Hydronic Unit Heaters: How to Read the Rating and Size One

A unit heater is about as simple as heating equipment gets: a coil, a fan, and a casing. Hang one from the ceiling of a shop, garage, warehouse, or entry vestibule, run hot water or steam to it, and it blows warm air straight into the space with no ductwork. If you already have a boiler on site, a hydronic unit heater is often the easiest way to heat a big open area.

How It Works

Hot water or steam flows through a finned coil, and a fan pushes room air across the coil and back into the space. That's the whole mechanism. Because the combustion happens at the boiler, the heater itself has no flame, no flue, and no gas connection to worry about in the room.

Output Depends on What You Feed It

Read the rating carefully. The Dayton 5PV35 shows two different outputs in its specs: 148,000 BTUH on 200 degree F water and 204,000 BTUH on 2 PSI steam. Same heater, very different results. The rating always depends on the medium and the temperature, so a heater sized from the steam number will fall short on a hot water loop. And if your boiler runs lower supply temperatures, for example with outdoor reset or a condensing boiler, the output drops below the 200 degree figure, so check the manufacturer's rating table at the temperature you'll really run.

Flow and Piping Matter

The same listing shows a water flow rate of 14.9 GPM and 1-1/2 inch threaded NPT connections. That's a lot of water for one heater. On a building with several of them, the supply main and the pump have to carry the combined flow, and the branches should be balanced so the last heater gets what the first one does. Our circulator sizing guide and the circuit setter guide cover that side.

Sizing a Unit Heater

• Start with the heat loss of the space, in BTU per hour at your design temperature.
• Choose a heater whose rated output at your actual water temperature covers it, not the output at the highest temperature in the table.
• Check the airflow and mounting. The 5PV35 moves 2,900 CFM through an 18 inch fan and is about 29 inches tall, 24-5/8 wide, and 13-3/4 deep. The heater has to throw air where people work, so mounting height and direction count.
• Don't oversize. A heater that's too big cycles on and off and leaves the space uneven.

Hydronic or Gas-Fired

If you already have a boiler and a hot water or steam loop, hydronic unit heaters let you heat additional spaces without adding combustion appliances or flues. If there's no boiler, a self-contained gas-fired unit heater, like the Dayton gas-fired wall and ceiling models in our catalog, is the other route. It needs a gas supply and venting, but nothing else. The deciding factor is usually what's already in the building.

Controls and Care

• A thermostat controls the heater, and many installations add a fan delay or an aquastat so the fan doesn't blow cold air before the coil warms up.
• Keep the coil and fan clean. A dirty coil moves less heat and strains the motor.
• Protect the coil from freezing. A water coil in an unheated space needs a plan for a power failure, such as a glycol mixture or a way to drain it.
• The 5PV35 comes with an OSHA-compliant fan guard, and any replacement or accessory should keep that protection in place.

National Boiler Supply carries hydronic and gas-fired unit heaters from Dayton. Send us the heat load and the water temperature you'll run and we'll help match a heater.