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Old radiant system

angang Posts: 2Member
I just bought a house built in 1964 with a hydronic system. The house seems reasonably tight for its age, but not well insulated. The boiler itself is only one year old (Dunkirk PWXL, standard efficiency). Question #1: should I consider adding an outdoor reset control, and what is the expected payback for doing so? There are three zones: two baseboard fintube, and one radiant. The radiant is in-slab copper tubing, that I believe is original. No leaks that I know of, and it heats well. I noticed that this system is piped with a single circulator (TACO 007) and three zone valves, but no mixing valve for the radiant zone. So, the floor can get as hot as the boiler high limit (in theory). This seems wrong to me, but as far as I can tell, it was designed this way from the beginning. There are neighbors with similar systems and they seem happy. Question #2: Should I worry about adding a mixing valve and separate circulating loop for the radiant zone, or just leave well enough alone?


  • Paul PolletsPaul Pollets Posts: 3,169Member
    Controlling the boiler

    Without a 4 way mixing valve and motor for the radiant, there is a very good possibility of thermal shock (and condensation) for the new appliance, which will cause the boiler to fail prematurely. The return temperature to the appliance needs to be above 140 within 4-5 minutes after startup. The acidic condensate caused by firing the boiler at low temps, will start to eat up the ferrous parts of the boiler...which is most of it.
  • Jean-David BeyerJean-David Beyer Posts: 2,609Member
    I am a homeowner, not a professional.

    My house had an old GE (1950) oil fired boiler with copper tubing in at grade concrete slab downstairs and fin-tube heat upstairs. All one zone, so all got the same temperature water. There was insufficient fin-tube upstairs.

    From what I have learned, mostly at this site, a boiler like that should not be run at less than 140F to prevent condensation. And 140F is too much for the slab. Also, that boiler had no reset, so the temperature swings were intolerable. I reduced the aquastat settings so that the boiler ran between 130F and 140F, and that helped the temperature swings. Boiler was about twice the size it needed to be, so rapid cycling was the rule. It must have been a tough boiler because it did not rust out, though it ate vent pipes.

    The way the water to the slab (and unfortunately to the baseboards) controlled its temperature was that there was a big loop that had the load (slab and baseboard) on the "left" side, and the circulator on the "right side. In the middle, across the loop was the boiler with a globe valve (later replaced by a ball valve). If that valve is closed, no heat is delivered to the system. Wide open delivers maximum heat, and in between is in between. There were no temperature gauges on it, so I do not know the actual temperatures delivered. I tested it by walking on my ceramic tile kitchen floor barefoot on a very cold day it it was uncomfortably too hot. I suppose a real temperature regulating valve would have been a better way. I have no idea if these were available in 1950, or if the builder knew to use one. I had the instruction book for the boiler and it did not say anything about adding such a valve, but it looked as though the boiler was meant to heat radiant slabs. The circulator was a big 3-piece one that finally died. The oil company who did my service at the time replaced it with a Taco 007 and it worked fine. I was surprised because it was so small compared to the original one.

    Look at the pages starting with the one labelled Page 11 here. My pdf program calls it page 10. That is my original boiler.

    (Now I have two zones, each with its own reset curve, and outdoor reset. Much more comfortable and cheaper to run than before. Gas fired mod-con.)
  • STEVEusaPASTEVEusaPA Posts: 2,820Member

    I think you would benefit both in comfort, and savings by changing some near boiler piping, and adding outdoor reset to all zones, and a mixing valve to mix down your radiant, and like Paul said, to protect your boiler. 

    Please make sure you have someone who knows what they're doing.

    Ask them to perform a complete heat loss for your home, to determine how many btu's you need for each zone, and at what temperatures.  If they dont want to do any of the calculating, don't let them do any of the piping/controls.

  • angang Posts: 2Member
    Avoiding thermal shock

    This is a followup to your kind response from several months ago. I am just getting around to planning the modification to my system to avoid thermal shock to the boiler, which I see happening regularly now that the heating season has begun. In your previous reply you mentioned using a 4-way mixing valve. I my old house I added a radiant heating zone to an existing hydronic system with three baseboard zones and I used a Taco 3-way thermostatic mixing valve. I had a circulator for the radiant circuit, and separate circulator and loop for all the baseboard zones. This seemed to work fine and I had no thermal shock issues.

    I am trying to understand the pros and cons of a 3-way versus a 4-way mixing valve. Any advice would be appreciated.

    My plan as of right now is to add a 3-way mixing valve to the radiant zone, and give each of my three zones its own Taco 007 (or grundfos equivalent) circulator, and a 3-zone switching relay to control it all. Does this seem reasonable? Also, I a curious about intergrate flow check valves versus separate check valves. Is there any real difference between them?
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