Calculating Radiant Infloor Operating Costs, Water Heater vs Boiler

I am looking to upgrade my radiant heating system from a water heater to a boiler and I wanted to get a rough idea on the estimated fuel usage so I could narrow in on how much if any money I would save from this upgrade.

Right off the bat let me say I do not encourage in any way shape or form running radiant in floor off a water heater. The only reason I am doing it currently is because my heating load is so small and I could not afford to install a boiler at the time. A typical boiler install can easily run you $5-$9k depending on what boiler you choose so using my water heater to heat the small zone of radiant in my home made sense. Its worth noting that hooking radiant infloor up directly to your domestic hot water heater can make you sick, damage your water heater and just plain cause you grief. So if you do not know how to do it the right way,  don’t! Call an expert!

NOTE: Although I am sharing the math on how I do the calculations I will move quickly so if you have questions or if I am not overly clear on the math leave me a comment

To figure out the cost of operation, my first step was to do a heat loss / heat gain calculation. Using some free software, I was able to calculate my heating load to be 10,400 BTUS for the area in my home being heated by radiant infloor. Now that I have this information we can use a slick little formula to calculate the estimated fuel usage and from there I can calculate my cost to heat it based off local fuel rates. The formula is as follows;

EFU = (Heating Load x 24 x DD x CF) / (AFUE x Q x DTD)

This formula translates to

Estimated Fuel Usage = ( Heating load X 24 x Heating Degree Days X Heating Degree Days Fuel Correction Factor ) / ( AFUE X BTU Content of Fuel X The Design Temp )

As noted earlier I am heating with a water heater currently so I needed to get the AFUE of it with some math. Based off of a 90 degree rise in water from ‘city water temp’ to ‘heated water temp’, an input rating of 36,000 BTUS and the gallons per hour recovery rate of 30.5 ( both numbers are on the side of your water heater ) the AFUE of my water heater is 63.3% ( calculate your own with this tool )

Another critical element is the BTU’s you get from the fuel you get. For reference here is the BTU content of fuel for the most common fuel types;

  • Fuel Oil = 138,500 BTU per gallon
  • Gas = 100,000 BTU per therm
  • Propane = 92,000 BTU / gallon
  • Electricity = 3,413 BTU / kw

I heat with Natural gas so I get 100,000 BTU per therm of gas. Lastly, I went and snagged my degree day data which translates to the number of heating days between October and April that number is 7566. You can get the number here for your own location

My Design temp in my home is 70 degree indoor and the design outdoor temp for my area is -2 so that makes my heating Design Temperature 72.  Now our formula looks like this;

EFU = ( 5,400 BTUS X 24 X 7566 X .640 ) / .633 X 100,000 X 72 )

Which becomes this once we do the multiplication / division;

EFU = ( 627,554,304 ) / 45,57,600 ) = 137.69 therms

I now know that my estimated fuel usage to heat my living room is 137.9 therms. This is fine, but my gas company does not bill me in therms they bill me in cubic meters or m3. So I need to convert the therms to cubic meters and I did so using this online tool. This now tells me that in a year I will use 380 cubic meters of natural gas fuel every year to heat my living room radiant project.

Dollars and cents wise that works out to $106.20 a year including tax, delivery, etc. Now we have the fun part of looking at what would the cost be if I upgraded my water heater to a boiler. My boiler would have an average AFUE of 95% instead of 63.3% so my new formula would look like this;

EFU = ( 10,400 BTUS X 24 X 7566 X .640 ) / .95 X 100,000 X 72 )

Let’s break that down into a smaller number;

EFU = ( 627,554,304  ) / 6,840,000 ) = 91 therms

176 therms converted to m3 or cubic meters = 252 which makes my final heating bill with a condensing 95% efficient boiler $79.00

So my heating bill would drop from $106.00 to $79.00. That is an annual saving of $27 dollars. So how long would it take for my upgrade to pay for the condensing boiler? Well over 10 years keeping in mind that I have also not calculated any of the electrical costs associated with heating my home.

One key thing to make a note of here however is the fact that I am only heating one room in my house. If I was heating my entire home my heating requirements and fuel usage would be much higher.

Upgrading your gas water heater to a boiler will definitely save you money but do not look at it from the standpoint of ‘return on investment’. Do it for other reasons like reliability, durability and lower cost of operation in general.

I will be upgrading to a boiler as soon as I finish adding JAGA low temp radiators to the remaining floors in my home because my little water heater will not be able to do the job and it’s not exactly efficient using an appliance that is 63% efficient. If I want to go full bore I will go geothermal and install a Water to Water unit that will provide me with hot and chilled water so my JAGA radiators can provide cooling and heating.



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