Circulator Sizing for Boilers and Indirect Water Heaters (IWH)
It would be a good idea if you have not viewed our Components Seminars Part 2 & 3 or reviewed and understand webpage Circulator Sizing for Boilers and Systems.
There is information you will need before determining the circulator size for the IWH. Let's assume the boiler and IWH are not installed yet. Before I can address circulator sizing you would have to gather some of the following information using the Lochinvar boiler manufacturer and HTP Superstor literature and installation manuals.
- Is the boiler going to be piped with manifold piping
- Is the boiler going to be piped with P/S piping
- Is the IWH going to be piped directly on the boiler pipes
- If the boiler is piped as P/S but the IWH will be piped on the system piping as a separate zone.
- The resistance through the IWH
- The resistance through the boiler
- GPM flow required through the IWH
- The total length of straight pipe and number and type of all fittings
I will show two examples, one applied to a high efficiency boiler piped directly off the boiler piping, the highest resistance, and off the system piping as a separate independent zone, normally the lowest resistance application.
Look at the information fro the manufacturers information. Below are excerpts of both.
HTP Superstor
For this example, we have to assume a boiler and IWH tank size. We will choose a boiler to meet out calculated heat loss of 92,000 btu’s and apply an EP110N and a SuperStor SSC-50 IWH. Next what Delta-T do we want to use? I will pick 25f. We now know the boiler spec is 8.4 gpm @ 5.6 ft. hd. The SSC-50 also needs 8 gpm with 5.82 ft. hd. This is good match. When the flow of the IWH falls between the minimum and maximum flow of the boiler this makes life much easier.
From our Pipe Sizing for Boilers and Systems webpage we know we should use a 1” copper pipe. We should add the Ft. Hd. Figures together. 5.6 + 5.82 = 11.42 Ft. hd. Calculate the Ft Hd. Now calculate the Ft. Hd. from the piping. I will add another 1.5. 11.42+ 1.5 = 12.92.
A good choice here would be a Taco 0015 on High speed. The pump will run on its curve so it will really be running at about 12.25 ft head at just under 8 gpm. I will not confuse this at this time with calculating different flow rates and how that applies to larger boilers. Just keep the IWH flow rates within the flow rates of the boiler.
Let’s look at another example where the IWH is piped as a separate zone off the system piping.
When the IWH is piped as a separate zone on the system piping you will not need to add the boiler heat exchanger resistance (ft. hd.) to the calculation. Looking at the above calculation and a total Ft. Hd. Of 12.92 and deduct 5.6 Ft. Hd. For the boilers we now will size the circulator for a total of 12.92-5.6 = 7.32 Ft. Hd. And will still need 8 gpm flow.
Again the pump will always run on its curve so the flow rate and Ft. Hd. Will be slightly higher but not enough to be a problem.
The above calculation would also apply to a cast iron boiler. The big difference is a cast iron boiler has almost "0" Ft. Hd. through the boiler.
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