Choosing The Perfect Hot Water Space Heater For Your Garage Or Workshop

If you already use a hot water system to heat your home, adding heat to your garage or workshop can be accomplished by installing a fan forced hot water unit heater. Hot water garage heaters transfer heat from the hot water to the air.

Operating Principle

The hot water from your boiler flows through a radiator coil and an electric fan pushes air through the radiator coil. The air entering the coil absorbs the heat from the water, resulting in a warmer discharge air temperature. The cooler water leaving the coil returns back to the boiler to be reheated for another pass through the radiator coil.

Benefits of fan forced hot water unit heaters in garages or workshops:

o Clean heat, no odors, no by-products of combustion to vent.

o No open flame to worry about.

o No messy fuel oil to contend with.

o Efficient.

o Easy maintenance.

o Very reliable. The only moving part is the motor and fan assembly. If a motor fails, it is very simple to replace.

o The fan speed can be adjusted on many models. This allows you to dial in the perfect airflow.

o Low clearance requirements on units with side piping connections.

o Horizontal units have adjustable louvers to help direct warm air where you need it most.

o Durable. All unit casings are treated for corrosion resistance and finished with a gray-green baked-on, high solids paint.

o Compact and lightweight for simple installation.

o Tapped holes in the units casing allow for simple suspension using inexpensive threaded rod.

o Large electrical junction boxes allow for simple power wiring.

o Huge range of sizes to match the heater to your heating load. If selected properly, you don’t have to worry about under or over sizing.

o The fan can run in the Summer to help circulate air.

Drawbacks of fan forced hot water unit heaters in garages or workshops:

o Water in a cold garage can easily freeze if there is a failure.

o A home boiler system is required. If your boiler is down, the heater will not produce heat.

o Hot water piping is required to distribute the water from your boiler to the hot water unit heater.

o Controls can be slightly more complex due to the integration of your hot water boiler and the unit heater. See the commonly asked questions section for typical control sequences.

o Fan forced units move large volumes of air which can kick up dust in your garage or workshop.

Unit Configurations:

Hot water unit heaters come in horizontal or vertical models. The “horizontal” or “vertical” describes the direction of airflow. A “horizontal” unit heater is typically placed along a wall and blows warm air into the space. A “vertical” unit heater is typically placed along a ceiling and blows warm air vertically down into the space. Vertical unit heaters are typically used in very large spaces. Horizontal units are typically used in smaller spaces such as garages or workshops.

There are several modifications available. Horizontal and vertical unit heaters are available with “power throw” high CFM fans, high pressure tubes, low outlet temperatures (for high steam pressure applications), special motors and custom coatings. These modifications are rarely used in the typical garage or workshop. Call us for information about customizing a unit to meet your specific needs.

Installation Overview:

1) Suspend the unit heater using threaded rods that thread into the self tapping holes located on the unit heaters casing.

2) Install hot water piping from your boiler or wood furnace to the hot water unit heater.

3) Wire the hot water unit heater’s fan motor.

4) Install controls.

Commonly Asked Questions:

1) How do I calculate the heating load for my garage or workshop?

ANSWER: Call us or use our simple heat load calculator. Do NOT fall for equipment that is advertised as will heat up to 400 square feet. Heating a 400 square foot insulated garage in Florida is a lot different then heating a 400 square foot un-insulated metal garage in Maine. 1 heater can not magically service both 400 square foot spaces. Call us; we are happy to run a detailed engineering calculation that will help you pick the perfect heater for your space. You do not want to undersize, and you do not want to oversize.

2) How do I control my hot water unit heater in my garage?

ANSWER: If there is always hot water running through the coil, you can simply use a switch to turn the fan motor on and off when you are using your garage or workshop. Automatic control is preferred over manual control. There are 3 common methods for automatically controlling a hot water unit heater in a garage or workshop:

Common Control Sequences

The following control sequence descriptions are typical for steam/hot water unit heaters in garage or workshop applications:

>>Intermittent Fan Operation – Hot Coil

When a room thermostat calls for heat, the motor is energized. Hot water or steam is continuously supplied to the unit heater, even when the motor is not running. When the thermostat is satisfied, the motor is de-energized.

