet without a drain.
If you want to force air into a room, you should also be pulling air from it.
(With exceptions... kitchens, bathrooms, utility/equipment rooms)

quote:

Originally posted by konata88:
But have no idea how to tell if I've done so. I'm measure about 105-110 degree F.

Is this okay?

First locate the equipment data plate and find the temperature rise range.
(30-60, 35-65, 40-70, 45-75.... or maybe 70-100 'old systems')
You want to be near the middle of that range with a clean air filter.

Then check the temperature rise across the furnace.
Supply air temp minus the return air temp.
Measuring the supply away from direct sight of the heat exchanger (so close supply register with good flow).
Given that most newer equipment has a low temperature rise of 30-35 F, your 105-110 is fairly low if the house is at 70.

Too low is just as bad as too high.

Kitchens and bathrooms do need air circulation, not pulling air from them builds up too much humidity and causes mold.

Most equipment rooms the same, commercially they require a vent in the lower half of the door. If you are sending in air, air has to come out, and a lot of times it's the undercut on the door leaving a 3/4" gap that accomplishes it.

One problem is installing carpeting in a house that was previously wood flooring and that undercut gets so small the door drags on the carpet. It also blocks the flow of air as it won't let any out.

While we don't often see a vent or grille letting air escape, door undercuts are the vent. It's the ones leading out of the house that shouldn't be there.

First winter in our new-to-us house was abysmally cold. A 1962 architect designed home well suited for California, but located in Western PA. Read: energy inefficient as hell. With two furnaces, we froze our asses off.

The next summer, we replaced both furnaces as part of an air conditioning project. Turned our our heat enchangers were corroded or calcified or something. Anyway, now I can cook us out of the house if I choose. Thank God gas is fairly cheap, because I like to be warm, and I can't do much to change the energy profile of the home.

Thanks guys. I couldn't find the info you were describing. To play it safe, I opened up one of the ducts again. And the other one, I opened to about 45 degrees (50% open?); this duct goes to vents that are downstream of the other duct (ie - closer to the air return and in the same path). I adjusted instead the air supply vent flows a bit (but didn't fully close any of them). Made sure each duct had some vents that were fully open and partially closed others in rooms where heat is not needed.

I'll avoid mucking w/ the duct valves and figure out some other solution. Flow to the desired rooms still seems decent. I'll re-check supply temp at the vents later.

ETA: not conclusive, but while the flow seems similar, the temp seems to have dropped. In rooms where heat is desired and I was measuring around 105 degrees, it seems now to be measuring around 98 degrees. I'll check again later; maybe it'll go up more if the inside temp gets cooler as the outside temp comes down. Maybe the furnace has a longer duty cycle / stays on longer and so the output temp rises.

quote:

Thank God gas is fairly cheap,.

Unfortunately those days are coming to an end.

The way I adjusted our heating/cooling duct controls was to open every one of them all the way, then incrementally close-down the ones in the ducts where the rooms they serviced were running much hotter/cooler than other rooms.

quote:

Originally posted by Excam_Man:
If you want to force air into a room, you should also be pulling air from it.
(With exceptions... kitchens, bathrooms, utility/equipment rooms)

When we had our ducts cleaned they recommended closing-off the cold air return in the kitchen. So I did. We noticed, after doing that, that the adjacent family room was neither cooling as well in summertime nor warming as well in wintertime as it had been. So I opened that return back up. No other changes. Problem solved.

There's a cold air return just the other side of the very large door-less doorway between the two rooms. But, with three output vents in that room, I guess that wasn't enough.

Here's something of an oddity: The furnace is in the basement. The original furnace had a return vent in the bottom of the cold air return plenum. Somebody, probably our next-door-neighbor HVAC guy, recommended blocking that off. I did. When the new furnace was installed they put no return there.

Is all your duct work in the attic? How's the insulation on it? Just curious, no expertise here.

All ducts in the attic. Insulated but probably minimum to code (2002). Our insulation sucks in general. They saved every penny possible.

quote:

Originally posted by ensigmatic:
The original furnace had a return vent in the bottom of the cold air return plenum. Somebody, probably our next-door-neighbor HVAC guy, recommended blocking that off. I did. When the new furnace was installed they put no return there.

Two possibilities:

1) some know codes and others don't.

2) some care and others don't.

No returns from kitchens, bathrooms, utility rooms or equipment rooms.

quote:

Originally posted by konata88:
I couldn't find the info you were describing.

There's a label on the inside, either on one of the sides or on the top/bottom panels, or it can be on the backside of the door (newer ICP/Bryant/Carrier furnaces). Or some (Janitrol) puts it on the blower housing or sometimes on a control panel cover.
It will have the model/serial numbers and all the important data on it (electrical, gas, temp rise, clearances, etc).

Temp rise is temp rise, no matter the outdoor temp.

The only way for the temp rise to be different is on two stage equipment. Then you have a temp range for low stage and a temp range for high stage. Low stage will have a lower range (normally by 5-10 degrees).

quote:

Originally posted by konata88:
All ducts in the attic. Insulated but probably minimum to code (2002). Our insulation sucks in general. They saved every penny possible.

This is a real possible cause of at least part of your problem.
If I had to guess the ductwork was probably insulated using R-4 insulation. The 2019 code now requires R-8 if the ductwork in in an unconditioned space.

Putting your unit and ductwork in an attic is, unfortunately very common in certain parts of the country but probably the worst place to get the best performance out of your unit. When I got my Energy Star Certification I remember our ASHRE instructor commenting that placing the unit in the attic is like trying to make ice cream in an oven (referring to air conditioning)…

Although this won’t help you in the short term but you might want to consider having a good HVAC company perform a Manual J calculation of your homes needs and install properly sized and insulated ductwork. We did that in two older homes that had their units in the attic. The units were only 2 years old but they still weren’t getting the house comfortable enough for them. Our HVAC contractor performed the Manual J calculation and determined that the units were the right size but the ductwork was all wrong…

Another possibility is that you may not have enough insulation in your attic as well…