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Author Topic: Replace standard central AC unit with heat pump tied to oil furnace?  (Read 291 times)

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Offline gspren

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  Our AC unit is approaching 20 years old and looks bad but still works, and our heat is an oil furnace, both use same ducting. Is replacing the AC with a heat pump and some how only use the oil below a certain temperature something I should consider?
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Offline TroyC

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Re: Replace standard central AC unit with heat pump tied to oil furnace?
« Reply #1 on: April 29, 2022, 09:57:12 AM »
In the south my heat pump does fine until it gets in the 30's. That don't happen a lot but the colder it is the less efficient the heat pump is, making it run longer. Heat pumps usually are installed with 'emergency' heating elements (think lots of electricity). I have a friend in lower Delaware with a heat pump but I think he also uses supplemental oil heat in the winter.

Offline Crusarius

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Re: Replace standard central AC unit with heat pump tied to oil furnace?
« Reply #2 on: April 29, 2022, 10:06:51 AM »
I can't speak for the efficiency or functionality of the heat pump / ac since mine was installed at the end of october and I have radiant floor heat as my primary. But I can say if you thin there is any chance of it going out this summer you should have ordered a new system a year ago. It took almost a year to get our system and 3 days for install.

Offline peakbagger

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Re: Replace standard central AC unit with heat pump tied to oil furnace?
« Reply #3 on: April 29, 2022, 10:42:30 AM »
Heat pump is a bit of generic term these days. The old fashioned central heat pumps with a coil in ductsystem is not very efficient. The heat pumps were typically single speed or two speed. They could produce limited heat but most had electric coils that kicked in over some low temp. A big problem was many of these systems were built or retrofitted to home without AC when duct heat loss was not regarded as important. That usually means poorly insulated ducts run through unconditioned areas. This frequently means 1/3 of the cooling is cooling an attic or a basement. New heat pump technology uses variable speed drive compressors and other methods to put out heat down to lower temps.

The most efficient options are cold climate mini split units that can heat and cool remarkable well down to about 20 F. They can put out heat down to -12F but its not great heat and in most cases they can not keep up with the heating load at the lower temps unless a lot of extra capacity is installed. The most efficient units have an outdoor unit that mounts on wall (if in snow zone) in a location that is sheltered from blowing snow. The outdoor unit is connected with "trunk" of tubing and wiring to an indoor wall mounted unit. Some outdoor units can run several indoor units but the overall system efficiency is bit lit lower. Many utilities offer rebates for these units. There are indoor units that can be mounted in ceiling "cassettes" but the efficiency is bit lower. Overall minit split are much more efficient than conventional ducted heat pump systems as there is no ductwork to leak. The "trunk is lot smaller than a duct for moving heat. I use 12,000 btu minisplit to heat my house during shoulder season anytime the temp is above 20 F. I live up northern NH so my heat load it lot higher than yours and I heat the house with a wood boiler in colder weather. I have solar on the house and generate more than I need so I used the minisplit to burn up extra power. 

Minisplits are around $2,500 plus accessories and far more expensively installation. Most firms charge roughly double the cost of the unit for what is usually less than 4 hours work. I did my own install and then hired a tech to hook up the tubing, purge the lines, vacuum the lines and open the valves to charge the lines. It cost me $250. The trade off is the two name brands Fujitsu and Mitsubishi only guarantee their equipment if installed by dealers so I have no guarantee coverage unless it was bad out of the box. I talk to heating contractors who do this on frequent basis and the vast number of repairs are owner caused. Usually the outside unit gets damaged by foreign objects like basketballs or snowplows. Some folks install them in bad spots outdoors where they fill up wth snow or ice. Once they freeze up so the fans to not turn they need to thaw before they put out heat. Some offer a drip tray heater that can help but if someone blasts it with snowblower, the tray heater may not be able to help. Note there are some 2nd tier brands from China with potentially far less reliability that various parties sell for cheap, I see some ads for as low as $2,500 but caveat emptor. I would strongly urge that you get a quality whole house surge suppressor on your main panel as there are expensive electronics in these units.

So depending on your climate a couple of minispits can cut your heating cost substantially compared to an older style system but if you expect temps below 20 F you really need a backup . Some folks put in electric radiators to keep the house from freezing up but if you are living there you probably want a backup like a wood or  pellet stove. A forgotten by many option is a vented kerosene heater. The big brand name was Monitor but they are long out of business but Toyostove still makes them. They are now compatible with low sulfur diesel. They throw out a lot of heat with an easy install. BTW leaky ducts also let in lot of moisture and sometimes as much of half the cooling load is getting rid of moisture. Getting duct out of the picture especially from basements can really cut down on that cooling load. 

