learn me: Heat Pumps for home HVAC

Our forced-air gas furnace and central air are both taking a dive and are coming up in need of either major repairs or replacement. We expect to retire in this house so I want to put in something for the long haul.

I've heard that while they're more expensive, Air Heat Pumps are efficient enough that they pay for the difference over the course of 4-5 years. I've heard conflicting things about whether they are effective in Michigan - some things online read like they are good down to 30 degrees or so, at which point you need a furnace or resistance heater. But other more recent articles say there are models that heat effectively down to 15 below zero, which would be pretty much worst case scenario. 

Can you completely replace a central A/C with a large heat pump?

I've also looked into geothermal but local friends who have gotten quotes say that the up-front cost is crazy, - like $30-50k. So that's probably out. 

Absolutely you can replace a central A/C system with a heat pump.  Pretty much wherever you call home, you will need supplemental heat for the coldest days of the year.  Here in Virginia, that is most often an electrical resistance heater in the air handler.  In your area, a gas furnace may be more economical.  See if you can talk to people in your neighborhood that have heat pumps and see how they like them.  A common complaint in the winter is that the air coming out of the vents seems "cold" since it's not nearly as hot as the air coming from a furnace. 

Watching this with interest. I have to imagine adding a heat pump to the AC would cut down on my annual oil usage, even if it only works above 30f. At what cost to my electric bill though?

A young guy I know told me spends $25 a month in winter for gas to run his furnace.  What?   (Chicago area)

When questioned he has a fancy wood burning fireplace insert that burns wood 24-7.  The gas furnace goes on when Chicago hits low temps.  

He spends his spring/summer time gathering free wood the rest of the year.  

Then one day I noticed his clothes tend to always smell like a camp fire. 

we went to a heat pump 2 years ago in our house. We are north central indiana so its a frozen tundra with winds averaging 20-30mph. At some point it'll switch over to the furnace in the unit. We went larger than the old unit because we were right on the edge of what would work well etc. Our electric bill went down about 20% from the switch over to a newer unit. 

While newer units are better at it, there is no way I would attempt a heat pump in MI. Wrong tool for the job. 
 

Before you do it think about your electric load. Whatever BTUs were being supplied by the furnace will have to be replaced by the heat pump. You will need to upgrade your electric feed to the unit (and possibly your service and panel to the building). And if you have strip heaters (which is a terrible idea in MI), the load will be huge. 
 

A heat pump/ furnace combo would work fine, but the installation would be pricey. 

Interested as well.  My utility is offering a slew of rebates... Something about helping the state reach its "green energy" goals (and selling more electricity to me in sure)

One thing to consider is that if any form of carbon tax comes into play in the future, whatever you burn will become more expensive and a heat pump will be more attractive.

We did a heat pump at our old house (1,900 sq ft raised ranch in CT). It gave us a secondary heating source (first was electric baseboard) and central A/C. It was basically the same coat as the A/C alone, so it was a no-brainer. We purchased back in '08, and the unit we had was effective until it was about 25 degrees F, and then we'd switch over to the baseboard heat. I've heard the newer units are efficient to a lower temp, but in MI I'd still want a secondary source of heat in addition to the heat pump. In the fall and early spring, it definitely cut our heating costs significantly, and the central A/C was a huge bonus, as we replaced our wall and window us it's which we're loud, and a pain to get the temps balanced throughout the house.

If you are having to replace your air conditioning unit anyway, I would recommend getting a heat pump. The only difference between a AC unit and a heat pump is a reversing valve. In cooling mode it pulls heat from the inside and dumps it outside. In heating mode, it swaps the position of the coils in the refrigeration cycle and pulls heat from outside and dumps it inside. They work great in the shoulder seasons and delay when you would have to start using gas heat.

Heat pumps are getting more efficient and while most of them can heat below 30°F, there is a de-rate for the lower temperatures. The lower the temperature outside, the less heat the unit can pull from the outside and the less heat will be available inside. I've designed around several units that advertise heating down to 0°F, but they are only able to provide ~30% of design heating capacity at those temperatures. Typically, we do not use heat pump only unless the winter design temperatures are above 40°F. Below 40°F winter design temperature, you need to add an auxiliary heating element. Typically these are resistance heat, but you can get a hybrid heat pump that has a gas or oil fired auxiliary heater. In milder weather, the unit will switch to heat pump mode and use the compressor and refrigeration cycle to heat the house. When the temperature inside drops below the auxiliary heat threshold (usually a few degrees below setpoint) it will turn on the auxiliary heat. The benefit to this is less energy use in the milder temperature times of the year. Your electric bill will be higher in the winter, but your gas bill will be lower and should offset the increased electric use.

In the south, the amount of time in auxiliary heating is low enough and electric prices are low enough that most people have electric auxiliary heaters. Since you already have gas at your house and are in a colder climate, I would go with a hybrid unit so you will still have the lower cost of gas compared to electric resistance heat when you need the auxiliary heat, but you can save money in the milder months using the heat pump instead of firing up the gas furnace.

Geothermal heat pumps are whole 'nother can of worm that transfer heat to water instead of air and use the relatively constant ground temperature to absorb/remove heat instead from the water. These require a lot of open land for a loop field, or a lot of deep cores drilled for a vertical field. The quotes you heard for a geothermal system are inline with what I have seen. A typical heat pump is within a few hundred of a cooling only system.

We've had to replace the a/c at our recently acquired place in WV a couple of years ago and went with air heat pumps. They were a little bit more expensive than regular a/c, but our run down to about 35F before switching over to the existing oil heat. Before we had the units replaced, we had to get the oil tanks refilled almost once a month, for the last two winters we were now able to make it through the winter with two refills of the oil tanks instead of at least four.

Based on how much it costs to refill those tanks, it looks like just the savings in heating oil consumption will pay for the whole system (not the price difference, the whole system) in about 6 years. That said, I think we're in a more ideal climate for air heat pumps because winters here aren't that cold - not like Northern NV where we had sometimes weeks of 10-20 degree weather or worse.

I have a heat pump and I don't like it. The furnace works by giving you an intermittent blast of hot air that you can feel from the registers. A heat pump works on a much lower differential, so to maintain 70 in our house the heat pump has to produce maybe 72 or 73 constantly, rather than 80 or 90 occasionally  from a furnace. So the effect in the winter is a constant cold draft blowing from the vents. I get tired of that so I often kick on emergency heat which cycles the furnace. It is nice in the summer for AC though.

It sounds like Heat pump + Natural gas backup is the most reasonable option for me. I would do heat pump -> radiant heat but a significant part of our home is on a slab so that's probably out.