Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit

2 probe aquastat for temp difference bet. source and load?

seems to me this is probably a kind of control found for solar work. I'm actually heating an indirect tank off a steam boiler to run some warm floor in this house.  for the longest time i had it on the worlds oldest indirect on this old coaler.

It was a big heat exchanger that was piped with 2" lines and hung right next to the boiler.  no pump.  I put a bypass and balancing valve in the floor circuit side and just tuned it not to make the floor too damn hot and let the amount that the steam boiler ran govern the amount of heat going to the floor with the floor pump running 24/7

worked great until one of the 2" pipes developed cancer.  Can't tell you how many tanks of oxygen and acetylene it took but we actually got the shards of the pipe to back out of tee right next to the boiler.  but it is time to go to a modern stainless indirect setup. so i'm getting that plumbed in.  meanwhile i'm adding more and more radiant (eventually i'll get everything radiant and go to a modcon but thats the 5 come 20 year plan.).

So I i'm going to use a pump to supply the indirect and i want the pump to have an aquastat to keep the tank from getting too hot (i'm abivalent about whether i'll actually use a setback controller right away or just a limit), but i also obviously don't want the pump to run when the water in the boiler is colder than the water in the tank.  but i don't want the pump keyed to the burner because then i'm going to be leaving all this hot water in the boiler to heat the basement after each steam cycle.

So i'm kind of looking for a dual probe aquastat that will close when the temp at one probe is maybe 5 or 10 degrees above the other.

in some ways this is like a hydronic boiler circulator controller that has a circulator cutout at a lower temp than the boiler cut in, but that works off one aquastat at the boiler and i'm trying to get one at the indirect tank and one for the boiler water for comparison.

this could be a relative controller and i could use another just to kill the whole circuit as a max limit for tank temp. or it might be a controller that could serve both those functions.

thanks for any notions.

archibald tuttle


  • SWEI
    SWEI Member Posts: 7,356
    Floor side

    should be fed by a mixing valve, preferably one controlled by Outdoor Reset.  Taco iSeries-R is probably the least expensive option there.  It wants its own circulator (non-ferrous) and you might be able to make it work with just a single pump and a 4-way valve.  Is the indirect for DHW, or some kind of buffer/separation for the floor zone?
  • archibald tuttle
    archibald tuttle Member Posts: 819
    indirect isolates radiant circuit from steamer

    i use the indirect to create the water temp i want for the floor instead of mixing,because the steamer has no pressure -- well 1/4 lbs or so when running but i don't use checks and try to get pumps to take it to the third floor.  i just pressure up the indirect water circuit with a circulator and use thqt for the floors.

    it does mean i'm running an extra circ as opposed to mixing valve but even when i get to the modcon, this house does not have barrier tubing so i'll still be separating. some is polybutyl, some is non-barrier pex.

    so i want to get heat out of the steamer during the warm up and cool down  cycle especially.

    i'm experimenting with firing rate on this guy to figure the best cycle.  when i fire about 250,000, once it makes steam i get 3 or 4 minutes on and 1 off on the vaporstat.  obviously that is short cycling, but when i back down to 175,000 it takes 25 minutes to steam from cold start but never hits the vaportrol which is set 10 inches or between 1/4 and 1/2 lb. from cold start (and i can really lower my stack temp (added a couple more bricks as well to the flue) and my efficiency goes up to almost 80, but i need to capture heat during the inrun and cool down for that to make sense.

    obviously i'm going to try 200,000 and see if splitting the difference shortens the time to steam while not hitting the vaportrol so frequently. 

    one problem is that the radiation is convectors.  you get no warm mass built up so cycle time to restart is always going to be short. and i'm busy converting to low temp hydronic radiant with floor and wall installs so i can't see going through the grief to change to cast iron rads, never mind the back and wallet exercise needed to collect them.

    a modulating fire conversion burner would be ideal but don't know that anyone has really gotten that down yet.   i think i saw a riello listed.

    but changing firing rates on the Carlin EZ is a modest PIA.  ideally, they should have made an irsi style adjustable orifice (like the way you used to adjust the diaphram on cameras back in the day.  or ate least some kind of union style fit out where you can change an orifice and pop it back together.

    i've got unions on either side of the gas valve so at least i can pop it all out without unwiring the valve.

    but i digress about that stuff.

    i guess though i am thinking outloud about whether running the pump for the indirect during heat up will stretch it out much.   i don't think so but want to get all that up and running before i finalize my firing rate.

    in any event, i want the hot water from the steam cycle to keep circulating to the indirect for the radiant floor until the desired set temp is reached or until the temp in the boiler approaches the temp in the indirect so that continued circulation would actually cool the indirect rather than heat it.

    archibald tuttle.
  • archibald tuttle
    archibald tuttle Member Posts: 819
    rebooting my search for two source aquastat

    . . . .that can compare heat source and heat storage and stop circulation when the source approaches the temperature of the storage.


  • archibald tuttle
    archibald tuttle Member Posts: 819
    answered my own question . . .

    this looks like a cute entry level version for $79 plus $13 shipping

    then there are the prepackaged units for about twice as much:

    i probably should have posted this on the solar thread as that is what the technology is most often used for.  but they are called differential temperature controllers or differential thermostats - i think i first search for differential aquastats and didn't find any, or got snowed under by the results for aquastats that all have setpoint differentials.

    of course if you've got any other clever ideas i'm all ears --  eer eyes i guess in this format.

    and while i was looking found this super affordable temperature controller on amazon.  looks great for control strategy stuff and finally a price you can afford for extra temperature control, $18. 

    merry [12th day of] christmas.

  • hot_rod
    hot_rod Member Posts: 15,048
    solar differential controllers

    is what they are called and there are dozens of them available these days.

    The more you pay, the more features you get. The more expensive ones will have some data logging, energy metering, timers, variable or fixed speed pump operation, etc, etc
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream