This is part of a series on electrification in New Mexico.
The most common distribution systems for heating and cooling are forced air, radiant floors, and baseboard radiators (including traditional radiators). More recent are the use of mini splits which have their own air handlers. For an existing home it is generally advisable to retain the existing distribution system since that is the least disruptive and probably the lowest cost alternative. My house used baseboard radiators so it did not make sense to change to a different distribution system such as that provided by mini splits.
There are air to water heat pumps, air to air heat pumps and mini splits. Another option is ground source heat pumps although these have a high upfront cost and generally are not an option for retrofits. Air to water heat pumps are what I would recommend in most cases. Air to water heat pumps can drive air handlers or work with radiant floors and low temperature baseboard radiators and they also can provide domestic hot water in addition to space heating and cooling. Usually one system to provide everything is going to be more cost effective since the upfront costs can be split over multiple uses and seasons.
When I started researching options before my gas boiler completely stopped working much of what I found was available in Europe or elsewhere in the world but there were not many options that I could find in the United States. The Department of Energy launched the cold climate heat pump challenge in 2021 to 2022. This program brought in more mainstream companies into the market and products were being released in late 2024 as the federal government changed administrations. Although this program could run into problems there are more options that are reaching the market. One product is not going to fit everyone’s needs so options are necessary to find something that is the right size and performance for each situation.
Finding installers for air to air or air to water heat pumps is still a challenge. Manufacturers think New Mexico is in the hot desert southwest and do not understand the high altitude Chihuahuan Desert of New Mexico can get very cold. I had to tell one manufacturer we are not Phoenix. In addition, we do not have a good vocational education system and most of what installers know is passed down with little curiosity about new technologies. Still, this is not going to change until we start bugging manufacturers and demanding installers become familiar with heat pumps.
Many places in New Mexico do not have access to natural gas so homes are heated with propane or wood stoves. There are few homes that either do not have access to the electricity grid or have an off grid source of electricity so heat pumps are possible almost everywhere. Replacing propane with a heat pump makes a lot of sense financially and eliminates the need to have a propane tank filled periodically. Heating with wood or a pellet stove takes a lot of work and requires someone to be present most of the time. It is hard to evaluate the cost or climate emissions from heating with wood but having a heat pump would probably be considered a better standard of living. Widespread use of heat pumps could make a significant difference in access to affordable heat in many parts of New Mexico.
Requirements
The first step is to determine requirements. Some systems can provide heating, cooling and domestic hot water while others can only supply one or two of these. If all that is needed is heating (and possibly cooling) through a forced air system then an air to air heat pump may make the most sense.
In New Mexico the night time temperatures may be low enough for night cooling. This option uses a forced air system with a fan to cool off the house at night enough so that it does not get too hot during the day. This is probably the cheapest option for cooling.
In the desert southwest the next option for cooling is using a swamp cooler. Swamp coolers use 60 - 80% less electricity than an air conditioner since they only require a fan and a small pump to keep the pads wet. They can lower the temperature 15 - 40°F. This may not be sufficient for Phoenix but it should be adequate for the Chihuahuan Desert of New Mexico. Typically swamp coolers are installed on the roof but since roofs can get hot it would seem better to install them at ground level. It is easy to direct the cooling from a swamp cooler to specific rooms by opening a window or door in that room so the cool air flows through it. But some house are not designed with swamp coolers in mind and they will not work for all homes. Personally I avoided houses in New Mexico that used air conditioning instead of swamp coolers because the utility bills are so much higher.
Heat pump water heaters are 2 - 3 times as efficient as electric or gas water heaters. Domestic hot water is a significant energy consumer in your house so the savings are significant. They cannot be installed in an enclosed space since they need to have a sufficient quantity of air to draw from or they end up recirculating the air. Heat pump water heaters are now generally available although there are still not that many models.
Insulation and weatherization
It is surprising just how little insulation and weatherization is in older homes. Single pane windows were common. With our intense sun adequate insulation is needed in the roof and should be addressed before looking at cooling options. Since most of the cost for heat pumps is the upfront cost, reducing the required size of the heat pump is needed to make them cost effective. Insulation does not wear out like pumps and compressors. It also increases the comfort level since drafts and cold walls can leave one feeling cold even if the house is warm.
Manual J calculations
Manual J calculations are used to calculate the heat loss (and cooling load) of a house. They should be done any time there have been significant upgrades to reduce heat loss when installing new heating equipment but it appears they are rarely done. For example, even with adding an addition to the my original house the heat loss was reduced 75% from the original heat loss calculations from 1965. Often installers will just install the same size heating equipment as they are replacing. This results in an oversized heating system that costs more than it should and operates inefficiently. One way to tell if your heating system is oversized is if it turns on and off a lot. This also results in a shorter life for the heating equipment. Installers will often error on the side of installing something too big rather than make the effort to figure out what is really needed. This becomes even more important with heat pumps since the upfront cost of the equipment is the big cost and there is less room for operating error than with gas.
