I have recently converted from oil to gas in my 2500 sqft 1930s era Home. We have a hot water radiator system. We left the original radiator main pipes and risers. They are large pipes 3” mains. and as such the plumber used a rather large 1/8 HP pump model taco 0011-f4. We would like to move to three zones in the House, and also lower the electricity usage of the system. That pump uses 202 watts while running. I can’t seem to find information about how to size the main pipes and how that relates to pump size.

2 Answers 2


1> install a constant delta-P pump ERP rated circulator (it'll adjust the power to the required delta-P instead of running fixed RPM)

2> install constant-delta-P thermostatic preregulation valves (Caleffi Dynamical)

3> Calculate each valve maximum flow allow no more than the needed flow to heat each room

This configuration can let your (GAS) consumption go down by at least 10% if you have a condensing boiler and you set properly the delta-T, also can push down the electric consumption by about 40% or more (up to 95%), (Pump will run maximum just for the first hours, when valves start to throttle down radiator flow, pump 'reads' less need and slows down it's speed accordingly, pumps are also 'intrinsically' much more energy efficient (EEI<0,23 when older 'A-class' had EEI of about 0,40)

Similar configuration may be achived (knowing diameters and lenght of each pipe) using delta-p compensation before zone valves and using 'standard' preregulation valves on radiators (But in this case, calculating each valve is much more difficult, with dynamical calculation is quite easy)

This is a reference for dynamical (it's in Italian, but I think google can help translating) Another reference (see summary: blue box on page 29)
ERP-related (1) ERP-related (2) information (English)

  • This is interesting. Do you have any more references for this in English? It’s hard for me to figure out how this may address my question.
    – rhl
    Apr 11, 2018 at 3:17
  • Sorry, but Caleffi workbook and magazine are (for what I know) the best explain on balancing hidronic systems available online (US version is focused on US-Sytle-OCEB systems). This is a link to the product datasheet. I think you can google 'radiator system dynamic balancing' online. NOTE: OCEB: Office for Complication of Easy Businesses. I know Danfoss is making the same product under Caleffi patent.
    – DDS
    Apr 11, 2018 at 8:03
  • I think the spirit of your suggestion is to installl a circulator pump with a variable speed based on pressure differences in the feeds and returns, and those pressure differences are made smaller based on TRVs is that the case?
    – rhl
    Apr 12, 2018 at 1:17
  • Usually yes, but not only, many pumps are hugely over-sized, so your system may work fine just with half the power you installed. Pumps have to be choosed not according to 'pipe size', but according to head and required flow. In a 'no TRV' system you can use a 'return-temperature controlled' circulator that keeps injecting only the water needed to keep return to desired temperature (said in another way: only injecting the heat needed and not always runing to satisfy the cooldest day request)
    – DDS
    Apr 12, 2018 at 8:50
  • I accepted this, although, i'm still not totally sure how to do this. But, I get the idea.
    – rhl
    Apr 16, 2018 at 0:17

If in fact the main delivery pipes are 3", then the old piping system was probably a gravity system. They usually worked well but were slow to respond to temperature changes and used a little more fuel than a pumped system. The contractor should have done a better job by re-engineering the whole system and replacing the oversized piping. As far as wanting to zone the system there are a lot of options depending on how the existing radiators are piped. You could just add a "Danfoss " type valve with an integral thermostat on each radiator. This would be the most simple zone system I can think of. Some type of thermostat would be used to cycle the boiler or you could run the boiler on an outdoor reset type device. The pump would probably run 24/7 as long as the water in the loop piping was above a minimum set point. Other than that you may have to re-pipe most of the delivery mains to achieve zoning. For that I would hire a heating engineer.

  • Yes it was a gravity system. We wanted to wait to replace the mains for budget reasons.
    – rhl
    Apr 11, 2018 at 2:55
  • If the system is heating well and you don't mind the oversized piping you could leave that piping installed and just zone the system using the piping as is now installed. Changing the pipe main size from 3" to smaller will not improve the system efficiency enough to allow you to re-coup your new investment cost. However if you are going to redesign the whole system, then yes, replace the mains, add zones, etc. MY 2 CENTS
    – d.george
    Apr 12, 2018 at 9:50
  • Right well I mean, I guess what I have is the brand new boiler and stubs for two more zones. The old mains attach to old risers. Since I won’t change the risers, “redesigning” to me amounts to changing the mains, and going with zones. A pump which changes speed based on temperatures in the rooms, that makes a lot of sense. The only question is whether a single thermostat (per zone) can drive that, and what is an example of a pump/valves etc..
    – rhl
    Apr 12, 2018 at 11:40

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