Author Topic: HVAC Overhaul  (Read 1438 times)


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HVAC Overhaul
« on: May 31, 2017, 09:32:06 AM »
A couple years ago, we bought a dream family house to raise our kids in.  It was built with efficiency in mind back in 2007, and generally is well thought-out.  It has 6" walls with good insulation.  It has two separate 2-ton geothermal zones--one upstairs and one down with both floors at roughly 1600 sq ft.  The downstairs zone works like a charm--you get about 4-6 degrees of temperature change per hour regardless of the temperature outside. 

The problem is the upstairs system.  It's a split system with the heat pump in the basement and typical copper refrigerant lines going to an air handler in the attic.  The attic is a massive space since it has a 10/12 hipped roof over the 1600 sq ft.  It has great airflow with soffit vents and a ridge vent.  There's also about 20" of cellulose for somewhere around R-70 insulation.  Unfortunately, the air handler is in the unconditioned space, and while they use insulated ducts, they're only R-8 or so.  The returns are fiberglass ducts and the supply ducts are insulated flexible.  I've measured the temperature gradient across the air handler, and it's performing near spec.  However, the heat/cool loss of the duct work is noticeable on the hottest and coldest days.  For example, when it's 10F outside, the airflow through the attic brings it down to close to that.  65F ambient air goes through the return, cools to about 58F before reaching the air handler, gets bumped up to 73F, and cools to 67F on the way back to the vents.  On  one occasion in the last 2 years, it's lost temperature while continually running.  I'm trying to think of a way to minimize the efficiency loss through the attic.

I've come up with 4 potential options:
1. Don't do much of anything except basic efficiency gains (make sure there's no air leakage in the ducts, keep the filters clean, keep refrigerant charge appropriate, etc.)  Although it's frustrating to notice the difference between the comfort downstairs and upstairs, it really doesn't cost that much to keep it going this way.  I mitigate this on the hottest or coldest days by setting the thermostat a few degrees hotter than normal and setting the thermostat to hold the temperature.  It usually doesn't lose temperature, but the system runs constantly.  For the whole 3200 sq ft house, the entire electric bill has never gotten higher than $220 in a month, and it's the only utility I have (no natural gas, propane, city water, etc.), so it's really not bad.  I can't imagine saving much more than $50 or $60 in a month, so the gains are minimal.  If I go this route, I might put in a pellet stove or propane fireplace upstairs for supplemental heat in the winter.  I'm also considering adding solar electric panels, so I can add a few panels to mitigate the extra electricity usage.

2. Seal the attic and remove the cellulose.  I'm not sure this will save me much space since the volume of attic is so large so that turning it into a conditioned space would require a lot of extra conditioning!  This would let me keep my existing system and duct work in place.

3. Build an enclosure around the HVAC system and ductwork and move the cellulose on top of the enclosure.  This is probably the most efficient solution, but would involve getting all of the building materials through a 2' x 4' attic access in the girls' closet ceiling.  It would also be tons of work.  This would let me keep my existing system and duct work in place.

4. Install some sort of mini-split hydronic geothermal system.  This would be my preferred solution provided one exists--pass power and heated/cooled water to each room separately from the basement heat pump and allow for each room to control the thermostat themselves.

Right now, based on the costs of each of those other systems and the fact that the current system works well enough, I'm leaning toward #1 until the system reaches its end-of-life, and then moving to #4.  Does this make sense?  Are there any other ideas that I should consider?



  • Stubble
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Re: HVAC Overhaul
« Reply #1 on: June 02, 2017, 07:31:16 PM »
Saw this and had 2 thoughts (surprised I had any left, am totally burnt out from work) - the return is duct board and the supply is just flex? No supply trunk, just flex? Maybe the duct just sucks, not just leaks or lack of insulation, but design issues, like long flex runs and loss of velocity. Also, if the cost of the insulation and other changes to condition the attic aren't outrageous, you could have someone do a heat load (Manual J) to see if the current system can handle it.

Jon Bon

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Re: HVAC Overhaul
« Reply #2 on: June 03, 2017, 09:15:02 AM »
Woah 220 a month and its the only utility you have? And its a 3200 sqft house? Do you complain about the mileage on your Prius too? :)

Seriously though there is not a ton you can do here. Like option 2 removing the cellulose and then conditioning the attic would cost a TON of money more then you would ever save.

So am I to assume you have an electric dryer, stove, and hot water tank? All those things are going to be a substantial load on your electric bill as well. Is your upstairs uncomfortable when its very hot and very cold? You could probably try to cover/wrap your ducts with more insulation but it would probably be more trouble than it was worth and might crimp them restricting airflow. Sounds like you have a pretty efficient set up unless there is more information.

Are any of the ducts taking sharp or unnecessary turns in the attic? That can really hurt your efficiency.


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Re: HVAC Overhaul
« Reply #3 on: June 08, 2017, 08:52:49 PM »
Can you improve your ducts without much trouble?  Replace flex hose with rigid, wrap it well, etc?  And/or bury the ducts in cellulose.

Building an enclosure shouldn't be too hard.  I would use foam board insulation, lightweight and easy to work with.  Cut it with a razor and assemble it with duct tape.  Half, third, or quarter sheets will fit easily through your attic door. It should be easy enough to come up with a design that can be taken apart for service.

Upgrading your system, especially to geothermal, will cost many thousands, and take a long time to pay off.  If the heat pump isn't enough, I would do a pellet or propane stove.