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Can I heat my house with a water heater? Or should I use a boiler?

Air Conditioner, Alternative Heating, Alternative Methods of Heating, Energy Efficiency, Flooring Installation, Floors, PEX, Plumbing - Directory, Radiant Heat May 2, 2014 Sonia

by Brian Whitehurst of

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 4 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.

radiant floor design

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

Example House

Heat Load / Flow rates

eht-pexheat-electric-bannerFirst 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

 Radiant House

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.

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.

A. 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.

B. 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.

C. 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.


Efficiency / Cost

Unfortunately efficiency is a non-exact calculation and after much deliberation most of our customers go with a personality choice rather than hard numbers on how efficient they want their heat plants to be anyway. However, we will run through some numbers on cost to give you an idea of what you need to consider.

On the market in the hydronic industry there are usually 2 efficiency categories; high efficiency and standard efficiency. High efficiency units can rise as high as 95-97% efficiency. That means 95% of the heat you burned in fuel was transferred into the fluid of the heating system. Standard efficiency max out at around 85% efficiency. Such that 15 cents of every dollar spent on fuel go up the flue. So we all want to use the highest efficiency unit available right? Well, the cost of high efficiency units may make them cost more and run over the life of the unit than a standard efficiency one, but they come with a much higher initial cost.

Let’s take a look at 4 different scenarios for our project house. I will be carrying the cost out to the life expectancy of a boiler at 20 years.

1. Tank style water heater dedicated to the radiant system, estimated life expectancy 5-7 years.

Water Heater, 85% Efficient $400
Venting $100
Pump & Controls $500

Material Total $1000
Estimated Annual Fuel Cost $2200 per year
Lifetime cost at 20 years $2200 x 20 + $1000 + $400 x 2= $45,800

Water Heater

2. Tankless Water Heater, estimated life expectancy 7-10 years

Tankless Water Heater, 85% Efficient, No standby heat loss $700
Venting $250
Pump & Controls $800

Material Total $1750
Estimated Annual Fuel Cost $2000 per year
Lifetime cost at 20 years $2000 x 20 + $1750 + $700 = $42,450

3. Tankless Water Heater, High Efficiency, estimated life expectancy 10-15 years

Tankless Water Heater, 95% Efficient, No standby heat loss $1500
Venting $100
Pump & Controls $800

Material Total $2400
Estimated Annual Fuel Cost $1800 per year
Lifetime cost at 20 years $1800 x 20 + $2400 + $1500= $39,900

Tankless Water Heaters

Tankless Water Heaters by Takagi. Designed for either Domestic Hot Water or Space Heating

4. Condensing Boiler, High Efficiency, estimate life expectancy 20-25 years

Condensing Boiler, 95% Efficient, No standby heat loss $3000
Venting $100
Pump & Controls $500

Material Total $3600
Estimated Annual Fuel Cost $1700 per year
Lifetime cost at 20 years $1700 X 20 + $2400 = $37,600

Weil-McLain boilers

Weil-McLain Condensing Boilers

So we can conclude; if we carry out the costs of the systems to the same life span of 20 years

Water Heater, Tank style, 85% Eff with 7 year life
expectancy, after 20 years         $45,800
Tankless Water Heater, 85% Eff with 10 year life
expectancy, after 20 years         $42,450
Tankless Water Heater, 95% Eff with 15 year life
expectancy, after 20 years         $39,900
Condensing Boiler, 95% Eff with 20 year life
expectancy, after 20 years         $37,600

The higher efficiency units will cost less to run over extended time and an investment in a boiler will yield almost a 15% return on your money invested in a stronger heat plant. Initial costs on a boiler will be over 3-1/2 times that of a tank style water heater. But you do get what you pay for. Will your budget allow for an investment for an investment in a high power, long lasting, high efficiency power plant or do you need to tighten the belt and take a shorter life solution.


This is a bit of a throw away point for many customers but it needs to be considered. What warranty will you receive when using your heat plant for your heating system? Since Water Heaters are designed and built for domestic hot water distribution, the manufactures will write their warranties for that application. If you install a water heater for space heating, you will either void the warranty or decrease the warranty to about 1 year. Read the Warranty fine print.

Boiler Manufacturers are usually offering warranties on the boiler heat exchangers for 15-25 years (electronic parts are less). However, if you are do-it-yourselfer and expect to install your own boiler, again read the fine print, you may be voiding the warranty and have to pay for any replacement parts need from installation problems or breakdowns.


Now that you have determined if the option of using a water heater is applicable to you, make sure you let us do a full heat loss calculation and a unit comparison to see what your best choice is. Visit our website or Contact Us at for more information. We are glad to help.


Brian Whitehurst is the Senior Design Engineer and Owner of a radaint floor heating design and sales company operating since 2002.

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