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Can I improve HVAC efficiency comfort in my home?

wayneslc | Posted in General Questions on

My wife and I live by ourselves in a two-story 5600 sq ft home in climate level 5B in Salt Lake City Utah.  The top floor is about 2800 sq ft and is at ground level. We live on the east bench of the Wasatch Mountains. The bottom floor of the house abuts the side of the mountain on the east side but on the west side it walks out to a terraced yard.

 

We have two air-conditioning and heating systems, one for each floor. The more efficient and larger units are for the top floor and frankly, we rarely turn on the heating or air conditioning on the bottom floor because we are rarely down there and because it is naturally cooler down there in the summertime because of the east wall of the bottom floor being nestled into the mountainside and because the top floor AC unit cools off the bottom floor as well.

 

Our master bedroom measuring 14.5 x 11.5 x 17.5 feet is on the top floor and faces west resulting in our room being one of the warmest in the house during the summer.  Because we live by ourselves most of the time except when our kids or friends are visiting, I feel guilty that in order to keep our bedroom cool, we must cool off the entire upper floor. This seems very inefficient. Also, if someone is visiting and staying in one of our other upstairs bedrooms, they seem to be too cold when we set the air conditioner for our comfort level in our bedroom.

 

Similarly, in the wintertime, turning the heat on to keep our bedroom warm enough for our comfort level also seems inefficient when we are by ourselves as we are heating the entire upper floor. Worse, when we have guests staying on the bottom floor, keeping the bottom floor warm enough for them results in too much heat throughout the upstairs area including our bedroom.

 

We have lived in this house for approximately 20 years, the last five of which we have had solar panels installed on our house which provide most of the energy for our two electric cars and for the rest of our electrical needs. It produces approximately 12 kW per hour maximum.

 

I’m thinking that perhaps installing a minisplit heat pump might be the best way to mitigate these temperature problems in our bedroom, hopefully improving our carbon footprint overall. Am I missing anything (we already have ceiling fans in all of our bedrooms and living spaces)? If someone agrees that a minisplit heat pump is the answer, is there a way to determine the correct size of a minisplit heat pump for a room of the size mentioned above? And if so, would this also be somewhat economical- note I didn’t calculate potential savings from installing solar as I felt that I should install it given our city’s air pollution issues and Utah’s dependence on coal for electricity generation.

 

Thanks

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Replies

  1. Robert Opaluch | | #1

    Rooms that face west tend to have a lot of solar gain in the summer months, depending upon the extent of shading of windows. (You probably know this from your experience in your home.). There are many ways of shading the sunlight, so it doesn't heat up your bedroom (window treatments, awnings, trees, covered decks/balconies/patios....). It probably makes sense to consider possible tactics to reduce the summertime solar gain, rather than install a system to counter the effects after the fact. That would likely be more carbon neutral that using energy to undo the energy gains in summer. I'd guess that you'd have to provide a lot more in the way of details to get more detailed suggestions, but others here are likely to provide additional ideas too.

    You would need to measure room and window dimensions, and (guess or list) the materials in your walls, roofs and below the lower floor, and do a bl0wer door test or at least estimate the air leakage (age of building? Number, quality and type of windows and doors?) in order to calculate the heating and cooling load of any of the rooms. Then a mini split, PTAC or even a small window air conditioner or heating unit could sized appropriately to solve these overheating and underheating problems.

    Yes a mini split is an affordable and efficient way to provide more heating and cooling, without knowing more specifics about your home and lot.

    You could do (or have someone do) a Manual J calculation per room to size a mini split or other system, but asking others to do it for you can result in a lot of estimates, likely to be large overestimates, of heat loss and gain.

    Sorry I can't provide more specific suggestions without a lot more details about your home and lot.

  2. Walter Ahlgrim | | #2

    The first step would be a good energy audit. Look for someone willing spend some time at your house. I say flat out ask how long they expect to be on site at your house and hold them to that estimate. You want a blower door test and for them to identify more than a few current leaks. Also you want inferred (IR) photography preferably on one of the hottest or coldest days of the year. The IR photo will show air leaks and weak spots in your insulation.

    One of pet peeves is duct work and or equipment in the attic. If you have them and can get rid of them you will be way ahead.