>>Continuous Fan Operation – Intermittent Hot/Cold Coil

When a room thermostat calls for heat, a valve is opened, allowing steam or hot water to enter the unit heater. When the thermostat is satisfied, the valve is closed. The fan runs continuously.

>>Intermittent Fan Operation – Intermittent Hot/Cold Coil

When a room thermostat calls for heat, the motor is energized. At the same time, a valve is opened allowing steam or hot water to enter the unit heater. An aquastat may be attached to the supply or return piping to prevent fan operation until the coil is adequately heated to avoid cold air delivery. When the thermostat is satisfied, the valve closes and the motor is de-energized.

3) How do I determine how much heat a hot water unit heater will generate with varying entering hot water temperatures?

ANSWER: Hot water unit heaters are also used with steam. The major hot water unit heater manufacturers rate their capacities using 2 psi steam. For example, a Modine model HSB33 will produce 33,000 BTUs when it is hooked up to 2 PSI steam. Another example: A Sterling model HS120 will produce 120,000 BTUs when hooked up to 2 PSI steam. Since you are most likely using hot water in your garage or workshop, you need to know how much heat you will get out of the unit with your specific hot water temperature. Call us. Within minutes we can tell you the exact output for every sized heater under any condition. Do not fall for advertisements that claim 290,000 BTUs!. A size 290 unit heater will produce 290,000 BTUs ONLY if it is supplied with 2 PSI steam. If you are using 160F water, this same heater will produce 152,000 BTUs, almost half the capacity shown on the advertisement.

4) Can hot water unit heaters be used with outdoor wood furnaces?

ANSWER: Absolutely. The coil doesn’t care where the hot water comes from. As long as the water is warmer then the air, the unit heater will transfer heat from the water into the space.

5) What are the different piping configurations on hot water unit heaters?

ANSWER: The most common hot water unit heater for a garage or workshop is the horizontal model. Most major manufacturers offer side piping and top and bottom piping arrangements on horizontal units. The side piping connection models allow you to install the heater closer to the ceiling. The top and bottom piping connection models allow you to easily rotate the unit if you need to change the direction it is pointing.

6) What is the best location in my garage or workshop to install my hot water unit heater?

ANSWER: The hot water unit heater in your garage or workshop should be located in the coldest area, and it should be angled slightly so it blankets warm air across the coldest wall.

7) What is the best location to install a wall mounted thermostat in my garage or workshop?

ANSWER: The thermostat should be mounted in a location that represents a good average temperature in your garage or workshop. If it is located in a cold spot, it will falsely run the unit heater more then it should. If it is located in an area that receives direct sunlight, it will falsely run the unit heater less then it should. A well insulated interior wall is the best spot for the thermostat.

8) What is an aquastat?

ANSWER: An aquastat gets strapped on to the supply or return hot water pipe. It gets wired in series with the fan motor. The aquastat will only allow the fan to run when there is hot water running through the pipes. This inexpensive device will prevent cold air from circulating throughout your garage or workshop when the hot water pipes are cold.

9) What kind of sound can I expect from a hot water unit heater?

ANSWER: While sound is created anytime fans and motors are used to move air, Modine unit heaters were designed to minimize their sound level through the careful selection of motors, fan blades and the design of the air intake opening. The information below shows typical types of buildings or rooms with a corresponding Sound Class rating. For a unit heater with a given Sound Class rating, when placed in the type of building or room shown below, the sound of the unit heater will be relatively comparable to the ambient sound level of all sounds within that type of building or room. When purchasing a unit heater, ask for the the sound class rating to be sure you are buying something that will fit your application.

Sound Class Ratings

Type of Building or Room Sound Class Rating

Apartments, Classrooms, Court Rooms, Executive Offices, Hospitals, Libraries, Museums: Class I

General Offices, Hotel Dining Rooms, Recreation Rooms, Show Rooms, Small Stores: Class II

Bank Lobbies, Grocery Stores, Gymnasiums, Post Offices, Restaurants, Service Stations: Class III

Factories, Foundries, Machine Shops, Packing Plants, Shipping Platforms: Class II-VII

Forge Shops, Steel Fabricating Shops, Boiler Works: Class VII

10) How can I compare the cost of heating my garage with different types of garage and workshop heaters?

