John Siegenthaler: Btus in a box

Have you noticed the strong interest in cold-climate air-source heat pumps over the last couple of years?

This past January, I walked the AHR trade show in Atlanta. Dozens of booths prominently displayed ductless “mini-split” air-source heat pumps systems, or their commercial equivalent — variable refrigerant flow (VRF) systems. These products define two of the fastest growing segments of the North American HVAC market.

Ductless air-source heat pumps are easy to recognize: Most have one or more “high wall” air handlers coupled to an outdoor condenser unit using one or more refrigerant line sets. Most ductless heat pumps now have variable speed compressors, ECM blowers, self-diagnostics, and of course, an app allowing you to control the system from your smartphone. Some of the ductless air handlers have become very stylish, with sleek cabinets and fancy wireless remotes. Some of the air handlers are disguised as (rather thick) “picture frames” for displaying the occupants favorite artwork. I’m sure very few people would ever notice that the artwork frame is an air handler — at least when the picture frame’s blower is off!

When paired with heat pumps, radiant panels should be constructed so they can deliver design-load output at the lowest possible supply-water temperature.

A recent report (bit.ly/2VpJsD7) commissioned by the Natural Resources Defense Council states: “With aggressive program and policy support to drive the market, air-source heat pumps could be installed in two thirds of New York State residences by 2030.” Based on U.S. Census Bureau statistics, that would require more than 5 million New York State homes to be heated and cooled by air-source heat pumps within the next decade. A lofty goal indeed, and it’s just for one state.

Ductless by default

Check out some of the building industry associations or government agencies that give out awards for low-energy and net zero homes. Chances are you’ll find ductless air-source heat pumps as the most common “comfort solution.”

Attend a seminar or a tradeshow that’s focused on low-energy and net zero homes and you’re likely to hear that ductless air-source heat pumps are all you need to know about heating and cooling. The common advice for such homes: Install two or three high wall air handlers, leave the interior doors open, and you’re all set. None of that overly-complicated hydronic “stuff” is required.

The commonly-inferred approach to design and installation of ductless air-source heat pump system is:
Determine the rate at which the house loses heat on a design day;
Select one or more interior units and a condensing unit that matches that heat loss; and
Do the “plug & play” installation, and enjoy the profit margin.

In simplistic terms, this approach treats every house as a “box.” All that’s required to maintain a stable indoor temperature is to put Btus into the box at the same rate they leak out of the box. This is sound reasoning from a thermodynamics standpoint. However, from the standpoint of human physiological comfort it’s a very over-simplified and incomplete model.

Air is still air

Nearly all ductless heat pumps and VRF products are “air-to-air” systems. In heating mode, they absorb heat from outside air and deliver that heat using one or more interior wall units. Hot refrigerant flows through the coil of an active indoor unit. A blower pulls room air from the top of the unit, forces it across this coil, and out through oscillating louvers in the lower front of the unit.

All air-source heat pumps require occasional defrosting of their outdoor evaporator coil. In most air-to-air ductless systems, the heat needed to defrost the outdoor coil comes from inside air. Some units slow down or turn off the indoor blower during defrost, others don’t. With exception of a polar bear swim during February in Fairbanks, nothing beats a stream of cold air blowing across your body on a winter evening to elucidate the word uncomfortable.

Some ductless heat pump manufacturers have attempted to improve the compromised comfort situation. One now offers high-wall air handlers with a small infrared sensor that scans room surfaces for cold spots and then directs air flow toward them — a neat high tech concept. Pardon me for asking, but how do you get warm air from a ductless heat pump over to a cool floor area or wall on the opposite side of a room without blowing it there, and perhaps past someone in that flow path?

Another ductless heat pump supplier now offers ceiling-mounted air handlers that discharge air through hundreds of small holes rather than open louvers. The idea is to slow discharge air speed to reduce drafts. I imagine this works well in cooling, but how does it not create room temperature stratification during heating mode? While I’m sure these features offer some improvement, they’re fundamentally limited by what’s achievable using conditioned air streams as a proxy for human comfort.

Air-sourced, water-delivered

Based on the above, you probably think that I’m opposed to air-source heat pumps. Quite the contrary. Over the last decade, major design evolutions such as enhanced vapor injection and variable speed compressors have greatly enhanced the thermal performance of air-source heat pumps in cold climate applications. The very concept of absorbing heat from outdoor air at cited outdoor temperatures down to -22° F amazes and delights me!

