Zero Carbon ® Blog

A: Yes, if you do not have ductwork for AC. If you have the ductwork for AC then the best solution is to connect your heat pump to that duct work. It will then both heat and cool via forced-air circulation, so your primary heating will now be by forced-hot air not forced-hot water. Not everyone likes forced-hot air heating, though I prefer it to forced-hot water because it creates better air circulation. If you do not have ductwork, you can add heat pumps that heat the water for circulating in the radiators. Please see the answer above.

Another alternative to adding ductwork is to install mini-split heat pumps rather than a ducted system. This is what we have at our rental property. Having experience with both ducted and ductless systems I prefer ducted systems. The air circulation is better, the installation cost is lower (if you have the ductwork in place) and the efficiency (COP) is higher, in our case the COP on our ducted system is about 3.0 and on our ductless system is about 2.5.

There are also hybrid solutions where you put new ductwork in the places that are accessible. Usually the first floor is accessible from the basement and so it is fairly cost effective to add ductwork to the first floor from below. Likewise it can also be cost effective to add ductwork through the attic to provide heating/cooling to the top floor. If you do add a heat pump to the attic the attic needs to be well insulated and the heat pump needs to be inside that thermal envelope. If you don’t do this you will be wasting a lot of heat in the attic in winter. This heat will melt snow on the roof and can cause ice dams to form and water leaks that can cause a lot of damage.

A: The question I asked during the webinar was not very clear.  We have a water based heating system with radiators and a boiler fueled by gas.  We also have an air-sourced A/C system with small tubes running throughout the house (a UNICO, high velocity system) without the hanging of mini-split systems in each room.  Is there any way of using an air sourced heating system to heat the hot water in our radiator based heating system (either through the existing boiler or directly to the hot water circulation system)?

Hi Alan, yes is the short answer, you can connect an air-sourced heat pump to a water radiator system. These are sometimes called hydronic systems. But the long answer is a bit more nuanced. These type of air-sourced heat-pump to hot-water systems are made by Jaga, Daiken and Spacepak and other manufacturers. I have never used one so I have no particular recommendations. The hot water in a boiler-driven radiator system is typically at 140F. Air-sourced heat pumps have a hard time getting the output side (whether it is air or water) above about 110F. So your radiators will no longer have that piping-hot feel (which may be good or bad – I have nearly burnt myself on hot-water radiators before). This means that it will take longer to heat your house up from cold – like when you come home from a winter vacation. But, as long as the heat output is matched to your heating load your rooms will stay warm, it will just take longer to get warm compared to what you are used to.

Are you thinking of using the UNICO high-velocity system with a heat pump so it would heat in the winter as well as give you AC in summer? We have a similar system in our rental house and I would consider replacing it with a heat pump if such a system exists. Let me know if you find one. Note, if you do this you will need to insulate the attic on the sloped part of the roof plus the gable ends and add window inserts to improve the insulation on any windows in the attic. If you don’t do this, the heat pump will be leaking heat into the attic in winter (no matter how well you insulate the ducts themselves) and this is not only wasteful but it will melt the snow on the roof which can lead to ice dams and water leaks into the house.

Q: Have you done any calculations (what % higher than air source) to show when geothermal is a better option from a financial sense? A: I have not done precise modeling, but if you can generate electricity from solar panels at 5c/kWh (doable with a sloped south-facing roof), and you can get the wells drilled a lot cheaper than my quote, and you get the federal tax credit then I think geothermal starts to make financial sense.

Q: Was a pressure test conducted on the building and ducts to determine their leakage rates? A: No, but I sealed the holes with aluminum tape so I think the system is pretty good.

Q: Before the heat pumps were installed, was a manual J & manual D conducted to properly size? A: No. But remember this was a retrofit and we already knew that the old system had sufficient heating and cooling output to create a comfortable house. Since I was significantly reducing the heating and cooling load with the fab four, I was very confident we had enough heat and cool in the system.

Q: My understanding if you do not size the HP system using manual J & Manual D and the building and duct leakage are not verified, then NO unit will meet loads. A: Not true, see above.

A: Electric back-up heating is feasible, and many contractors push it as the source of back-up heat when heating with heat pumps. But they are not paying the bills. Using utility electricity at 23c/kWh of electricity to run an electric resistance heater means heat would cost me 23c/kWh of heat in the house (electric-resistance heating elements are 100% efficient at converting electricity to heat) but my oil furnace costs about 8c/kWh of heat in the house. Natural gas costs about half this.

