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Adding panel radiators to existing hydronic system with old cast iron radiators

todddos | Posted in General Questions on


I have an existing circulating hot water system in my 1920 row home with original cast iron radiators and a relatively new gas boiler. The system feeds the radiators with 2″ black pipe, that stems off from the main 4″ black pipe heat and return lines.

I’m renovating my basement floor (~40% below grade) that has no existing heat system. I’d like to modern panel radiators to the existing system, but I’m not sure this is possible.

For one, can I even feed a modern panel radiator with 2″ pipe, then adapt it at the inlet/outlet? Or, if I run smaller pipe to the new panel rads, will this cause a problem with the flow rate, considering the older radiators with 2″ pipes would be running on a much higher flow rate than the new radiators?

Would a separate zone be necessary to pull this off? Hoping this is not the case, as it may be cost prohibitive.

Thanks all.

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  1. Expert Member
    Akos | | #1

    You generally should not mix high mass and low mass radiators unless you are running outdoor reset with near constant flow. If your new boiler is a modcon with outdoor reset, you should be fine to add them.

    With older systems like this with piping sizied for gravity feed, you do need larger pipes to get some flow rate, even then, it sometimes tends to bypass zones that are bellow the main trunk as there is just not enough pressure to push the water through. You can add balancing valves to increase the pressure drop through the existing runs to make it work but it does require some tweaking.

    The simples is to just add in an extra pump. If you want to keep the cost down, you can use one of the taco priority zone circulators that can be driven by a thermostat directly. If you size the rads a bit larger than needed, you can get away with not tying it into the existing controls, just let it run on its own.

  2. todddos | | #2

    Thank you for the prompt reply @Akos

    Unfortunately my boiler is new, but not a condensing boiler and I'd highly doubt is has an outdoor reset. My boiler is from 2016, model

    I did just realize that I had overstated the pipe size. The main feed and return pipes are 2.5" and the pipes that spur off are 1.5", rather than 2".

    Attached is a pic of my current boiler setup, if that is helpful.

    I will likely wind up in a situation where the potential added panel radiators would indeed be slightly below the main trunk that enters the boiler. They would wind up on the same floor as the boiler room.

    Can you elaborate on the priority zone circulators? Not sure I follow.

    As for upsizing the panel radiators, I was planning on doing that. My thought was that even if they are not on a separate zone, I could just add thermostatic valves to each unit and have control to tweak it from the radiator itself. Would this be solid logic?

  3. Expert Member
    Akos | | #3

    Can you take a picture where the pluming is, specifically where there are pumps. Without adding in a throttling valve, I doubt the system has enough pressure drop to feed a panel rad + TRV directly.

    Priority zone circulator is just what Taco calls them. This is a standard pump that comes with a built in relay module and transformer for a thermostat. Makes it much simpler to add in.

  4. todddos | | #4


  5. todddos | | #5

    @Akos, this is a shot of the Taco circulator that's currently plumbed into the boiler.

  6. Expert Member
    Akos | | #6

    That is a very basic setup.

    The simplest is to add a set set of venturi Ts ( on the water out of the boiler and connect your rads with TRVs to there. This would be much simpler than trying to T into the 4" black pipe and adding a pump. Best would be to source a cast iron panel rad, but it should work well enough with a low mass provided you have enough runtime on the boiler.

    Even simpler might be to add a bit of extra insulation to your basement to your basement (say R15/R20 rigid on the walls). With a bit of air sealing this can reduce your heat loss to the point that the standby loss from the boiler and pipes can supply the bulk of the heat. Add in a couple of small electric baseboards in case and you are done. Much cheaper to install and you might never make back the difference in operating cost.

  7. todddos | | #7

    Thanks @Akos

    The Venturi T route seems to be my best option indeed, and a great suggestion.

    Unfortunately, I don’t think bolstering insulation will entirely work for the space. The house is one a steep grade, and although the front of the basement is 75% below grade, the rear of the same floor is 100% above grade an has a 20ft exposed wall with several windows. We see regular winter temps of 20 degrees F, and the boiler room and much of the exposed piping for the existing system will be walled off and isolated from the space I'm trying to heat.

    In my scenario, the circulator I have is plumbed on the supply side coming out of the radiator, correct? So with the Venturi T, would I install this on the supply line coming out of the boiler, and just after the pump?

    Or, When you say a set of the Venturi T’s, are you saying that both the supply and the return lines each get one, right after they exit the boiler?

    I’m planning on adding 3-4 additional panel radiators to the system for this floor.

  8. joshdurston | | #8

    I would post this on, the guys there are pretty good with the technical details and will help you avoid some common pitfalls.

  9. Expert Member
    Akos | | #9

    You install the Ts on the outlet of the boiler itself. You'll have to do a bit of math to figure out the size of the venturi T and weather you need one or two. Keep in mind that you must still maintain the minimum flow through your boiler with the existing pump, so it is important to get the sizing right. Most likely you can feed a short run of rads with a single venturi T+regular T pair.

    There is more info about these here:

    I would also run your place through one of the online load (ie calculators to figure out the actual heat load your basement. This would save you having to install extra rads. Typical well insulated basement needs between 3000 to 6000 of heat (so 0.3 to 0.6 GPM flow).

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