Help me keep my cool...
June 27, 2015 6:29 PM   Subscribe

We own a house. It's a kind of large, built-in-the-50s, added-on-to-in-the-80s, sprawly ranch-style house, but with 2 bedrooms, with vaulted ceilings, on the second floor. (Our kids' rooms.) In the summertime, they get hot, because of a.) vaulted ceilings - I've blown in insulation where I could, but there's still only 4 inches between them and the roof; b.) open stairway to downstairs - keeping the doors to the rooms closed helps a little, but kids don't really "get" that; and c.) crappy ductwork - the furnace is new, but the ducts are old, and it's a long, bendy run from the basement to the kids' rooms. I'm toying with some options for trying to improve the comfort level in our house - you are not my HVAC specialist (and I'm not opposed to talking to one), but am trying to see if my logic is sound. Special melting snowflake details inside.

(There are a bunch of factors here, sorry if things are disorganized.)

When we bought the house, there were two furnace/ACs in a (sort of) zoned setup. One furnace/AC was ancient and inefficient, the other AC was slightly less ancient and leaked Freon too rapidly, and its associated furnace was fairly inefficient. We talked to a couple HVAC companies about replacement of both, and the one we went with talked us into replacing the two with a single merged system. (In hindsight, this may have been a mistake.) We went forward with the replacement, and while the heat works well enough, despite being somewhat expensive, in Missouri summers the AC struggles to keep up, and, well, costs a small fortune. If the temperature in the house goes above the set temperature during the day, it can't bring it back down until night. If it gets above 95, it can't really keep it at the set temp, which is usually about 74, for cost reasons (I'd prefer closer to 70). When the temp is above 85, the AC runs more or less constantly (though it's a variable speed/cool so it's not always on full blast).

HVAC company we've worked with previously says system is working as designed - modern systems are supposedly designed to keep a 20-degree differential between inside and outside. They claim installing some new duct-work will help but be expensive (and probably only help so much). It's the largest heat-pump/AC unit they have, so it's not like we could up-size now (and it's been 5 years since it was installed).

Overall, our house is not overly energy-efficient - there are a lot of windows, some of which were originals from when it was built in the 50s - single pane, with storm windows. The family room has two walls of windows (which are 80's style dual-pane, which remarkably seem to have retained their seal) and a vaulted ceiling - that room is a problem both winter and summer. We did have the windows in the upstairs rooms (and the downstairs bedroom) replaced with fairly energy-efficient modern windows a couple years ago. There is at least some level of insulation in most of walls, but probably not as much as there should be.

We're currently using a window unit to keep one of the upstairs rooms cool, but it's noisy, and doesn't fit the window very well, so is not sealed in the window very well, and it only cools one kid's room (though it does take some load off of the AC for the rest of the house).

So, without rambling too much more, here are the things I'm considering.

1. Installing a zoned ductless mini-split system for the upstairs two bedrooms, with expansion capability to either the downstairs 3rd bedroom or the family room eventually. The cost on these has gone down to a point where I could do it for under $2k. It would replace the window unit we're using upstairs, and ideally augment the main system. It would allow us to keep the bedrooms cooler at night, and not cool the rest of the house as much while we're asleep, so hopefully the collective cost of running both systems instead of just the one would balance out. (Does this logic make sense?)

2. Continue with the window unit upstairs for the first bedroom, and get a second window unit or free-standing (vents through the window) in-room unit for the second bedroom. Possibly mount one or both units through the wall instead of the window - the windows upstairs are horizontal sliders, so I had to build a frame to hold the window unit which was designed for a vertical slider. This is the cheap option, and would probably be combined with one of a, b, or c below.

3. Install a new, larger duct from the basement to one of the upstairs rooms that gets the poorest flow, and possibly install one of those in-duct booster fans to get some more air to the upstairs as well. This would also probably be combined with one of a, b, or c below. I could put one duct in to the upstairs pretty easily myself, but it would still be messy, and would not solve the overall problem of cooling the whole house at night.

4. Something I'm not thinking of?

2 and 3 above would probably be combined with one of the following:

a. Replace a few more old windows with newer high-efficiency ones - this will need to be done eventually anyway, as some of them are in poor condition, but would be moved up on the schedule. I don't foresee this making a huge difference upstairs but might impact overall heating/cooling bills.

b. Install a gable fan to cool the attic more, possibly add a couple of attic vents in one area that needs them.

c. Something I'm not thinking of?