Modern low ambient or cold climate air-source heat pumps have largely displaced long-held beliefs within the HVAC industry that air-source heat pumps aren’t suited to cold-climates. That limitation, when it was widely accepted, became the driver for development of ground source heat pumps. The latter demonstrated the ability to overcome the limitation of air-source heat pumps in areas where outdoor temperatures frequently drop to or below 0.

The performance gap that once existed between air-source heat pumps and geothermal heat pumps has been significantly narrowed. Cold climate air-source heat pumps are now very competitive with geothermal heat pumps when the two are compared on a market-based unsubsidized life cycle cost basis.

What hasn’t been sufficiently narrowed is the comfort gap between heat pumps that use forced air rather than hydronic heat delivery.

There’s more to comfort than reducing air temperature variations in a room. More to it than balancing the rate at which Btus enter and leave the “box” (e.g., interior space). More to it than hanging a ductless air handler in a hallway and relying on open doors in adjacent rooms for adequate heat dispersion.

Ideal heating comfort involves imperceptible air currents, adequate mean radiant temperature and eliminating objectionable operating sounds. It requires provisions for keeping different areas of a building at desired comfort levels, even when doors are closed. It necessitates a way to satisfy equipment needs, such as defrosting an outdoor coil without blowing cold air into heated spaces.

Opportunity awaits

It is possible to deliver the substantial performance gains of modern air-source heat pumps without compromising comfort. The machine at the heart of the solution is a cold climate air-to-water heat pump.

Just like a ductless heat pump, an air-to-water heat pump absorbs heat from outside air, even air that’s well below 0°. Unlike a ductless heat pump, the output is a stream of heated water, in some cases up to 130°. Give me a source of such water, access to modern hydronic technology and I will deliver unsurpassed and uncompromised comfort.

Why not use this complementary relationship to go after the rapidly developing market for low-energy and net zero homes? Show the architects, builders and incentivizing organizations how a heated floor slab supplied by a low ambient air-to-water heat pump can provide energy efficiency, reduced carbon footprint and superior comfort. Show them how that same heat pump can provide domestic hot water and chilled water cooling. The latter can be delivered by a combination of radiant cooling and a small air handler that satisfies the latent cooling, ventilation and heat recovery requirements of these homes. Trust me, there are lots of people making decisions about comfort systems that have no idea that any of this is possible.

The Japan Air Conditioning, Heating & Refrigeration News (JARN) recently reported the following: “In 2017, global air-to-water heat pump demand reached 2.66 million units, a continuous increase from 2016. The major markets are concentrated in China, Japan and Europe, accounting for 68.8%, 16.8%, and 10.8% of the global market respectfully.”

What about the U. S.? JARN reports: “In 2017, the U.S. market scale of air-to-water cylinder-integrated heat pump models (e.g., heat pump water heaters) was 89,000. Monobloc or split-style air-to-water heat pumps (e.g., those used for space heating or cooling) are not very common in the U.S. market.”

There’s work to be done if that previous sentence is going to change. Air-to-water heat pumps are an emerging market for the U.S. hydronics industry. Several more makes and models of these heat pumps will enter the U.S. market in 2019 and 2020.

It’s not just the companies that sell those heat pumps that stand to gain. Every system with an air-to-water heat pump needs piping, valves, circulators, air and dirt separators, expansion tanks and controls. Many systems also need buffer tanks, piping insulation, PEX tubing and panel rads. That’s what the hydronics industry offers. Why not sell more of it into this emerging market? Why not display it at trade events that draw professionals interested in low-energy, net zero and carbon reduction solutions? Why not show how hydronic solutions leverage modern air-source heat pump performance to deliver comfort without compromise?

Air-to-water heat pumps are a “bridge” that the U.S. hydronics industry needs to cross to participate in the rapidly growing market for low-energy/net zero houses. Without a proactive approach from our industry, this segment of the housing market will slide further toward ductless air-source heat pumps as their default solution. The occupants of those houses will continue to “tolerate” compromises such as cool floors, blower noises, drafts, cold air during defrost and bedrooms that remain 5 to 6° below the temperature of spaces where the air handler is mounted.

An opportunity stands before us — take notice and go after it.

John Siegenthaler: Btus in a box

There’s more to comfort than balanced energy flows.

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Plumbing and Mechanical

2020 November

John Siegenthaler: Btus in a box

There’s more to comfort than balanced energy flows.

Recent Articles by John Siegenthaler, P.E.

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