Retrofitting a Ducted Minisplit in Existing HVAC System
ericelf
| Posted in Energy Efficiency and Durability on
I keep coming back to this question every few months hoping to find an answer that pleases me. Is today the day?
I have an 800ft*ft 4 room (2 bedrooms, kitchen, family room) ranch house in Cincinnati, OH. Simple gas-force air traditional HVAC system. I really want to get rid of the gas and go completely electric.
I expect that most people will view my efforts as a waste of money because I have a fairly new high-efficiency HVAC system. I will be replacing the expiring hot water heater with a hybrid system in the very near future and I have a kitchen remodel design that replaces the gas range.
The ranch house has a full basement where the HVAC and ducting is located so just floor vents in the 4 rooms + 1 for the basement. Two returns located on either end of the house.
Before I spend too much time on this, I’d like to get some experienced opinions on what might be possible. Which of these options do you think are most feasible and cost-effective?
Option A) Decommission existing HVAC, but do not remove it. Install properly sized Single Ducted Mini in the supply plenum. Plan to have existing DC blower run on low(?) to help with pressures and move air.
Option B) Remove existing HVAC, but keep ductwork. Install properly sized Single Ducted Mini in the supply plenum. (My previous research seems to indicate that this will take a specialized unit.)
Option C) Since I have a decent A/C already, is there any energy-efficient way to just replace the heating?
Thanks in advance for any quick words of advice! Just trying to help target my solution search at this time.
Eric
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Replies
If you have energy usage history, you can go through this and figure out roughly what your actual HVAC loads are:
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
Chances are, in a small house like that, the existing hvac is at least 2x oversized, which is a good thing as it means big ducts.
Once you have the actual load, measure the pressure drop in your existing ducts. You can get a cheap differential manometer from your standard online sources for this. You can look at the specs of your HVAC blower to see what the flow rate of the unit is. You can now scale this number to figure out the pressure requirement of the new unit.
For example, this will typical be around ~0.8" of water at 1000 cfm. So if you connect up a unit that only needs 500cfm, the pressure drop would decrease to 0.4".
There is a good chance that any mid static ducted unit (Mitsubishi PEAD) can handle this, if you have extra large ducts, sometimes even a low static unit can work.
There are also some high pressure air handles that are very similar to a standard furnace (ie Mitsubishi PVA-A). These are not cheap though.
You can also look at just replacing the existing coil with a heat pump. Something like an Infinity greenspeed. These only go down to 2ton, which most likely is oversized for your house, but can probably made to work.
@Akos Thanks for the link and information. I'll do some calculations and investigation. Good to hear that there is a reasonable chance of success!
Eric,
I reused the ducts serving my top floor (approximately 1,000 square feet) by installing a Daikin ducted mini split. I suggest working with a good contractor who understands minisplits and how they should be sized to optimize comfort.
@Akos and others - I have completed the HeatLoad analysis (link below) and have come up with a current HeatLoad of ~21,000BTU/hr (64℉ inside, 4℉ outside). I have not completed the static pressure analysis yet, but expect it to be fine or it could easily be resized.
I'm thinking a complete replacement of my gas HVAC system is what I should consider (this is still a hard pill for me to swallow as I will be replacing a reasonably good A/C as well and a nice blower). I'd replace with something like an LG LV361HV4.
Soon, I'll talk with local professionals, but would like to understand the sizing of HeatPumps. My understanding is that I'd want to multiply the HeatLoad by 1.4 to size a standard gas heater. Let's say that is 30,000BTU/hr for me.
Now my question is, for HeatPumps where the heating capacity decreases with outside temperature, do I need to consider the units capacity AT MY DESIGN TEMP? Which of the units below do you think would be a better fit for my house?
The LG LV361HV4 model has a heating capacity of 35,000 (70℉ inside, 5℉ outside).
The LG LV241HV4 model has a heating capacity of 24,000 (70℉ inside, 5℉ outside).
I assume it is a bit of a coincidence that the heating capcities above closely match the named sizes of the units for LG. When I looked at Mitsubishi they seemed to drop more.
Thanks bunches for any additional input! Eric
Analysis: https://docs.google.com/spreadsheets/d/103b3LDfhmgfNuZ9ruNaFgCOCkXLzaCbyhku54AAG1zM/edit?usp=sharing
Is your indoor design temp 64° or is that a typo?
I suggest you do the math about this fuel switch. My guess is if you deprecated you new system over 20 years along with fact it is likely to cost more to operate a heat pump it will likely cost $1000 a year more for cooler less comfortable system. Your money your choice if it make you feel good go for it.
At 400 sqf per ton it sounds like you would be much better off putting your money into insulating and air sealing.
Walta
Walta,
The 65F is the balance temperature of the house. This the the outdoor temperature where typical older construction houses don't need any heating to keep the inside at 70F.
Walta - Thanks so much for your reply!
I hear you about putting money into insulation+sealing first. Current walls are brick + solid block + dry wall. Very little (~ 0.25") air space. Would love to have expert opinion, but yet to find the right person. I have created a "simple box model" to try to estimate where the heat is going and thus where I'd get the most bang for my money. Link below to sheet, but may be difficult to understand.
My analysis shows a heatpump at COP=2 should cost about the same.
I have had a dual fuel heatpump, so I understand "less comfortable".
Simple Box Model: https://docs.google.com/spreadsheets/d/1ioERLXaWfwSa_5-iJ5MzHroXojh72OldQT8XelV16wc/
Original heating analysis with cost analysis added: https://docs.google.com/spreadsheets/d/103b3LDfhmgfNuZ9ruNaFgCOCkXLzaCbyhku54AAG1zM/
Energy use history over months effectively uses average infiltration which is far less than infiltration under design conditions. Your +40% bump may be enough to cover the difference, but who knows. Yes, use capacity at design temp.
> not completed the static pressure analysis yet
1/2 the flow in your ducts will mean about 1/4 the pressure drop.
Jon R - Thanks for mentioning about increased infiltration at low temps. I had not thought about that. I'll keep it in mind.
Generally all hyperheats provide their nameplate capacity heat at 5F. They are usually able to provide bit more heating and cooling above their namplete number outside of this. Since your 99% temp is above 5F rating, using that it should be quite safe.
From the size of the house and your heat loss, it feels like your place has some major low hanging fruit for energy reduction. If your basement is not insulated, that alone could be 15% to 20% of that number. Putting even 1.5" of rigid insulation on the basement walls makes a pretty big difference and significantly warms up your floor upstairs.
You can also try to do a bit of blower door directed air sealing. If they find big holes, plugging those would probably get you with the range of the 2ton heat pump. There a couple of models that run a bit higher than 24k at low temps that might be worth while to look at:
https://ashp.neep.org/#!/product/33644
https://ashp.neep.org/#!/product/31987
https://ashp.neep.org/#!/product/25275
https://ashp.neep.org/#!/product/25272
Some of the units don't come with a pan heater (check if it is an accessory), that is generally a good idea especially if you see a lot of snow.
The min modulation on the some of the furnace style air handlers is not that great, I would aim to oversize as little as possible (or pick one with better range).
Akos - Thanks for the links and additional information!
With brick + solid block + dry wall, my walls are going to be hard to improve, but I like the idea of insulating the basement. It was in the longer term plans for when I was to finish that space. I just added the needed egress window recently.
I was hoping to find a free evaluation that would include blower door, but they seem to all be gone. It may be that I go another season before updating.
Thanks again!