Home is heating up in late evening
We live in a rather energy efficient 5 year old, two story, east/west oriented home. HERS rating, iirc, is in the 40s.
In the summer, the central a/c can maintain a temp of 72F most of the day even when outside temps go up to 95F. But in the late afternoon, just as the sun goes low in the sky, the a/c cannot maintain that temp and it will rise to anywhere from 76 to 79 on the upper floor. The a/c runs pretty much constantly during the afternoons.
I’m perplexed. What is causing this? Any suggestions to mitigate it?
I thought it was residual heat, but the a/c is running all day, so if it can handle radiated heat while the outside of the house is being heated by direct sunlight, why can it not maintain temp when the outside cools down?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
It sounds like afternoon heat gain through your west facing windows. The heat gain combined with the AC setting (72 degrees is pretty chilly) may be overtaxing your system.
It's just thermal lag caused by the thermal mass of the house. Radiant barrier would be the best bang for the bucks here.
(get a return duct on the 2nd floor if there's no intake there already.)
Thanks for your responses.
- We keep it at 72 in the afternoon so that when it rises in the evenings, it is still a tolerable temperature. If we keep it at 75 in the afternoon, it goes much higher in the evenings.
- I haven't heard of a radiant barrier - will look into it.
- Unfortunately, we don't have a return on the 2nd floor. I'll have to look into what it would take to add one without destroying the aesthetics of the house.
Thanks again.
Cedy,
You haven't provided us enough information to diagnose your problem. So far, everyone is guessing.
Does your house have unshaded west-facing windows? If so, Steve may be right (especially if the warmest rooms are the ones with the west-facing windows). It would be useful to know the solar heat gain coefficient (SHGC) of your west-facing windows -- if you know the home builder, and are able to do some research, that would be useful information.
Are some of your home's ducts located in a vented unconditioned attic? If so, Anon3 might be guessing correctly. But his or her suggestion to install a radiant barrier (presumably, somewhere in your attic) is only one possible solution to this flaw (if your house has this flaw), and not necessarily the best solution at that.
"we don't have a return on the 2nd floor", well there you go. Hot air rises, cold air sinks.
If the second floor is fairly open (or the doors are kept open), I don't think the lack of a return is that big a deal. I don't have any returns in my house, but my floor plan is open and any intervening doors are louvered. I'd be more suspicious of the afternoon sun and ducts running in a vented attic. But as Martin pointed out, we are all guessing. It probably would be worthwhile to have a Resnet rater check things out first-hand.
Thank you all for your responses. If you need any additional information please ask. We are the first owners of the house and have all the docs.
Our house was rated and I have attached the report.
To answer questions already asked:
- The SHGC of our windows is .270 & U-Value is .290. Our first floor windows have solar shades and drapes. The upstairs windows have hex-cell insulated blinds. I put temporary drapes upstairs(Thick sheets) but they don't seem to have much or any effect over what's already there.
- There are no a/c ducts in the attic except - see below
- The upstairs has 3 bedrooms and a loft area. We keep all doors (not louvered) open and there are small vents above each door to allow air-flow if the door is closed.
There is one duct that is in the attic. It connects the MBR to the landing area. I was told that this allowed air flow from the MBR suite to the outside if the MBR door is shut. This is a problem area because I can see that the temps (using a temp gun) at the vents is much higher than the ceiling dry wall. I haven't gone up to the attic to see if the insulation (blown) has fallen off this duct.
Thanks again.
Afternoon direct solar gain from unshaded west facing windows would be the prime suspect here. Unlike absorbed radiated heat heat coming through the walls or roof, directly radiated energy coming through the windows doesn't change (much) with outside air temperature. Exterior operable shades reflecting a large portion of that radiation (light, both visible and IR/UV) before it can come through the window can do quite a bit of solar gain mitigation.
Radiant barrier will buy you next to nothing if the attic-R is already at code minimum or higher (probably is if your house is really HERS 40-something), and exactly nothing if the phenomenon is due to direct solar gains from west facing glass.
As long as there are return paths through halls & stairwells it's not likely to be an HVAC ducting problem, especially given that it only occurs late in the day, a few hours after what would be a "normal" peak cooling load. But viewed another way it could be considered a supply duct sizing or AC compressor sizing problem if the peak load wasn't accurately calculated. But killing the PM gains with window shades is still the best mitigation strategy.
It sounds as if you have " jump duct" return path for the AC from the MBR. Hopefully it's buried in the attic insulation, or at the very least the duct itself is insulated? (If not, it's worth insulating over the jump duct, but it's not likely to affect this problem very much.)
Unless the jump duct is undersized or obstructed, opening or closing the door won't change the air flow very much. Testing it with a manometer would be useful if all else fails, to see if increasing the size the return path is in order.
