Hydronic Heat Pipe Sizing

Hydronic Pipe Sizing

When installing indirect water heaters, duct coils, etc, all you have to do is run the same size pipe as the tapping's right? For example if the tapping's are 3/4" and the boiler is rated at 60,000 btu's and the coil is 50 ft from the boiler run 3/4" pipe. This could be no further from the truth. Not only does the pipe need to be sized properly so does the pump for indirect water heaters. The same applies to duct coils along with the air flow of the blower. There is no relationship of tapping sizes and pipe sizes. You will find some tapping's are smaller than the pipe requirement and other tapping's are larger than the required pipe.

Theres a maximum amount of btu's and gpm flow carried in a pipe. The average flow rate in a pipe is normally 2-4 fps. This equals about 3 to 5.5 gpm for 3/4" pipe. There is also a maximum amount of btu's of radiation one can use on a given pipe size. See Copper tube aluminum fin chart, Cabinet convector chart, and the cast iron baseboard chart.

  We see many installations with too small of a pipe installed or improper flow and the baseboard does not perform to it maximum potential due to a lack of btu's. There are a few things which keep baseboard from performing to peak output at any water temperature.

       Water temperature affects btu output. The hotter the water the higher the btu output. We do not always need 180ºf water temperature such as a day not at design outdoor temperature.  The warmer the day the lower the water temperature in the heating pipes needs to be. As the day gets colder the warmer the water needs to be. For instance, I installed two boilers in a 4 unit apartment house with outdoor reset (see glossary). I stopped back when the weather got colder to check the operation. It was 36ºf outside and the water temperature in the system was 117ºf. I made one more stop when the outside temperature was 23ºf and the water temperature in the system was 129ºf. These temperatures would have been higher with standard baseboard. This job had cast iron radiators and about 50% more than required for the heat loss of the building. Excess radiation is a plus as long as the contractor does not size the boiler off the radiation.

Another reason the btu output can be affected is the flow in the pipe may be too slow. When we slow down water we reduce the btu output. See cast iron baseboard chart above. There is a similar chart for all baseboard and heating coils. These charts show the btu output per foot of radiation at different flow rates such as 1 gpm and 4 gpm. To achieve the proper flow rate is determined by proper circulator sizing.

   The BTU output is also affected by the pipe size. A certain size pipe can only carry a certain amount of btu's. If the connected load (radiation) is greater then the pipe size can carry the end of the loop is going to be a lot cooler than the beginning of the loop. Heating loop are normally designed for a 20° delta-T or temperature drop. This means when the water enters the loop at a given temperature  it will give off 20° by the time it exits the end of the loop. It only gives off 20° at a design water temperature (usually 180°). When the water temperature is less than 180°, if designed at 180°, the delta-T will be less than 20°. Some systems today may be designed at lower temperatures than 180° and maybe less than a 20° delta-T. Radiant infloor systems are designed less than 180°, some as low as 90°, and maybe a delta-T of 10° - 15°. When determining the amount of radiation on a pipe we do not measure the cover we measure only the pipe in the cover which has the fins on it. The amount of pipe with fins may be far less than the amount of cover you see on the wall.

When looking at the chart below we see how many btu's the pipe can carry and we do not want to put more radiation that that on that pipe. We also have to have a pipe big enough from the boiler to get the heat away from the boiler when needed. The boiler pipe may be larger than the system piping.

Pipe Size Total BTU's
1/2" 15,000
3/4" 40,000
1' 75,000
1 1/4"



As stated above this will limit the amount of radiation allowed to be installed on a given size pipe. With what we call copper tube aluminum fin radiation there are limitations.

Pipe Size Rating @ 180°f
Total Feet
3/4" 590 67 3/4'
1" 800 100'
1 1/4" 850 164 3/4'

So lets watch how many feet of radiation we put on that 3/4" pipe. We see a problem usually with a system where there is an addition put on a home and the addition has a loop of baseboard installed and connected into the existing loop of the room the doorway was cut into to get to the new room. Now the heating season rolls around and the bedrooms now don't seam to get as warm as they used to. The added radiation for the addition exceeds what that 3/4" pipe can carry. The addition should have been made a separate zone.

Disclaimer: The information found on this website is for informational purposes only. Any and all preventive maintenance, service, installations should be reviewed on a per job situation. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or it's personnel accepts no responsibility for improper information, application, damage to property or bodily injury from applied information found on this website as it should be reviewed by a professional.