How to Size Piping And Pumps in a Hydronic Snow Melt System.
When it comes to hydronic heating math is everything.
It is absolutely crucial to properly size piping and pumps in a radiant snow melt system. If your installer cuts corners to come in with a lower quote he will most likely do so under-sizing the piping and omitting circulator pumps from your system. He may even go as far as to use drainage copper tubing instead of L-Copper which has a much thinner wall and will wear out in record time springing leaks all over. It is up to you the home owner to educate yourself and ask the crucial questions about a quote.
Velocity, Flow Rate , BTU/h, Head Loss
These are the most important numbers to consider and to familiarize yourself with. They are very simple concepts so don’ be intimidated.
Velocity refers to the speed of a fluid in a tubing, the optimal hot fluid speed being 4 foot/second. Higher velocity means more fluid that flows through if the tubing size remains the same. Above 6 foot/second hot fluids will start to flow through the center of the tube with the fluid near the wall being colder. This means the heat from the fluid will not transmit to the target (floor, heat exchanger, driveway etc…) High velocity of fluids will also result in increased wear on all system components and the amount of noise and vibration created.
In this context flow rate refers to the amount of fluid that can pass through a pipe or tubing in a certain amount of time. It is most often expressed as Gallon per Minute or GPM.
While the material of the pipe matters, the most important factor is pipe size. The larger the inside diameter of a pipe the more fluid can pass through it if the pressure remains the same.
|Size||PEX Flow Rate at 4 ft/sec (gpm)||Copper Flow Rate at 4 ft/sec (gpm)|
British Thermal Unit/hour refers to the amount of heat per hour that is considered. When a heat source heats up the water or glycol (we care about hydronics here) that heat then must be transferred from inside that heat source to where we want it; the floors or the driveway to heat. Larger diameter pipes can transmit more fluids carrying more heat from the heat source in a given time to the target location if the circulator pump is sized properly. Generally a maximum of 30 F temperature difference (30°ΔT) between the supply and return side of a boiler is acceptable. Anything greater than that may result in the heat exchanger eventually cracking.
|Pipe Size||BTU/h Capacity at 4 foot/sec and 30°ΔT Copper||BTU/h Capacity at 4 foot/sec and 30°ΔT PEX|
Also referred to as back pressure, flow resistance, friction loss. It is basically the resistance due to friction fluids must overcome to flow through a pipe or tubing. The longer the pipe or smaller the diameter will increase head loss. Shorter or larger pipes will have lower head loss. Having fittings, valves, circulating different fluids (glycol) than water and different temperatures will all affect this value. You will find a very good head loss calculator on the Plastics pipe Institute’s page
Let’s find out the required pipe sizes for a typical snow melt system
For us to size piping and pumps we will use these criteria for our calculations:
- 1,00 foot² driveway insulated with R 10 Insulation in Toronto
- 3/4″ PEX tubing at 9″ spacing
- 50% Propylene Glycol as heat transfer fluid to be circulated
- 180 BTU/h/foot² heat requirement (150 BTU/h/foot² + 20% heat loss to the ground below)
- aiming to have no snow accumulation during a snow fall
We will need a boiler that will have at least 180,000 BTU/h heat output. To be able to transfer that much heat to the driveway we will need to first figure out our boiler loop pipe sizing.
- Looking at the BTU/h Pipe Size chart above we find that a 1-1/4″ copper tube will be able to carry that much heat. That is our boiler (primary) loop tubing size. When we plug the 180,000 BTU/h and the 1.25″ tubing into Bell & Gosset’s System Syzer for a 50% Propylene Glycol filled boiler we get 17.31 GPM Flow
- The total length of the 3/4″ PEX tubing at 9″ spacing will be 1,000 ÷ 0.75 = 1,333′. In reality that will be 1,400 give or take due to converging tubing and loss of area coverage at the manifold. We divide that 1,400 into 6 circuits and get 6 x 233’/circuits. Note the maximum circuit length for 3/4″ PEX tubing is 300′
- Since we need to distribute 180,000 BTU/h to the ground we also divide that into 6 and get 30,000 BTU/h per circuit.
- We also divide that 17.31 GPM into 6 and get 2,885 GPM per circuit flow. We know from the flow chart above that we are good up to 4.6 GPM on a 3/4″ PEX so all is well.
We now know the total flow requirements and can calculate the total head loss for our system. This is important to size piping and pumps.
Sizing our circulator pumps
- To calculate the head loss of the boiler (primary) loop we plugin the 1-1/4″ L Copper boiler loop size and 17.31 GPM and we get the head loss of 9.74′
- We now need to find a pump that will be able to deliver 17.31 GPM of glycol against 9,74′ head loss.
- We go to Bell & Gosset’s Pump Selector page and enter those numbers along with the fact that we will be using 50% Propylene Glycol and we get a list of applicable pumps
- We now need to find the pump we need to supply the circuits of the manifold. Lets assume the manifold will be close to the boiler and an 1-1/4″ L copper pipe will connect it to the boiler loop through a hydraulic separator much like our 1/1-4″ Schuller™ for about a 4′ Head Loss
- To calculate the circuit head loss we enter the circuit length of 266′ and the results are a total of 7.37 Head Loss for the circuits
- We add that to the 4′ we got for the supply and returns to the manifold and we get a total of 11.37′ Head Loss. To play it safe and account for head loss incurred at the manifold body and fittings along the way we round it up to 15 and get another list of pumps we can chose from at B&G’s Pump Selector