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Community and Q&A

How can one adjust the temperature on an indirect heater?

user-1119494 | Posted in Energy Efficiency and Durability on

I recently moved to the NE from the west coast and have no experience with these things, but my client has water that barely reaches uncomfortably warm, even at a bathroom 10′ from the tank. The output pipe on the tank is also easy to grab and hold, while the pipes bringing heat to the tank are momentary-contact only.

What I see looks like someone has plumbed the tank as one of the three zones of baseboard heat.


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  1. GBA Editor
    Martin Holladay | | #1

    An indirect water heater is equipped with an aquastat. The aquastat is wired to the boiler. When the aquastat calls for heat, the boiler fires and a dedicated circulator delivers hot water from the boiler to the heat-exchange coil in the indirect water heater.

    Either your aquastat is set too low, or the aquastat is defective. If you don't know where the aquastat is or what it looks like, call a plumber.

  2. user-1119494 | | #2

    Thanks, Martin. Knowing the name of the "aquastat" is very helpful. The thing mounted to the water heater and wired to the zone control box looks nothing like an aquastat: much simpler and non-adjustable. I have a great photo that would remove all doubt...

    The local plumbers (yes, plural) she has called come out and tell her there is nothing they can do. Now no one in town will even return her calls.

    I think her contractor worked with lousy subs who undercut the pros and the pros are now letting her stew...

  3. GBA Editor
    Martin Holladay | | #3

    See photos below.


  4. Expert Member
    Dana Dorsett | | #4

    Some have covers with no external adjustments visible, others can be as simple looking as a knob (but you will probably have noticed if that were the case.)

    Show us that "...great photo that would remove all doubt...". I've yet to see an indirect that had no temperature adjustment.

    A great picture of the boiler and zone manifold might also be useful for debugging purposes.

  5. user-1119494 | | #5

    I found the link to attach files hiding below the posting notes!

  6. user-2890856 | | #6

    Picture of the indirect and the boiler also . Depending on whether the indirect has an aquastat or a sensor will tell the story . Please show the relay also

  7. user-1119494 | | #7

    Here is a pic of the boiler, zone controller, and indirect water heater.

  8. user-1119494 | | #8

    Sorry, Richard, I have no idea what relay you mean unless it is one of those in the zone controller.

  9. Expert Member
    Dana Dorsett | | #9

    Go ahead and take a close up of the zone controller, and indicate which zone is set up for the indirect.

    BTW: The way the near-boiler plumbing is configured isn't all that great, but if the system is working it's probably not worth re-plumbing until/unless it's time to swap the boiler. Installing a vent at an ell on each of the returns before every zone pump is something I've never seen before- probably a band-aid attempt fix some air accumulation problems created by the expansion tank being on the "wrong" side of the pumps or something. From the color and look I'm guessing the boiler is an oil-fired Smith (probably series 8 ?), most likely 3-4x oversized for most houses, which is a separate issue.

  10. user-1119494 | | #10


    The heater MAY be reasonably sized: apparently the house is a bear to keep warm.

    The heater is connected to zone 2.

  11. charlie_sullivan | | #11

    See fig 8 page 7 of this manual. Is that the one? It looks like access to adjust is from the opposite side from the picture you posted. Might be a pain to get to but should be possible.


  12. user-1119494 | | #12

    Thanks, Charlie, but it does not seem to be the same thing. I see no sign of a dial face or any sort of toothed ring in the pics I took. Very close match, however. I wonder if they have the same sort of part, but one that has a fixed temperature setting rather than a variable one... and might that be what I am seeing?

  13. charlie_sullivan | | #13

    It's possible that the one you are up against is fixed, but if it's adjustable, the access is from the other side. The side you show has the screws poking out. The side they show the access from has the heads of those screws.

  14. Expert Member
    Dana Dorsett | | #14

    The SR301 zone controller is usually set up with a the indirect HW heater on zone 1 rather than zone 2 , since it can be configured as a "priority zone". When a zone has priority calls for heat from other zones are ignored until the priority call is satisfied. Zone 1 is currently jumpered as a priority zone on your system. If it's a space heating zone it will ignore the call for heat from the water heater until the thermostat on the heating zone is satisfied.

    See page 4 of the manual, and try to verify all of the connections:

    Try to find a part number for the Honeywell on the indirect. It can be replaced with an adjustable aquastat if it's toast. The whole thing unscrews from the well after back off the lock nut behind the back plate of the sensor box.

    Separate discussion, but unless your house is one of the largest & least insulated houses in the state it's highly unlikely that the boiler isn't oversized for the actual space heating load (even if you lived in Churchill Manitoba). Being "...a bear to keep warm..." is more often a function of insufficient radiator or baseboard capacity, or the water temperature settings on the boiler being set too low (which would also affect the domestic hot water heating recovery times & operation slightly.) What is the model number & D.O.E. BTU output of the boiler? I'm guessing it's a 3 or 4 plate sucker, with over 90MBH (90,000 BTU/hr) of DOE output. The smallest 3-plate Smith is good for 91MBH, unless a somebody jetted it down to something lower (which can be done if you have a stainless liner in the flue and is adjusted by a qualified tech who knows which end of the combustion analyzer is up.) see:

    It was common to oversize boiler that had embedded hot water coils to get decent domestic hot water performance out of it, then when the coil limes up from use many people just retrofit an indirect tank operated as a priority zone.

    [edited to add]

    I just noticed in the zone controller picture that zone 1 was calling for heat when the picture was taken, as indicated by the LED being on. With the priority jumper installed that means that even if the tank aquastat was calling for heat as zone 2 it would be ignored until the thermostat for zone 1 was satisfied. If there is minimal radiation on zone 1 the boiler could be cycling on/off during that zone call, and would still be ignoring the other zones despite having burner capacity to spare.

    The easiest and first experiment to run would be to pull the priority jumper, since you almost certainly have excess boiler capacity. That way the water heater would be served even if it was taking forever for zone 1 to heat up (due to insufficient radiation, or whatever.)

    How much radiation or baseboard do you have, broken down by zones? If baseboard, just the length of the baseboard to the nearest foot is good enough. If radiators you'll have to do some measuring and calculating:

  15. user-1119494 | | #15

    I'll take a look or ask someone there. It may be a while before I get that far west in MA again.

    I guess the answer we've come up with is that this is an unusual aquastat, but may be just fine.... Just not obvious...

    My thanks to all.

  16. user-1119494 | | #16


    Great stuff. I'll try your suggestions sometime in the next month or so.


  17. pardaanthony | | #17

    I have a peculiar situation. Our restaurant is having issue meeting up to code as 4 sinks can reach the ideal temperature. The boiler rest on the rooftop where water in then rerouted to a storage tank 6 floors below (the basement) before being distributed throughout the restaurant. The only sinks that do not meet up to code are the one that are motion sensors to turn on preventing us from being able to control the hot/cold output. We had an engineer come in to take a look at the boiler and determined that everything was functioning fine. We then had a plumber come in to take a look at the sinks itself to see if it was the settings on the thermostatic mixing valves, but he claimed it was no the issue and that it was because of the length of time it takes for the hot water to come down from the roof. Pretty much zero solutions from both of these technicians, and we need to get it fixed soon before our next inspections. Any thoughts on how to resolve this issue?

    1. Expert Member
      Dana Dorsett | | #18

      A recirculation loop on the hot water side to keep the local hot water above some pre-set temperature is a common solution to the water delay issue when the water heater is distant from the tap.

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