Going Ductless: Heat Pumps without Forced Air

Millions of homes are heated by fossil-fuel boilers and radiators. How can these systems switch to clean electric heat?

Photo courtesy of Todd Gardner.

This article is Part 4 of our ongoing series: The Ultimate Guide to Low-Carbon Heating in Minnesota.

You can go back to Part 3 here: Clean & Efficient Heat: Air‑Source Heat Pumps.

2022 UPDATE

When we wrote this article back in 2019, we realized the low-carbon heating options for radiant systems were lacking. So, we decided to build a new type of heat pump, designed from the ground up for boiler & radiator hydronic systems.

Click here to learn more about RenewaBoiler – The Heat Pump for Radiators.

So far in this low-carbon heating guide, we’ve focused mainly on forced-air systems. But millions of homes don’t have forced air; instead they have radiant hydronic heat.

Radiant hydronic heat systems are powered by a boiler that heats water, which is circulated to radiators throughout the house. This was the primary way that homes were heated until the 1950s, when central air conditioning (which requires a forced-air system) became mainstream. But retrofitting ductwork into a home is very expensive, so there are a ton of radiant-heated homes still around, especially in Minneapolis & St. Paul. And hydronic in-floor heating, which works on the same principle, is commonly built as a premium feature in modern homes.

When it comes to environmental impact and operating costs, boilers are no different than furnaces. The vast majority run on natural gas where available, and otherwise use propane or heating oil. There are condensing and standard gas boilers, just like with furnaces. And ultimately, BTUs are just BTUs – whether it’s warm air blowing through a duct or hot water circulating through a radiator.

So what are the low-carbon options for radiant heating systems?

Mini-Split Heat Pumps

The most common way for radiant-heat homes to go low-carbon is with mini-split heat pumps. These are small (“mini”) heat pumps that are “split” between an outdoor unit (mounted on a slab or metal frame) and one or more indoor “heads” (mounted on walls or ceilings). Refrigerant is piped between the outdoor and indoor units to perform the heat exchange.

The outdoor unit of a mini-split heat pump.

Homeowners can install as many indoor units as they need to keep their house comfortable. But just like ducted heat pumps, backup heat is required for mini-splits in Minnesota. So unfortunately, installing them doesn’t mean you can get rid of your boiler.

There are still a lot of reasons to like mini-splits:

• They perform both heating and cooling, so you don’t need a separate window air conditioner.
• They tend to perform better than ducted heat pumps at very cold temperatures. For instance, the Mistubishi “Hyper-Heat” mini-splits work all the way down to -13°!
• They are much quieter than a window air conditioner. (Or rather, the noisy part is outdoors so it’s not bothersome.)

A wall-mounted indoor head of a mini-split heat pump.

The biggest drawback to heating with mini-splits is that they are a step backwards in comfort. Radiant heat, especially in-floor, is generally considered the most comfortable way to heat a home. Radiators are also spread throughout an entire home to cover every room evenly, whereas with mini-splits you will have a much small number of heat sources. For most people, a box on the wall blowing warm air is simply no comparison to a large cast iron radiator, or to a floor that warms your feet as you walk on it.

For historic or architectural homes, mini-splits may also be considered an aesthetic problem. (But in my opinion, they look a lot nicer than a window-mounted air conditioner – and they don’t block your view to the outdoors!)

Environmental Impact & Costs

When it comes to energy efficiency, mini-splits are comparable to ducted air-source heat pumps. Depending on the outside temperature, they add 7-14 BTUs of heat per hour for every watt of electricity they consume. But the overall impact on the climate and on your wallet depends on how many you install, and the layout of your home. To attempt an apples-to-apples comparison, we’ll consider two hypothetical scenarios:

1. Single Unit – A single cold-climate outdoor unit with one or two indoor heads covering the largest downstairs rooms.
2. Full Coverage – Multiple cold-climate outdoor units with 6-8 indoor heads covering every living space and bedroom.

The single-unit scenario could cover about half of a typical home heating load, and the full-coverage scenario would cover more like 90%. Essentially, these are equivalent to the standard and cold-climate scenarios with ducted air-source heat pumps. (The remaining heat needs to be provided by the boiler and radiators.)

The installation costs for mini-splits is highly variable depending on your home layout, electrical requirements, and where you locate the indoor heads. But they are more expensive than ducted heat pumps – especially when you consider their costs relative to a cooling-only solution. You can expect the single-unit scenario to cost $4-5,000, and the full-coverage to cost $12,000 or more.

Air-to-Water Heat Pumps

Another way to get low-carbon radiant heat is to use a heat pump itself as the boiler. This requires something called an air-to-water heat pump, which uses the air as a heat source to heat up water for radiant heating. This seems like the ideal solution – and even has the advantage of being able to use the heat pump for your home’s hot water. But there are quite a few challenges:

An air-to-water heat pump in action.

First off is the economics. For ducted and mini-split heat pumps, a big part of the value proposition is the cooling ability, since it can replace your air conditioner for a very small price premium. But in a hydronic heating system, the heat pump’s cooling ability is useless. (Running cold water through the system will cause condensation on the pipes and radiators, which will lead to mold and rot.) To take advantage of the heat pump’s cooling ability, your home would need a specialized, i.e. expensive, distribution system for cooling.

Because of the difficulty with cooling, air-to-water heat pumps are a bit of a fringe market. (In the US, anyway…they are actually quite common in both Europe and Asia.) The biggest HVAC companies don’t even manufacture the equipment, so you have to go with lesser-known specialty brands. This creates a couple more problems: hardware costs are high and experienced installers are rare.

And since air-source heat pumps (in Minnesota) always require backup, so you will still need a traditional boiler to go with it.

A Note about Water Temperatures

Another problem with air-to-water heat pumps is water temperatures. Air-to-water heat pumps become less effective as you the water gets hotter, and they typically max out at around 120-130°. But radiators are typically designed to run at 160° or higher.

The good news is that radiators are usually oversized, especially for old houses that have upgraded the original insulation and windows. (The boiler industry is already capitalizing on this with high-efficiency condensing boilers – which don’t actually run at high efficiency unless the water temperature is below 130°!) And on the very coldest days of the year, when you need the hottest water, the system will be running on backup heat anyway.

So in some cases, you can get by with the lower water temperatures provided by air-to-water heat pumps.

In-Floor Radiant Heating.

And if you’re lucky enough to have in-floor heat, the design water temperature is typically 100° or less. So an air-to-water heat pump is a perfect fit!

Environmental Impact & Costs

An air-to-water heat pump used as a boiler has similar efficiency to a mini-split heat pump. But the amount of fossil fuel usage that it can replace depends greatly on the design temperature of your radiators. For a typical cast iron radiator design, the heat pump we install at 2040 Energy will cover around 50% of the load. But for an in-floor heating design, which requires much lower water temperatures, it will cover more like 75%.

Installation costs vary widely because there is such a small market. We have found that $15,000 is typical.

Overall Ratings

We don’t have any specific ratings for boilers themselves – their costs and carbon emissions are essentially the same as the equivalent furnace (see here for natural gas or propane and heating oil).

The overall ratings for the low-carbon ductless options are below. Note that the fuel costs and carbon footprints include running a standard-efficiency natural gas system as backup.

Just like with ducted air-source heat pumps, the mini-split and air-to-water heat pumps offer huge carbon savings and a slightly lower operating cost. But they are quite a bit more expensive to install.