Using a Water Heater for Space Heating Part 1
Part 1 of 2

Using a Water Heater for Space Heating Part 1

We at get asked this question a lot. I would like to take this chance to outline some of the important details worth considering when selecting a heat plant for your home, shop or business. After looking at these 5 points you should be able to determine if a water heater is applicable to your project.

  1. Heat Load / Flow rates
  2. Health & Safety 
  3. Efficiency / Cost
  4. Warranty

These are only some of the detail you need to consider when selecting a heat source. But, if any of these don't eliminate a water heater as an option, then proceed to getting a detailed quote from your design engineer. Note: Most of the details in this article should be discussed with an experienced heating engineer to find the correct model. But I will give some "rules of thumb" so you can make an educated guess if a water heater can work for your project.

Design Services

In this article I will be using a small example project house designed for Long Island, NY with a full basement.

Basement - 1000 sqft, 50 sqft windows, 21 sqft of doors, radiant in slab
First Floor - 1000 sqft, 120 sqft windows, 40 sqft doors, radiant in gypcrete thin slab
Garage- 250 sqft, 10 sqft windows, 80 sqft doors, radiant in slab


Heat Load / Flow rates

First things first. Will it work? Will the water heater do the job? You need to know the Heat Load on your heating system. Without that, you are just guessing. We at Pexheat do free heat loss calculations for all customers. However, to keep them free we just ask that you fill out one of our project specification forms found here;

If you want to calculate your Heat Load yourself, you can approximate your Heat Load using these common load factors.

Basement (below ground room) 10-15 BTU/ hr/sqft
First Floor 15-20 BTU/ hr/sqft
Second Floor 20-25 BTU/ hr/sqft
Shop (slab on grade) 15-20 BTU/ hr/sqft
Garage 5-10 BTU/ hr/sqft

If you have a lot of windows, high ceilings, many doors, fireplaces, etc. you should approximate to the higher value. If your space has many load factors (ex; high ceilings with lots of glass French doors) then approximate at the next higher load level. Again I would like to stress that this is for approximation, before you buy a heat source have a Load calculation done on your space.

Multiply your load factors by the area being heated and you will get the Heat Load on your space.

ex: Basement @ 12 BTU/hr/sqft X 1000 sqft = 12,000 BTU/hr

Add up all the spaces to get the total approximated load on the heat plant.

Basement = 10 BTU/hr/sqft X 1000sqft = 10,000 BTU/hr
First Floor = 17 BTU/hr/sqft X 1000sqft = 17,000 BTU/hr
Garage = 8 BTU/hr/sqft X 250 sqft = 2,000 BTU/hr
Total = 29,000 BTU/hr

Water Heaters and Boilers are sized by INPUT rating. That is how much fuel it burns. NOT how much heat it puts out. Another "rule of thumb" is to take the Input X Efficiency to get the OUPUT of the heater. If you are looking at water heaters, most tank style water heater burners max out at 30,000 to 40,000 BTU/hr INPUT. They are usually sold at 85% efficiency.

40,000 BTU/hr Input X .85 efficiency = 34,000 BTU/hr Output

You can usually use a 40,000 BUT/hr burner to heat a 29,000 BTU/hr Load. So we can continue discussing the possibility of heating our example house with a water heater. Please see below for some recommendations on Load Only heater considerations.

Tank Style Water Heater up to 30,000 BTU/hr
Tankless Water Heater up to 40,000 - 60,000 BTU/hr (depends on the model)
Over 60,000 BTU/hr; use a Boiler or multiple water heaters

What's this Flow Rate thing?

Flow Rate is how fast we can move fluid through the pipes. All devices have flow rate characteristics where they resist higher and higher flows such that there is a maximum flow rate through the device. We need to consider the flow rate through the water heaters. Tank style water heaters, since they are just large tanks of water have very low flow resistance such that we can move relatively high rates through them. However, Tankless water heaters have heat exchangers in them with very high surface area. This is needed to move high amounts of heat into the fluid in short periods of time. This results in very low maximum flow rates.

Small Tankless Water Heater - 5 gpm maximum
Larger Tankless Water Heater - 8 to 9 gpm maximum

So why do we care so much about flow? Flow is directly related to heat transfer. One can approximate the heat transfer of a heating system using another rule of thumb; Heat Transfer = Flow rate X 10,000 (at a 20 deg temperature change). Therefore, I can approximate the heat output of my Tankless water heaters at:

Small Tankless Water Heater - 5 gpm X 10,000 = 50,000 BTU/hr
Larger Tankless Water Heater - 8 gpm X 10,000 = 80,000 BTU/hr

A Small 5 gpm Tankless water heater has enough capacity to heat my 29,000 BTU/hr project house too. Note: Even though the 5 gpm Tankless water heater may have been rated at 140,000 BTU/hr, we will only draw a maximum of 50,000 BTU/hr when used for space heating.

Floor Heating


Health & Safety

Now that we know we can heat the space with our water heater, we question should we heat it with a water heater. Is it safe to use a water heater as a space heater? Water heaters were designed to heat water for domestic hot water use. They were designed to heat up to 120-140 deg F and run for short periods throughout the day. If your heat demand falls into the design characteristics of a water heater then it may still be worth considering.

  1. If you live in a city; NYC, Chicago, LA, NO! You can not use a water heater as a space heater. There are building codes that require ASME rated heat plants and water heaters are usually not ASME rated.
  2. What temperature will you be heating the water up to? If you are using baseboard or cast iron radiators to heat your house, they usually require 160-180 deg F water temperatures. Although some water heaters can go to 180 deg F, there are many pressure and temperature issues with these high temperatures. You would do well to stop and look into a wall hanging boiler for your system. Radiant floor heating usually operates from 90-140 deg F which is the ideal range for water heaters.
  3. Domestic hot water; are you looking into circulating your domestic hot water through your heating system? This is usually an unsafe practice.
    1. Most of the components in heating systems are not domestic water quality. Corrosion susceptible parts and lead content will make heating systems incompatible for drinking water. You will usually incur a significant increase in cost of your heating system to use all domestic water parts.
    2. Eliminating algae and bacteria growth in a heating system is difficult if not impossible. So separating the domestic hot water and the space heating system by a heat exchanger will protect the domestic water system and keep the costs under control.
    3. However, using two separate water heaters; one for domestic hot water and another for space heating is usually your best choice.

Our project house in Long Island can use a water heater for space heating under local building codes. Because the slab installations will require less than 140 deg. F water, it is safe to use a water heater to heat the radiant slabs. We will also be installing 2 separate water heaters; one to supply domestic hot water and a separate water heater to supply the radiant floor.

Domestic Water Supply
Brian Whitehurst is the Senior Design Engineer and Owner of a radiant floor heating design and sales company operating since 2002. 
See Part 2 to Look at Efficiency and Cost 
Using a Water Heater for Space Heating Part 1, Brian Whitehurst 24 September, 2021
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