Comfort Without Radiators: How We Think About Moving To Air-Based Heat

By Brendan Kavanagh

Most of the homes we work on around Boston were built between the 1850s and the 1960s, and nearly all of them deliver heat with water or steam. In pre-war houses, that usually means cast iron radiators on a steam system fed by an oil boiler that’s been rebuilt and re-piped at least once or twice. In newer houses, it’s most often hot water baseboard radiators fed by a gas boiler that is larger than the home needs. Whatever the vintage, it’s common for the boiler to be on its last legs, or if not on its last legs, extremely inefficient. This is why we routinely incorporate mechanical upgrades into our remodeling and retrofit projects. 

Air-based heat pumps are our go-to equipment for these upgrades. This is true regardless of whether the home’s existing system is a furnace or boiler, and whether or not it has central air. 

But homeowners who are used to radiators are often reluctant to get rid of them. They like the feel of their hot water or steam heat, and they worry that air-based heating will be loud, dusty, and drying. 

The good news is that newer air-based heat pump systems are none of these things, and when coupled with building envelope improvements, they can be extremely comfortable. Air-based heat pumps also offer some air quality benefits that you can’t get with radiators. That said, switching from radiators to air-based heating is a major change. It’s worth walking through what that means before we start pulling out your radiators.

Thermal Comfort Used to Mean Something Different

Before about the mid-twentieth century, homeowners had very different thermal comfort expectations than we do now. With little to no insulation or air sealing, houses quickly lost heat to the outdoors during cold weather. Central cooling didn’t become common in the residential sector until the 1960s. Indoor temperatures above 60 in winter and below 80 in the summer were acceptable, and differences of a few degrees or more between rooms were the norm.  

As building practices and technology have evolved, so too have comfort standards. Today, homeowners expect consistent temperatures from room to room and goldilocks humidity levels year-round.

Improving the Building, Not Just the Equipment

In our opinion, heat pumps are ideally suited to meet these expectations. They provide all-in-one heating and cooling, and when properly sized, remove humidity very effectively. Ducted systems also offer key air quality benefits that radiators don’t. 

But building envelope improvements are equally important. If we don’t air seal and insulate the home first, the new heat pumps may disappoint. Not only will they use more energy and cost more to operate, they are also much more likely to fall short of modern comfort standards. Installing heat pumps in a leaky, poorly insulated house is often a recipe for complaints and callbacks.

What Actually Changes

Temperature. With a properly designed system room-to-room swings are typically within a degree or two, which is often an improvement or at least similar to what the old radiators provided. What tends to feel different is the sensation of heat: when you sit or stand next to a radiator it warms your body directly in addition to warming the space around you, whereas forced air only warms the space around you. Some people miss the sensation of radiant heat (while it is possible to pair heat pumps with hydronic distribution, installation is significantly more expensive so we rarely install water-based heat pumps). 

Humidity. This one tends to surprise people. There’s a common belief that forced air “dries out” a house more than radiators do. In reality, both do the same thing: warming the air allows it to hold more moisture, so relative humidity drops unless moisture is replenished. Winter dryness has little to do with your type of heating system and much more to do with air infiltration. 

In a leaky house, indoor air is continuously replaced by cold, dry outdoor air, and the moisture you generate from cooking, showering, laundry, and breathing gets flushed out along with the escaping indoor air. When you tighten the envelope that exchange slows considerably, so household moisture stays put and relative humidity settles into a more comfortable range. 

In summer, heat pumps dehumidify as well as cool. Clients who’ve never had cooling are struck by how much better a dehumidified 74°F feels than 70°F at higher humidity. 

Similarly, clients who have had central air are often surprised by how much better their new heat pumps dehumidify than their old central air systems. It’s very common for central air systems to be oversized, which leads them to cycle on and off relatively frequently. This “short-cycling” leads to insufficient dehumidification, leading spaces to feel cool but clammy. 

Air quality. This is where ducted air-based systems have a real edge over hydronic systems, though in the first few weeks following the transition some clients report that their homes seem a bit dustier. Moving more air through rooms that haven’t had much air movement sometimes stirs up dust initially. The good news is that it’s temporary, and the longer-term trend runs the other way. 

A ducted system with good filtration pulls dust and pollen out of circulation every time air passes through the filter (changing filters regularly is key). Before long, we typically find that homes have fewer airborne particulates. Ductwork also mixes air between rooms - a bedroom with the door closed overnight can build up more CO2 than you’d expect from one sleeping person, and modest mixing helps people wake up with a clearer head in the morning. We hear this anecdotally, with some clients reporting that the house feels fresher, and we see it in data from the air quality monitors that more and more of our clients are using.

Ducted vs. Ductless

However, “air-based” isn’t just one thing, and ductless heat pumps have fewer air quality benefits than ducted systems. They help with mixing just in the room they’re mounted in, and they don’t filter the air. They often also require more frequent cleaning. On the flip side, ductless systems allow for more zones, allowing different spaces to be set to different temperatures. 

Because of their air quality benefits, we aim to get at least one ducted system in a home whenever feasible. That said, all ductless is sometimes the right call for homes without a reasonable path for ductwork, for homeowners with tighter budgets, or who want several controllable zones. Either way, homes also need a steady supply of fresh air which is why we routinely incorporate balanced ventilation into our remodeling and retrofit projects.

Whether the heat pumps are ducted, ductless or a combination, it’s important to carefully think through register and head placement. We aim for placement that mixes the air without blowing directly on where people sit or sleep.

Establishing New Habits

Everyone has thermal comfort habits. When we change a home’s systems, habits may need to change too. For example, if we swap out a large radiator for a ductless heat pump in your TV room, you might find that you need to set the thermostat a little higher, or snuggle under a blanket to be comfortable on a cold winter’s night. This may be the case even if we have made significant envelope improvements.

Heat pumps also operate differently than steam and hot water systems, and they have different maintenance needs. For example, most homeowners are used to setting their thermostats systems back at night and while they’re at work and school. Heat pumps in highly efficient homes work best with minimal setbacks (less efficient homes may still benefit from setbacks; they may run less efficiently but will use less energy overall). This is why we include heat pump maintenance and operations guidelines in our Use and Care Guides. 

The more we can prepare and educate people, the better. And we typically find that households settle into new routines within the first year.

How We Prepare Clients

• We talk through the differences early so nobody’s caught off guard when the radiators come out and the heat pumps go in.
• During the design phase, we have homeowners talk to past clients, and if possible, visit a home with similar systems so they can experience heat pumps firsthand.
• We’re upfront about tradeoffs rather than only selling the upsides.
• We don’t swap out equipment without first doing what we can to improve the building envelope. 

Switching from hydronic to air-based distribution is a significant change. It’s a bigger conversation than which heat pump you should buy, and it’s one we’d rather have long before we start any work on site. Heat pumps may also take some getting used to. But every homeowner we’ve worked with has found their own way of living comfortably with heat pumps, and we’re confident that you will too.