It is possible to do your own Manual J calculations. The calculations do not have to be that accurate and for an existing building the uncertainty about what really is in the walls and roof is probably more than the calculation errors.
Heat loss calculations and an energy audit may identify areas that will provide the best way to reduce heat loss. In my case I had upgraded the windows, walls and roof but discovered that half my remaining heat loss was from the uninsulated floor because I had overlooked the crawl space.
Supply temperature
The advertised efficiency of a gas boiler is measured with 80°F return water temperature. The idea is that the return water is passed through the flue gases condensing the water thus extracting some of the heat lost through venting. The dew point varies with several different factors but is about 140°F in the Bay Area and 125°F in Denver. The supply temperature in my gas boiler being replaced was 150.2°F so for a typical temperature drop of 20°F between the supply and return temperatures the condensing boiler probably never actually condensed. I would guess the setting for the supply temperature was not changed from the factory setting. In a survey of gas boilers in the UK the measured efficiencies of installed gas boilers implied they probably were not actually condensing.
Simply reducing the supply temperature is the best way to increase your efficiency and save money on fuel costs. The supply temperature should be reduced as low as possible while still providing enough heat on the coldest day of the year. Reducing the supply temperature improves comfort since the heat is on for longer periods of time and also prolongs the life of the gas boiler. Night time setbacks have to be reduced with a lower supply temperature.
Air to water heat pumps usually operate best at about a 120°F supply temperature. Some go up to 130°F with a lower efficiency. The U.S. Boiler Company has a heat pump that is aimed at retrofits that can supply 140°F water. By reducing the supply temperature for your gas boiler it is possible to get an idea of what supply temperature you need and then choose the right air to water heat pump for your house.
If the supply temperature required is still higher than you would like, reducing heat loss or upgrading emitters such as baseboard radiators will allow the supply temperature to be reduced further. I upgraded some of my baseboard radiators to ones designed specifically for low temperatures that increased the heat supply by 60%.
Outdoor Reset
Outdoor reset goes by the more descriptive name weather compensation in the UK. Regardless of what it is called, the idea is to couple the supply temperature to the outside temperature. When it is not as cold outside not as much heat needs to be supplied so the supply temperature can be reduced. Settings specify the outdoor reset curve and can be adjusted to be as aggressive as possible while keeping the house warm. Most modern gas boilers have outdoor reset but it may not actually be installed. There should be a wire running from the boiler to the north side of the house where a temperature sensor is located so it usually is easy to figure out if it has been installed. If your boiler does not support outdoor reset there are kits to add it. As seen above, lower supply temperatures increase efficiency. For heat pumps outdoor reset is usually even more essential than for gas boilers.
Modulating gas furnaces also are tied to the outside temperature and are condensing. Unfortunately this cannot be added to an existing furnace. Two stage gas furnaces are an intermediate option; they are more efficient than a one stage furnace that operates at full blast all the time.
Outdoor reset is also important for the efficiency of heat pumps. My original installer put it on the south side off the house where on a sunny January day it nearly shut down the heat. He obviously had not installed outdoor reset before. I had to crawl under the house and run a wire to the north side of the house to get it working properly. My current system is connected to the internet so it can pull temperatures from there and no longer needs a physical wire and temperature sensor.
Heat Pumps
Knowing the requirements for the heat pump, the heat loss, and the required supply temperature the choice of the heat pump can be made. In New Mexico a backup system is probably not needed for modern cold climate heat pumps. While it can get very cold the low temperatures do not last long like in the northern tier of the United States or Canada. Not having a backup system can reduce the required upfront cost and simplifies installation. For apartments and condos or just heating and cooling one room there are window units from Gradient and Midea. These use a regular outlet and only block a very small part of the window unlike window air conditioners.
An evolving area of heat pumps is smart heat pump systems. Some heat pumps may support demand reduction programs where the electric utility can turn them off if there is a spike in usage of electricity. Others can support time of use electric rates. But heat pumps also operate more efficiently in the middle of the day when it is warmest so it is possible to run them at that time and store the heat in a tank for use during the coldest part of the day around dawn. Since heat pumps run on electricity it is pretty easy to add such functionality. I have such a system which is the first of this product installed in New Mexico. It also is the first anywhere designed for low temperature baseboard radiators. The heating system is on the internet and can be accessed remotely. This allows problems to be diagnosed remotely either saving a service call or better preparation for those coming out to service the system so they do not have to make additional trips. It also allows for monitoring where homeowners can be notified of a problem before they notice it.