Insulating Attic for Better Air-Handler Performance
Hi Everyone,
About 4 years ago I had a Mitsubishi Hyper Heat system installed in my home. There is a condenser outdoors, two wall units heating and cooling downstairs, and I have a ducted air handler in the attic (1.5 Ton Capacity Ducted).
My major issue is that on fairly hot days (83°F or higher), the 2nd floor gets quite warm. The warm out of the ducts is 71°F until the weather gets really cool at night. On days where the outdoor temperature isn’t too hot, the air out of the ducts is quite cool (58°F). I will say that no matter the outdoor temperature, the wall units are great and put out really cold air and have no problem keeping everything cool and the air coming out of them is still quite cold. The outdoor unit doesn’t seem to be running continuously even on the hottest days.
Currently my attic is unconditioned and gets really hot on those days (130-140°F). I suspect that is really causing the problem. I guess my question is this, is it reasonable to expect that if I have my attic rafters spray foam insulated and remove the blown in insulation to make my attic a conditioned space, that the ducted unit in the attic should be able to keep up ok?
Specs:
My house is ~2100 sqft
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Replies
Not an expert, but it seems weird that the ducted portion of the system is not delivering cold air on hot days. (That's kind of its job, right?) Are these metal or flex ducts? Are they insulated? How large is the area this unit is serving?
Using spray foam will be expensive, and then you will need enough capacity in your system to deliver some conditioned air to the attic space.
What does your HVAC contractor have to say about this situation?
I like your plan so long as you understand your conditioned attic is likely to only be marginally better than the current situation. Mostly because the roof & gables you want to insulate may have twice the surface area of you ceiling getting the even code minimum R value become very extensive given the large area. If you did manage to keep the same R value when you move the thermal boundary the larger area means you will lose/ gain more heat.
What the current ceiling R value?
To my ear it sounds like the return air duct in your attic are very leaky and pulling in hot air from the attic and trying to cool it and deliver that air to the house. See if you can measure the air temp as it enters and exits the unit and the air temps at the supply and return grills.
You may want to look for someone to test your ducts with a "Duct Blaster” and fix the leaks.
Walta
Assuming your ducts are insulated and reasonably sealed, you might have a similar configuration issue:
https://www.greenbuildingadvisor.com/question/multi-zone-mitsubishi-system-not-performing-with-all-zones-running
My contractor installed tight fitted polyiso boards between the rafters of my conditioned mechanical attic which houses my heat pump air handler, then level 1 drywalled over that.
So far, on hot 90F+ days, the attic stays in the low 80s, and the air handler (which is also highly insulated) and flex ducts contained therein manage to bring cool air everywhere.
From what I understand PolyISO is better than spray foam from many perspectives, including cost, toxic off-gassing, and much lower GWP. The installers used simple tools like a bread knife to cut and install it.
Wow really great responses! I called one of the Mitsubishi Diamond Elite contractors and they seemed pretty useless, frankly. He told me that having a 3 ton condenser outside and a 1.5 ton air handler in the attic makes no sense and that the indoor unit should always exceed outdoor unit. Regardless I'll follow up on these suggestions and on the next hot day (should happen within the next week) I'll try the following:
1) Turn off all wall units and keep on only the air handler to see if I have the same issue that Akos linked to (perhaps low refrigerant or improper configuration).
2) Try to inspect the return to see if there are any leaks. My very amateur plan is to spray some old cologne around the return line in the attic and ask my wife to see if she smells it coming out of one of the supply vents.
3) All else fails, try changing the configuration items also as suggested by Akos' link.
Maybe a simple diagram could help some of us understand the attic duct location or configuration.
If the attic ducts lie across the floor of the attic, why not incapsulate those ducts rather than trying to insulate and condition the whole attic space? If they are hung up high in the attic, could they be relocated down to the attic floor, and placed below rather than above the attic insulation?
Just wanted to leave a note that I read the thread that Akos linked to and I suspect I have a similar issue. Usually when it is > 83°F outside, my upstairs can't maintain setpoint. However on Sunday when it was 89°F, I turned off everything except my ducted air handler and I replicated the exact same results; the unit performed great when the other units were off, the air vents coming out of the air handler were quite cold (57°F) and were able to keep the upstairs really cool. I also just changed the setting as mentioned in that post (SW6-7, SW6-8, Target ETm(°C) = 6). Its not super hot today, but I will post back once another hot day occurs to see whether it now works even with the downstairs ductless units are on.
I just wanted to post a reply since in the last week we have had about 5 days in the mid to high 90's here in Massachusetts. After I made the setting change as I previously mentioned, the air coming out of the ducts remains fairly cool. I didn't measure the output temperature, but previous to the setting change, my upstairs would reach ~78 °F and not start cooling off until late at night. Now the ducts are putting out a decent amount of cold air and the max temperature reached is ~74°F, it starts cooling off in the early evening and is at setpoint (72) by 8pm or so, which is great.
So if anyone else has a ducted Mitsubishi air handler in the attic and it does a terrible job on warm days, change the setting on the outdoor unit (SW6-7, SW6-8, Target ETm(°C) = 6)