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Advice: Recommended Boiler Temperature Reset

I've had great luck using the Tekmar injection mixing control in a similar system. I was able to lower the distribution temp at the radiators and keep the old cast iron boiler safe from cold return water. I've never had any complaints from the customer even on very cold stretches. Very easy to pipe!


  • Al Roethlisberger
    Al Roethlisberger Member Posts: 194
    Advice: Recommended Boiler Temperature Reset

    Along with some other Fall tune-ups to my old hot water boiler system(ca 1929) which includes a newish ca 1991 boiler, I wanted to add an outdoor temp reset.

    I have been researching this since last year, but now that I am looking at making the purchase, I thought I'd check here for any recommendations on particular brands or models.

    I would like a model that is DIY friendly, simple to use/setup, and effective and reliable.

    As I mentioned, this is an old overhead gravity system, converted in the early 90s to circulated system with a then new gas fired boiler.

    I do not have domestic hot water provided by the boiler at this time, and don't really have any plans to do so.


    Just a DIY'er trying to learn, and improve and maintain his converted ca 1929 overhead gravity hot water system since there is no one local that can.
  • tim smith
    tim smith Member Posts: 2,752
    DIY outdoor reset?

    My guess is that it is a simple cast iron boiler, straight flow through no mixing valve etc. This would require you to maintain a minimum return temp of 140 degrees. Depending on your type and size of radiators would determine if there is much benefit to this. If radiators are quite oversized then you may only require 140 or so to heat the house at coldest outdoor temps. If this is the case then there is no benefit to outdoor reset as you can't set the control any lower than what your max temp needed is. Just my .02 cents worth. Tim
  • Al Roethlisberger
    Al Roethlisberger Member Posts: 194
    Minimum or recommended hydrostat temp setting then?

    Thanks Tim,

    Yes, this is a simple cast iron straight through boiler, with no primary/secondary loop or mixing valve. I don't know if the radiators are oversized for the application, but they are the original heavy cast iron units.

    That is an interesting point you make about not requiring a OTR in this application, and is the first I have heard someone suggest that it may provide no benefit.

    One point I would like to make is that I plan to repipe the supply/return next year to likely include a pri/sec loop with a mixing valve if that makes a difference in your recommendation. The current setup is "plumbed backwards" into the original system from a past DIY job, so I'll be putting that right next year. The system works but I suspect is less efficient.

    But to your point about the minimum temperature of the return water, when we bought the house last year I discovered that the previous owner had the aquastat set to almost 200 degrees. Based on some research I had done here, I rolled that back to 170 in the interim.

    I know there are variables to consider, but in general, should the aquastat temp be even lower?

    I think I recall that for a hot water system, the average suggested temp at the radiator is 160/170 degrees, but perhaps that is incorrect?

    Is there a recommended average temp one should set the aquastat, or similarly should one take an average of the radiator temp and set the aquastat such that the optimal radiator temp is reached(again is that 160-170 degrees?)


    Just a DIY'er trying to learn, and improve and maintain his converted ca 1929 overhead gravity hot water system since there is no one local that can.
  • tim smith
    tim smith Member Posts: 2,752
    Regarding repipe & reset

    If you repipe w/ 4 way mixing valve then you will have a good option for reset w/ mixing. Regarding your heat requirements today, you could be well oversized and your requirements for water temp are only 140 or 150. I might suggest you try setting boiler at 150 operating temp and see how it keeps up. Through the years you probably have made envelope improvements which in turn have reduced the output requirements of your radiators. As I said in prior post, if you max temp needs are not above 140 or 150 then a reset would not be benificial due to the min. return required with no mixing system. Tim
  • Brad White
    Brad White Member Posts: 2,398
    The best of all options

    short of replacing the boiler with a condensing type, was touched on by Tim. This is my take:

    I agree that the boiler itself has limited turn-down range. If 180 F. is needed at say a zero F. design day, indeed re-setting that with a 140 F. limit is your option.

    IF the house has been insulated and the radiator to heat loss ratio is high, indeed, as Tim pointed out, you could get away with even cooler water on a design day. Say that is 150F per Tim's example. You now have only ten degrees of reset before you either damage the boiler or have to add a mixing valve to protect it.

    The four-way valve is to me a very highly recommended but still a second-best approach.

    You still have to protect the boiler and a return sensor would interact with the 4-way valve to give boiler return protection priority. But when satisfied, the 4-way valve would only then supply an indexed temperature to the radiators. How deep it can go depends on it meeting the boiler's needs.

    To me the best deal is to decouple the boiler and radiation loops and use a form of injection mixing. In this way, the boiler can use it's ideal setback range, however limited (180 to 140 or even 150-140) but the radiation loop can enjoy a much deeper reset curve. From the 180 or 150 F or whatever it may need on the coldest day, down to 70 degrees -room temperature!- on a mild day just before the system goes into Warm Weather Shut-Down.

    A 4-way valve will do similar things but if set up for boiler protection, it serves two masters and gets a little distracted shall we say.

    In the end, if you go "4-way valve", you will still be far better off than you are now. But if you are re-piping anyway, you have a choice to go deep, go long, go injection.

    My $0.02

    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Al Roethlisberger
    Al Roethlisberger Member Posts: 194
    Injection eh? Very interesting...

    Thanks for the tips on injection guys.

    Do you have any reference links offhand you can point me toward so I can study up on this setup?

    I'll Google in the meantime as well.

    Since the boiler is only 16 years old, and running fine at it's ~80% efficiency rating, I doubt it will get replaced any time soon. So any of these other solutions may be very interesting.

    Thanks again!
    Just a DIY'er trying to learn, and improve and maintain his converted ca 1929 overhead gravity hot water system since there is no one local that can.
  • Dave_4
    Dave_4 Member Posts: 1,405
    Injection selection for boiler correction

    Tekmar.com has several nicely worded essays on control strategies as does taco-hvac.com.

    Taco has a comprehensive control line including variable speed circulators, boiler reset controls, mixing valves, fittings and other neat stuff to make it happen. Some of the controls Taco has are or at least seem to be of Tekmar origin. Both are outstanding companies.

    Check out both. Good reading if you like this sort of stuff. Heck, I do.

    Ya think? :)
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