Manually Calculating Head Loss & Flow in my Radiant In Floor System

I designed my radiant infloor system which uses REHAU RAUpanels using LoopCad
I designed my radiant infloor system which uses REHAU RAUpanels using LoopCad

I recently installed REHAU radiant in floor panels in my living room and I used Avenir’s LoopCAD software to do the layout and design for it.

LoopCAD makes it easy to do the math but it is also important to know how to do the math without software which leads to today blog post. What happens when you need to quickly figure out how to size your pump without access to a computer?

More importantly the Loopcad software only sizes your pumps for the pipe and manifold not the extra items in the system like valves, expansion tank, boiler, etc.

Sizing a pump requires you to know two things. The flow rate which is incredibly easy and the head loss so you can size a pump that can do ‘X’ amount of GPM at ‘X’ feet of head.

How to Manually Calculate Head in your Radiant in floor System

The formula to estimate head loss is as follows;

Length of the longest circuit x 1.5 x .04

What does the formula mean? Well the first bit is obvious. Look at your radiant in floor layout and how long is the longest radiant infloor circuit?

The second number ‘1.5’ is to take into account for valves and any other components in your system. The last number in the formula is the head in the circuit, which is 4 feet of head per 100 feet of pipe.

NOTE: It is important when you design your systems that you design them for minimum velocity of 2 feet per second and maximum of 4 feet per second. If you have less the system will ‘gurgle’ and if you have more it will sound like a freight train. 

In the case of my radiant system my longest circuit is 158 feet of 3/8ths pipe. So our formula looks like so;

158 feet ( longest circuit ) x 1.5 ( add some buffer for valves, etc ) x .04 ( velocity per 100 feet of piep ) = 9.48 feet of head.

How close is this to my loopcad calculation? Its about double. My radiant infloor system was designed for a 10 degree delta T with the highest head loss circuit being 4.9. When I add the head loss for my REHAU ProBalance manifold that adds an extra .01 feet of head making the total head in my system 5 feet of head.

So using the rule of thumb will definitely not result in me under sizing the pump if anything it encourages me to use software and do it right so I dont accidentally oversize my pumps!

How to calculate the GPM.

Calculating the flow or gallons per minute (GPM) in your radiant system is just as simple as long as you know the Universal Hydronics formula. The formula is as follows;

GPM = BTUH / delta T x 500

Lets quickly break down what that all means. The BTUH is the heat required every hour to heat the space you are putting the radiant into. This number will be provided to you by your radiant designer or installer and is a critical number. Get this number wrong and all bets are off so dont even try to calculate it if you do not know how.

The DELTA T is the temperature drop in your radiant in floor system. Most residential systems are designed to work at a 10 degree delta T because it provides for an even temperature across the floor. In commercial applications or in areas we dont live in a 20 degree delta T is common.

The last number is the specific characteristics of the fluid you are using. In this case it is pure water which is 500. This represents the weight of 1 gallon of water ( 8.33 ) , times 60 minutes in an hour times the specific gravity of the water. You do the math and it equals 499.8 which we round to 500. It takes 1 btu to raise 1 pound of water 1 degree in 1 hour.

In my own radiant in floor project the heat loss was 3,931 BTU’s and as we discussed residential systems generally use a 10 degree delta T and my system uses pure water. So to figure out my flow rate the math is

GPM = 3,931 BTU’s in the room / 10 degree delta T x 500 ( Specific gravity of water )
GPM = .78 gallons per minute

How does this compare to my software calculation? Loopcad calculated my GPM at .79gallons per minute so safe to say the math works.

This means that for my radiant infloor system I need a pump that can do .78gallons per minute at 9.48 feet of head. Using this number we can now look up the pump curve for a Wilo or Taco pump and select our pump for the job. ( I am a big fan of Wilo and Taco Pumps )

Important Note: If you use glycol in your radiant infloor system the specific gravity number 500 will change! 



  1. Hi,
    thanks for the posting…how do the head loss calculations or factors (e.g. the 1.5 multiplier) either include head loss for moving heating fluid up 1 or 2 or more stories? We have a three story project with mechanical in the lowest room, and manifolds up ~20′ on the top story. BTW, the headloss calcs we get from Viega show 8.7′ head loss at 9.3GPM.
    Thanks, Steve

    1. Its a closed system so the 1 or 2 stories is not relevant to the design and the head loss.

      Out of curiosity you say VIEGA did your design what exactly did they calculate for you and what information did they have? That head seems high but for all I know you live in a monster home.

      Full Disclosure: Where I work we sell a competing product to Viega.

      1. Viega did all flows, layout, btu calcs from our title24 and floor plan data, as well as head loss.BTW the Rehau calculations were much higher with Raupanel and 3/8″pex. In addition to the headless from Hydronics we will also have another 7 feet of loss going through the Hubbell tankless heater (project is 100% electric). Will appreciate if you LMK of other electric tankless heaters as this is expensive and has significant head loss.

        Thanks, Steve

      2. Just to be clear the RAUPANEL system is the highest performing radiant panel system on the market verified by two separate independent studies.

        28-35 btus per square foot is serious output at 110 degree water and far higher then anything else on the market. You can see the report yourself its linked to on my site in the various blogs on raupanel.

        I used it in my own home and my logic was I pay more up front and pay less every month heating my house using 100 degree or less water.

        Where are you located out of curiosity? Perhaps I can help steer you back to RAUPANEL. Clearly biased on the company and brand but the numbers on its performance does not lie.

        The head loss sounds better now that I know we are talking an indirect 🙂

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