A: If the electricity costs more than about 12.5c/kWh then the natural gas will be cheaper, see the answer above. In most of New England and the West Coast electricity is much more expensive than this (it is 23c/kWh for us). In these states you will need solar and a heat pump to both save money and cut your carbon footprint.  Part of the reason electricity is so expensive in these states is because the electricity bill includes many subsidies for energy efficiency, solar panels and heat pumps. These subsidies include generous net-metering tariffs, generous solar panel and battery subsidies and low-interest-rate or zero-interest-rate loans, which are often even more subsidized for low-income owners.  The best way to cut bills and carbon footprints in low-income communities is to use these subsidized loan programs. If the homeowner does not have a credit score high enough to get a loan, then a leasing arrangement for the panels is always possible. These leases can be very expensive for the homeowner but in the last year or so I have seen lease rates come down, making them at least reasonable options if the homeowner can’t get a loan. In a band of states in the middle of the country from the Dakotas to Louisiana electricity is about 10c/kWh and so there is little money to be saved with solar panels. For homeowners in these states, it will be cheaper to heat with a heat pump than to heat by burning natural gas. It will also be very cheap to run an electric vehicle. 

A: A mini-split heat pump head (i.e., the part that is inside the house or basement) WILL dehumidify the air inside the house but only when it is operating in AC mode i.e., during the summer months. Since basements are generally cooler than the rest of the house, it may not be operating as an AC unit very often and so it may not actually be dehumidifying the basement much at all. In the winter, when it is operating as heater it will not be dehumidifying the basement air. 

How effective this is for you is going to depend on what the source for the humidity is in your basement. If the humidity comes in summer (which is quite common because warm humid air enters the house and then condenses on surfaces in the basement because the basement is colder than the rest of the house), then the heat pump will dehumidify the basement air but only when it is in AC mode, which as I mentioned it probably is not doing very often even in summer. However, if the source of humidity in your basement is due to rising damp, i.e., from a high-water table or from rainwater leaking in from the soil then this tends to happen in winter when the ground water is highest or we get rain on top of thick snow. In winter the heat pump will be heating your basement. So now you will have warm humid air in the basement. Warm air rises. Humid air rises. Warm humid air really rises. When this warm humid air contacts a cold roof surface it will condense. This will lead to mold and rot. This is a common problem in cathedral ceilings. Because of this, I think relying on a mini-split heat pump to dehumidify your basement is not a good idea. I think you will be much better off installing a heat-pump hot water tank. These dehumidify the basement air whenever they are heating the hot water, which is year round. Heat-pump hot-water tanks are much cheaper to run than fossil-fuel hot-water heaters. The dehumidification comes as a side benefit, but in my opinion, it is a major benefit. If you don’t want a heat-pump hot-water tank, then just getting a dehumidifier will probably do the job. Dehumidifiers provide a small amount of heat in the basement too. 

A: See Chapter 2 in the book. I recommend keeping the furnace and replacing your AC units with heat pumps. Heat pumps cost less to run if you power them with solar panels. See below for the likely difference to your heating bill and carbon footprint from adding heat pumps:

Adding a heat pump will have dramatic effect on a home’s heating bill and carbon footprint, but the effect varies considerably depending on what heating fuel is currently being used and what the source of electricity is for the home.

Effect of adding a heat pump:

	If currently heating with natural gas	If currently heating with heating oil
If using Eversource or National Grid electricity (at 23c/kWh and 0.87lbs CO2/kWh)	150% increase in heating bill 10% cut in carbon footprint	25% increase in heating bill 45% cut in carbon footprint
If using Municipal Light and Power electricity (at 14c/kWh and 0.87lbs CO2/kWh)	50% increase in heating bill 10% cut in carbon footprint	25% cut in heating bill 45% cut in carbon footprint
If using Rooftop solar panel electricity (at 8c/kWh and 0lbs CO2/kWh)	10% cut in heating bill 100% cut in carbon footprint	50% cut in heating bill 100% cut in carbon footprint

This assumes that the heat pump achieves a year-round COP (coefficient of performance, or efficiency) of 2.3, which is the average found in a review of the work of several researchers on Mitsubishi, Daikin and Bosch heat pumps in New England.  Both ducted and ductless systems were included. In general, ducted systems had higher COPs than ductless systems and Mitsubishi had the lowest COPs, as low as 1.3 in one case. The manufacturers claim COPs of between 3 and 5 for these units. None of them perform to the manufacturers’ specifications under real-world year-round conditions in New England.

A: It takes electricity to run a heat pump. There is no fuel. A heat pump is like a water pump, it moves heat from one place to another just like a water pump moves water from one place to another. A water pump does not create water and a heat pump does not create heat. A water pump needs electricity to turn the motor and a heat pump needs electricity to turn its motor too. When I calculate the cost savings from using the heat pump it is the savings on the heating oil bill less the cost of the electricity needed to run the heat pump. The same is true for the carbon footprint.

A: Where we live in Dover a generator is standard issue because we get frequent power outages in winter. However, no generator will produce enough power to run your heat pumps. This is one good reason to keep your fossil-fuel furnace when you add a heat pump. The back-up generator will run the circulating fans so the combination will keep you warm. I have recently bought a battery to replace my defunct back-up generator. The subsidies are very generous on batteries in MA right now, so if you are considering a back-up generator now is a good time to consider using a battery instead. Please see other answers on batteries.