ANSWER: Call us. We will first run a heat load analysis, and then we can run an energy comparison analysis based on your local fuel rates.

Water Heaters – Electric Vs Gas

My goal as a water heater advisor is to save you money. How does your water heater relate to your wallet? Most likely, one utility will be less expensive to operate than the other. My guess is most households do not have a preference. The area in which you live might dictate the type of water heater you will install. Certain areas of the country are predominately gas, while others are mainly electric. If you have a choice, hopefully this info will help you decide. You can save from the start by choosing the correct heater for your household.

Assuming you have a choice, how do you choose? Ask yourself if the present heater does a sufficient job. Does it supply enough hot water for your family? If you’re replacing an existing water heater, check to see what type you have now. Is it gas, electric or even propane? If your heater has done a good job for your family, why not stay with the same type? Usually that will be an easier and less expensive installation.

Many factors can enter the decision. But for this comparison, let’s keep it simple.


It’s hard to get simpler than an electric water heater. It is recommended to power your electric heater with a 10-2 wire run directly from the electric panel, and connected through a 30 amp breaker. Basically, you have two heating elements and two thermostats. One of each are at the top and one of each are at the bottom. Replacing either component is simple. Most elements just screw in, making replacement easy.

Thermostats can be replaced by the novice by changing wire for wire. The cost is reasonable as well. Most home improvement or hardware stores sell both elements and thermostats under $20.00 each.

The negative factors about an electric water heater are that it heats water slower than gas, and commonly electric rates are higher than gas.


Gas water heaters are slightly more complex. All gas heaters must be vented. You can vent an atmospheric vent heater into a chimney or a vent stack, which should extend through the roof, and should be at least 2 feet higher than the highest point of the roof. A Power-Vented heater will have a blower on the top, and push the gas vapors out the side of the home through plastic pipe. A chimney is not required.


Today’s atmospheric vent storage heaters commonly have a pilot light, so you need a thermocouple or thermopile (a type of electric thermocouple). An igniter is needed to light the pilot. And all makes have a main gas valve assembly. Thermocouples can be found most anywhere for under $10.00.

If your model has a thermopile, you may pay us much as $40.00. Various manufacturers have their own style of igniter. They can run up to $35.00 each, but rarely are found to be defective. The main gas valve assembly can cost around $85.00.


A Power Vent style of gas water heater is becoming more popular by builders today. Because these heaters do not require a chimney, most contractors will choose this style because they are not restricted in design, and cam locates the water heater anywhere, and extend the plastic pipe to the exterior wall. This is usually an inexpensive Endeavour.

If they chose an atmospheric vented style, it would have to be near the chimney, restricting the homes design. Power-vented heaters can cost as much as twice the price as a standard storage type of heaters. As well, most builders would rather pay $1000 for a power-vented tank than $10,000 for a chimney.

Power-vent heaters are quite complex. For starters, unlike standard gas heaters that run completely with gas, the burner operates with gas, and the blower runs on electricity.

We already said that to purchase this style of heater would cost about twice the cost as the standard. Choosing this style will allow you more flexibility in locating the water heater in your house. However, repair can be varying costly. For starters, there is a blower that can cost up to $200.00.

The main gas valve also cost two to three times as much as the gas valves on a traditional atmospheric type of heater. Additionally, there are numerous smaller but essential components such as a diaphragm, pressure switch and wiring harness.

What about size?

As mentioned previously, an electric heater will commonly heat slower than a gas heater. Therefore, if you need a 40 gallon gas heater, you will probably need a 50 gallon electric heater to provide sufficient amount of water ready to use.

Now that you know whether you want a gas or electric water heater, to buy smart, determine the size you need.

To do this, compare the recovery ratings of each.

Common industry estimates show that – if your house has:

  • 1 – 1 ½ baths – up to 3 beds……. your recovery should be 40 to 60 GPH
  • 2 – 2 ½ baths – up to 4 beds……. your recovery should be 60 to 90 GPH
  • 3 – 3 ½ baths – up to 5 beds……. your recovery should be 70 – 90 GPH