So, after that ramble, does my logic for any or all of the above make sense? Is there some option here I'm missing?
posted by jferg to Home & Garden (8 answers total) 3 users marked this as a favorite
It's probably worth it to talk to multiple HVAC companies about this. They do not all sell the same equipment and brands, so there might indeed be a larger system available even if this one company can't sell it to you. The way you describe how your A/C is working sounds like it is not properly sized for your house.
posted by cubby at 6:39 PM on June 27, 2015

My HVAC tech this spring told me that they look for a 20 degree difference between the intake and output of the AC (interior for both). He measured the air going into the system at the filter, 75 degrees (reasonable for ceiling air) and the air coming out of the ducts, at high 50s for us. Unless your system is intaking outside air to cool, it shouldn't care what's going on outside, only whether it is accomplishing its task on interior air.
posted by Dashy at 7:27 PM on June 27, 2015 [1 favorite]

You need an energy consultant who can come to your home and evaluate the house as a system. A energy consultant can advise you on the most cost effective way to maintain your desired comfort levels. Look for someone who calls themselves an energy consultant, energy advisor or similar. Such a person will likely have a blower door to test air leakage.

Don't just spend a whole lot of money installing more A/C systems. Don't spend a bunch on new windows. Find out what will be the most effective steps to take. No one here can tell you what will work best, because no one has seen your house.

Your HVAC company is covering for the undersized system. Your local design temperature is 100F, meaning your system should be able to cool the house to 75F on a 100F day.

It sounds like you'll need your ducts balanced and you'll need to take steps to reduce cooling load and/or increase cooling capacity, but you need an expert to help you with this.
posted by ssg at 7:32 PM on June 27, 2015 [3 favorites]

Also, if some of these walls of windows are facing south, you may want to consider exterior window shading to reduce solar gain and/or windows that reflect more solar energy.
posted by ssg at 7:34 PM on June 27, 2015

Having the same system for up and down is rarely satisfactory for a number of reasons.

As you have noticed the heating and cooling loads are different, yet you have one thermostat located one place telling the unit what to do.

Heat rises, esp. up open stairways. Here in south Georgia, that can be OK in the winter, but not so much in the summer. There are "zoned" systems that switch where the air goes from a single unit, but extra expense and equipment to foul up. You can sort of do this manually by say shutting the dampers on the downstairs registers in summer and making all the cold air go upstairs - to hopefully sink back down your stairway.

The other problem is ductwork length and size. There are inherent losses due to friction in the ductwork. Given two ducts of the same cross sectional area, you get more air out of the shorter one. The effect is greater than you'd think. This effect is much more pronounced for flex duct. (Search "ductulator" for some calculation tools.)
It is possible - and a bit of a PITA - to put in-line fans inside ductwork to counteract the losses due to distance.

I have a 2200 SF 2 story house, there was only HVAC downstairs when I bought it from the bank. The old unit had gotten stolen, Bank had put in a new 4 ton unit. That size unit should be able to handle 2200 SF.
I have lived in houses with one unit for both floors, and did not want to have to deal with poor performance again.
I ran the calculations for just running a trunkline upstairs from the 4 ton unit, I could not make them work, the runs were just too long unless I ran a HUGE duct upstairs. Since I work as a designer in an Architectural office, I called my buddy over at our mechanical engineer's office, who suggested either the co-axial fans, or, better yet, just put a separate unit upstairs.

Getting someone who is not selling equipment to have a look is a very good idea. Consider buying an hour or two from a mechanical engineer.
posted by rudd135 at 12:49 AM on June 28, 2015 [2 favorites]

We have a similar problem and the window unit is the best fix for us. A mini split system will get you the same results with better aesthetics and reduced noise but with increased cost.
posted by jmsta at 4:19 AM on June 28, 2015

Yes! What ssq and rudd123 said! Get someone out to your house who's not trying to sell you something and they can tell you way more than we can. Do it! You won't regret it.
posted by patheral at 9:23 AM on June 28, 2015 [1 favorite]

I'd say your #2 option of sticking with the window units and dealing with the upstairs rooms as separate entities is the easiest and maybe the best solution, certainly the cheapest. Use window units, the freestanding ones that vent out the window are incredibly inefficient and don't work very well, and I'm guessing with those things, you'd need to deal with the condensate, like a dehumidifier.

I had a very similar problem with 2 upstairs rooms, one with a vaulted ceiling, the other not. I had casements instead of the slider you have, so I removed the casement sash, easy, and built a fixed window, screwed into the frame, that took up about 3/4 of the opening, and left the bottom open to receive a window unit. The fixed window was just a simple frame with a rabbeted ledge the glass fit in, the glass held in place with glazier's points and putty. In the fall, I pulled out the A/C unit, unscrewed the window I made, and slid the casement sash back on it's brackets.

I put small 5000 BTU units in at first, that cooled one room fine, but the room with the vaulted ceiling required a 10,000 BTU unit to cool it, my guess is there was no insulation in the vaulted ceiling.

You don't mention if the upstairs rooms have return ducts, but if not, you might try to figure some way to get the hot air out of those rooms. With only a 4 inch space between the ceiling and roof, there's obviously no attic there, but you may be able to run a flex line into a nearby attic to vent. I think you could vent hot air directly into an attic space with no problem in warm weather, of course you would not want to do that in the winter. Venting your attic with a gable fan would also help.

I'd try the window units first and see if that solves your problem, you may not need the additional venting.
posted by PaulBGoode at 10:49 AM on June 28, 2015

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