The DOE is pushing to electrify home heating in the next decade and I expect there will be bigger and better incentives in the next few years. The trade off if the current administration gets voted out then there go federal incentives.

If you have won the lottery, variable refrigerant flow systems are worth looking at but they are pricey for a regular residential home. 
  

Offline gspren

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Re: Replace standard central AC unit with heat pump tied to oil furnace?
« Reply #4 on: April 29, 2022, 07:04:18 PM »
My oil furnace works fine other than the cost of oil so that would be the backup, I just need to find out if a heat pump could be installed instead of the AC unit and still utilize existing duct work, I'd be fine with the oil kicking on at 35 degrees or whatever they say is appropriate. I'll ask a hvac person someday but no hurry, I've got too many irons in the fire now but I like thinking options.
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Offline SawyerTed

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Re: Replace standard central AC unit with heat pump tied to oil furnace?
« Reply #5 on: April 29, 2022, 07:33:50 PM »
We installed a heat pump over an oil furnace in my MILs home 6 or so years ago.  The heat pump works down to 38 then the oil furnace kicks in.  

It saved a lot of money the last several winters.  

We had to switch the duct work around.  The 54 year old house had individual returns in each room.  The returns were on the outside walls.  Conditioned air was supplied on inside walls.  We switched the duck work so supply was on the outside walls and returns on the inside walls.  Worked like a charm 
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Offline TroyC

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Re: Replace standard central AC unit with heat pump tied to oil furnace?
« Reply #6 on: April 29, 2022, 07:50:50 PM »
Looks like you should probably pay an AC guy for a quick look to see what you have. I have a 5 ton split system heat pump with the air handler in a closet in the middle of the house. The difference between your A/C and a heat pump primarily is that the heat pump has a reversing valve to allow the coolant to reverse so that it will heat instead of cool all the time. If you replace a central A/C with the same size heat pump I would think it should work.

I am assuming you have a split central system A/C with the compressor unit outside and the cooling unit somewhere inside the house.

I did replace a package unit (everything in one box, like they use outside of mobile homes) a few years ago. The ductwork was a tad small for the 5 ton unit but it had a jumper that would lower the fan to run somewhat slower. So far no issues at all. I ordered the unit and did the install myself. Not that difficult on the package unit. Like the previous comment, the warranty is void unless installed by a licensed A/C guy, but for what I saved I'll take my chances.

Offline Larry

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Re: Replace standard central AC unit with heat pump tied to oil furnace?
« Reply #7 on: April 29, 2022, 10:15:38 PM »
I built a new house about 30 years ago.  It had a heatpump with propane backup.  Worked great except propane had a big price increase about a year after I put the system in.

Last house I built was 11 years ago.  Heatpump with electric heat strip backup.  Just replaced the system with another one a few months ago.  This last one will heat into the low twenties and probably even lower.  I don't like it though because it runs a long time to do the job.  I programed it to switch from heatpump to heat strips at about 25 degrees.

This last house has all insulated ducts in the attic, I built on a slab.

Talk to a couple of HVAC guys in your area.  The best systems I've had experience with are Trane or American Standard which I think are the same today.
Larry, making useful and beautiful things out of the most environmental friendly material on the planet.

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Offline SwampDonkey

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Re: Replace standard central AC unit with heat pump tied to oil furnace?
« Reply #8 on: April 30, 2022, 03:27:19 AM »
Up here in the glaciated north we have them installed with electric furnace backup, if the temp goes below 20F you need the furnace. Electric heating up here is way cheaper than oil, by leaps and bounds. Electric heat is the most efficient heat to. I could heat with electric for 10 months what it costs for a 250 gallon barrel of oil. And that included lights and hot water and electronics in that electric bill. Some one questioned it one time and we showed them the electric bills for several months, end of argument. We have frigid winters in NB, 20F is a mild day up here. :D

I heat with wood at this new house, and electric backup. When I get a lot older, those switch around, wood for power outtaged in winter. 2 cord of wood should do me for 20 years unless the cars all take the grid down. ;D
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Offline Ianab

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Re: Replace standard central AC unit with heat pump tied to oil furnace?
« Reply #9 on: April 30, 2022, 03:40:08 AM »
Yeah, heat pumps start to lose efficiency below about 10C (50F), the good ones can handle down to freezing, but less efficient. 

The usual units here kick in electric heating elements if it's too cold outside, but then the efficiency drops to about 1. Same as any regular electric fan heater. Fortunately we seldom get down below freezing, and even then it's short periods. Most of the time that you need heating here it's actually warm enough for the heat pumps to work OK. Just make sure you have that backup when the day only gets to 40F. It doesn't happen often, but you need a Plan B for when it does. 
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