What its the temperature you see at the jump duct grille in the landing area (with the MBR door closed) and what is the adjacent ceiling temperature?
A touch off topic but does a HERS of 32 strike anyone else as awfully low for SEER 13 ac, 92% furnace (unlikely to be an ECM blower), 62% ef water heater, R30/38 cathedral(?) Ceilings, R-20 walls (presumably no continuous exterior nsulation?), and R3-4 windows. I'm not picking on your house CEDY and I'm not an energy rater but 32 is a pretty good HERS for energy attributes that are closer to standard than exceptional. Where is your home located climate zone wise?
Denver CO. New Town Builders (now called Thrive) is a leader in these energy efficient homes & have won a lot of awards. I am no expert on these ratings though. The Solar panels may have something to do with the low rating.
Cedy,
If you have unshaded west-facing windows, and if the windows are the cause of the problem -- a likely explanation, but one that is still based on guesswork -- the best solution is exterior shading, not interior blinds. (Once the sunlight has passed through the glass, the solar heat gain has already occurred. Your blinds are heated up by the sun.)
The traditional solution, in southern climates, is an awning or top-hinged shutter that keeps the sun off the glass.
The other solution is to deliver more cool air to the room -- a solution requiring a bigger air conditioner or a bigger duct, and usually more electricity.
.
I suspect it's indeed the rooftop PV that lowers the HERS rating.
The rooftop PV also lowers the peak attic temperature too, since it shades large parts of the roof that would otherwise get the most intense sun exposure.
There are many variations on exterior shades, some easier to deal with than others, some fully darkening, others allowing a screened view, some fully automated, others strictly manual, but they all do the same thing- they limit the amount of energy that makes to the glass, and thus lower the solar gain (a LOT!). Coolaroo is one manufacturer with a decent reputation distributed widely in the US, but there are others.
We've given shutters and awnings a thought, but haven't acted (or agreed :) on it yet.
What about using solar films (that inhibit solar radiation) on the west facing windows?
Putting reflective window film on the interior side of sealed glass pane can cause them to heat up enough to break the seal. Putting them on the exterior doesn't last as long due to exposure to weather, and can also cause the seal to leak, but the risk is lower, and performance higher.
But sure, they'll "do something".
With an SGHC of 0.27 the existing windows are already fairly low gain, but with a reflective window film on the exterior that gain could be cut by about half. But that also means the beneficial gains in winter will be similarly reduced, which in a US climate zone 5B climate (Denver, and most of eastern CO) would usually mean higher heating energy use. Operable exterior shading would be a better (albeit more expensive) solution.
Dana, I missed the rooftop PV. That makes a lot more sense in terms of HERS ratings.
Thanks all. This has given me some options. I will follow up here with what we decided to do and the results. Thanks again.
A cheaper solution is build some solar screen yourself, they are like the insect screens but block out heat better. You'll want to get them out during the winter so it's extra work.
You can buy the screen from HD. You could attach them frameless using something like 3M dual lock for even cheaper.
An update. We've done a couple of things and have a plan for the future. Thanks for all your input
1. We've set the a/c fan upstairs to run all the time, while downstairs is on auto (we have 2 zones). I find that this creates a better circulation of the hot air from the 2nd floor. First floor is also set 2 degrees higher than the 2nd.
2. We have un-insulated ducts in our unfinished basement. We're going to have those insulated. The basement gets very cold especially near the heat/ac unit, so we know we're losing a lot of capacity there.
3. If the above doesn't fix the issue, we'll install external solar shades for the west facing windows. We'll start with the first floor which has 3 large west facing windows & then move to the 2nd floor if needed.
Only if the above doesn't solve the issue will we look at adding a return on the 2nd floor and/or increasing the capacity of our a/c unit.
Thanks again.
2.
The coolth in the basement may be as much or more due to duct leakage than lack of duct insulation. Be sure to mastic-seal every seam & joint in hard piped ducts before insulating them!
Also, tape the seams of the air handler with a decent quality temperature rated aluminum tape (eg Nashua 324a, available at most box stores.) Tape can be used for air sealing shiny-bright and clean galvanized ducts too, but mastic does better on most pre-existing ducts.
The cooling loads of most basements are quite low, so the fact that it feels cold next to the air handler isn't the most reliable indication of how lossy it is, but sealing and insulating the ducts is still "worth it", even if it doesn't quite buy you all of the additional capacity you're seeking.
Thanks Dana. Our ducts are sealed with aluminum tape & mastic. I'll ask the insulators to check again, but we've checked before - no leaks.
We have a lot of ductwork. See image.
Keep in mind the AC fan can easily use 400 watts, so it's essentially a 400 watt heater.