    It sounds like your current equipment is way over sized for your house adding a mini only make that problem worse.

    Walta

  3. wayneslc | | #3

    Thanks for your reply. The room dimensions are 11.5 ft high by 17.5 ft long and 14.5 ft wide. I've attached a picture of the west wall dominated by two ~ 8' by 30" doors, most of which is double paned glass (79" by 191/4"), and 2 windows, each about 6' by 3'.

    There are two other doors leading to the master bath and rest of house each 78" by 32".

    There is a covered deck outside of the doors and we keep the sun from coming in with our window treatments. The stucco and deck floor heat up however and I believe cause much of the room's heat gain in the evening.

    The house was built in 2001. It is well insulated with fiberglass initially and then blown insulation in addition about 10 yers ago. Wood flooring, not sure what was placed between the two floors otherwise.

    1. Charlie Sullivan | | #7

      Your window shades might still allow substantial heat gain, if they absorb some light including IR and heat up, transferring the heat to the room. You've got a good overhang there, but you might do better with some shades you can lower in the afternoon at the edge of the balcony. That would keep the balcony itself a little cooler, reducing heat flow through the large-area windows and doors, as well as reducing the direct solar heat gain through the windows.

      Otherwise, a good part of the heat gain may be from air leakage through the door and maybe other places. An energy audit with a blower door test would be a good next step.

  4. Tom May | | #4

    Well each of the rooms should have registers that deliver the cool air. Try adjusting or closing them in the rooms that get too cold to deliver more of the air into your bedroom. Same goes for heating. Thermostat location can be a factor also.

  5. Expert Member
    Akos | | #5

    The space is borderline for a small mini split but doable with a good 6000btu wall unit. Check the specs on the unit you go with and make sure the head can deliver close 100CFM on low speed, some are around 200CFM which creates comfort issues in smaller spaces.

    It is not a cheap option, but would give you full zoning control for the room.

    Before going that road, I would look at re-balancing your HVAC. It sounds like the room, since it was an addition, it was never properly integrated into your HVAC and not getting enough flow. Fixing this would probably cost less than the mini split.

    You can check this with a budget airflow meter from your favorite on-line marketplace (make sure to get one that can read low enough less than 100FPM). Measure the velocity at the register in FPM * AreaOfRegister(sqft) to give you flow rate in CFM. You can compare this to the other registers in the house to see how well the system is balanced.

  6. Jon R | | #6

    Worth doing, but ultimately fixed balance heating/cooling is a myth. What is balanced under one set of conditions will be significantly wrong for others. I'd try a "Flair Smart Vent" or bedroom thermostat controlled booster fans.

    > keeping the bottom floor warm enough for them results in too much heat throughout the upstairs area

    For this, look into increasing the isolation between the areas. Probably an air flow issue.

  7. Expert Member
    Dana Dorsett | | #8

    >"I’m thinking that perhaps installing a minisplit heat pump might be the best way to mitigate these temperature problems in our bedroom, hopefully improving our carbon footprint overall."

    Be aware that most bedrooms in newer houses don't have sufficient load to run a 3/4 ton (or even half-ton) mini-split at high efficiency.

    Without calculating the room by room load numbers it's a bit hard to make solid recommendations.

    Assuming it's low-E glass, what is the SHGC of those double-panes? (You may have to do a bit of research or even some home-archaeology to figure that out. ) A heat rejecting window film might be appropriate, or not, depending on the answer. Operable EXTERIOR shades is always a good option for rejecting direct solar gains. There are many options that are somewhat see-through.

    >"...we have had solar panels installed on our house which provide most of the energy for our two electric cars and for the rest of our electrical needs. It produces approximately 12 kW ..."

    >"...I didn’t calculate potential savings from installing solar as I felt that I should install it given our city’s air pollution issues and Utah’s dependence on coal for electricity generation...."

    The cooling load is probably peaking during the hours the solar is operating, so it's actually powering the AC more than it's charging the cars.

    I thought it was possible to buy 100% wind power in SLC through the utility for a slight up-charge from the base rate, or is that no longer the case? (A friend of mine in SLC a decade or so said he was buying wind power.) See:

    https://www.rockymountainpower.net/savings-energy-choices/blue-sky-renewable-energy.html

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