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Hot Water Radiators: You Can't Beat 'Em

Updated: Apr 27, 2020

Hot water radiators are the most common central heating system you’ll find in North American homes built before around 1935. You’ll also occasionally find steam radiators—although steam is more common in apartment and commercial buildings. And while radiators have been in common use since the late 1800s, they are still the best method of distributing warmth throughout a house. Radiators use less energy and are quieter than forced hot air systems. They provide a more consistent temperature and they don’t blow dust and allergens around the house.

So why is forced hot air so common in new construction? Simple: While walls are open, ductwork is cheaper to install than water pipes. And that same ductwork can also serve the air conditioning system.

But it is almost always impractical to retrofit ducts into an existing house. Radiators were commonly retrofitted into houses built before the late 1800s—they simply ran the pipes on the outside of the walls.

When hot water systems are installed in new houses, they almost always use long baseboard radiators instead of those old-fashioned cast iron units. True, baseboards take up less space and distribute the heat more evenly around a room. But I’ll take those old cast iron radiators over baseboard any day.

Cast iron radiators have lots of thermal mass. In other words, because they are big and heavy they can store a lot of heat. So, when the thermostat turns off the system, radiators continue releasing heat for a lot longer than baseboards (and a whole lot longer than ductwork which stops producing heat as soon as the system shuts off.) This makes radiators more efficient and prolongs the life of your boiler because it will cycle on and off less often. Plus those ornate old radiators are part of the charm of an old house

The drawing shown here a simplified illustration of how hot water radiator systems work. Of course a house will have many radiators rather than just one as shown here, but like here it will be a loop. Water flows from the boiler into a radiator, through that radiator to the next and so on. When it gets to the last radiator the water flows back to the boiler to get heated again.

This system works okay, but because water cools as it moves from one radiator to the next, rooms at the far end of the loop can be chilly. The good news is that old-timey radiators can be coupled with a new super-efficient boiler. That’s what we’ve got in my 1860s-era home in Warwick. You can tell from the chimneys that the house originally had stoves. At some point radiators replaced the stoves. And then, maybe 30 years ago, new pipes were configured into six loops creating six zones with six thermostats—about one zone for every two rooms. This is much more efficient that the old single-loop systems because there’s a direct line to every two radiators. This means the water doesn’t get cool as it does at the end of a single loop. And the independent zone lets us heat just the rooms we are using.

So, if you go down in the basement and see something like what shown in the photo below, you are in luck! This photo shows six zones. Each zone has a red manual turn off valve. The metal boxes with the white labels are valves controlled by the thermostats. Next look for a water heater. If you don’t see one that means one of these zones is being used for domestic hot water. That’s a nice feature—it means the boiler is creating hot water as you use it, so you’ll never run out. And you won’t have to replace a water heater every 10 years or so.

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