Boiler Manifold Piping

Welcome to Air-Water Heat Pump Primer

Welcome to the world of residential air-to-water heat pumps (AWHP). From a 10,000 foot view there are an estimated 12 million of these installed in Europe, while the United States have about 6 million installed and market is growing at an estimated rate of 270,000 per year. With states discussing reducing or ending the sale of new fossil fuel units at some point in the future the estimated 270,000 units a year could increase in the next few years
There are many contractors who believe heat pumps of any kind may not be good in their areas. While there may be some truth in some areas, heat pumps in general work in most areas. I have heard comments like, it won't heat the home in my area, it's too cold here for a heat pump, I heat with natural gas, nothing is cheaper than natural gas. The above comments may also have some truths, but most are not a concern.
Natural gas is about the cheapest way to heat a home today, but heat pumps are more efficient. Natural gas boilers are 95% to 99% efficient and a heat pump is on average 100% to 450% efficient. How is that you ask? How can anything be more than 100% efficient?
We will start simple; we all know the term kilowatt (KW). That is how we pay for electricity. One KW = 3.412 btu's. Heat pumps remove heat from the air and absorb heat from the compressor and refrigeration process. The heat is transferred into an air or water heat exchanger. The heat exchanger removes the heat from the refrigerant and transfers it into the water for the hydronic system. Heat pump ratings for heating are listed as Coefficient of Performance (COP). If a heat pump has a COP rating of 4, it means it produces 4 times more heat than bought or for every KW it produced 4 KW of heat. Let's convert this to btu's. One kilowatt is 3.412 btu's and 4 kilowatts is 1365 btu's. If we use a number that may make more sense look at 1000 watts or 1 Kilowatt. Purchase 1,000 watts or 1 Kw and produce 3,412 btu's of heat. This is more efficient than natural gas, but the COP will drop as it gets colder.

Watts to BTU Calculator

Heat pumps are available as air-to-air heat pumps, air-to-water heat pumps and water to water heat pumps. The first part of its name is where it absorbs the heat from and the second is where it transfers the heat to. For instance, air-to-water heat pump removes heat from the air and transfers it to water in the home heating system.
Which is best air-to-air or air-to-water heat pumps? Water to Water heat pumps is best due to the constant temperature of ground water. Next would-be air-to-water heat pumps, heat being transferred to water can be 3500 times more heat than what is transferred to air. Let's learn more about what an AWHP is and how it works. There are two styles available, while the operation is the same the installations are different.

Monobloc - is a self-contained unit with all the refrigeration parts like the air coil to absorb heat from outdoor air, the compressor, refrigerant accumulator, and refrigerant to water heat exchanger are in the outdoor unit. The heat exchanger will have hot refrigerant and hydronic system water with opposing flows passing through the heat exchanger. The water for the hydronic system is piped from indoors to outdoors to the heat exchanger collecting heat and returning to the hydronic system within the structure.
Since the system water is going out to the AWHP the water will be exposed to outdoor air temperatures so the water will require an antifreeze solution to avoid freezing and damage to the heat exchanger and outside system piping.

Entire system requires antifreeze

Using a flat plat heat exchanger


Only the flat plate heat exchanger and piping to outdoor unit needs antifreeze

Split System - is like an air-to-air heat pump. There is an indoor and outdoor unit connected with refrigerant lines. The indoor unit has the refrigerant to water heat exchanger built into it so there is no need to use an antifreeze solution unless there are reasons due to the heating system application. Many times, the split unit will supply slightly warmer water temperatures to the heating system or DHW production. When installing a split system AWHP the system piping is about the same as connecting to a boiler with few changes. The changes are usually an AWHP system may require a buffer tank, a booster boiler that could be the existing boiler dependent on heat loss, amount of radiation in the structure, and minimum water temperature needed. Don't fall into the old way of thinking that all systems require 180f water temperature all the time. Many duct coil systems can heat with 120f - 130f water temperature.

No antifreeze is required for the heat pump unit

Here are a few considerations you should be familiar with when installing an AWHP

  • If you are going to produce DHW use a larger tank, most DHW tanks are already oversized. Maybe use AWHP to just preheat DHW before going to a water heater
  • Pipe sizing is more critical – flow rates are higher than boiler systems
  • Circulator sizes are going to be larger
  • Delta-T’s are different. In North America, the delta-T is a standard 20ºf, AWHP’s is 5°F
  • Best applied to homes with a heat loss between 10-15 btu’s per square foot, but not limited to this rule
  • If a booster boiler is needed it can be a gas, oil, or electric boiler.
  • If the AWHP is of modulating design it may be beneficial to oversize the AWHP by one size
  • Larger jobs - there may be multiple AWHP's used
  • Should still consider OD reset

Hydronic Cooling – caution must be used to verify the radiation has provisions for condensate removal. This could be an air handler with a water coil in it for cooling and heating. There are also heat/cool cabinet convectors that can have provisions for condensate. There are also terminal units that look like mini-split air-to-air heat pump wall units. Remember, if the units do offer condensate trays, they must be piped somewhere to get rid of the condensate.

Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, installations should be reviewed on a per job situation. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or it's personnel accepts no responsibility for improper information, application, damage to property or bodily injury from applied information found on this website.