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Run circulator off Taco Fuelmizer or boiler control temp based circ delay?

RollCNY Member Posts: 4
Hello. Long time lurker, first post, and I have a decision point that I need help with. Sorry, this will be long, but I don't know how to explain it short.

I am a homeowner with limited boiler experience, but decent industrial controls experience, and good understanding (I think) of heat transfer and concepts. Up until last October, I had a 40 year old boiler for home heat, and a separate 18 year old natural gas hot water heater. I wanted to replace the hot water heater with an indirect, but struggled to find an amenable contractor.

So after lurking and researching on HeatingHelp (thank you all so much!), I installed myself a Taco 3 zone controller with DHW priority, a Taco Fuel Mizer SR501-OR), new Zone Sentry valves for two heating zones (3/4") and the DHW (1"), a Taco 007e circulator, a Triangle Tube Smart30 indirect, and a mixing valve. I have a single system circulator pump, controlled and powered from the Fuel Mizer. I set the reset dial for 10 deg design temp (Taco doesn't let you adjust the curve, only design temp), even though Central NY is more likely 0, but everything worked swimmingly. A closed end switch signal, heat or dhw, goes from the zone controller to the fuel mizer, the fuel mizer powers the pump, and closes the TT contacts to the boiler provided the supply temp from the sensor is at or below desired temp based on OR. At desired supply temp, TT drops out, but pump keeps running if thermostats or aquastat want heat.

The only real concern that I had with the system is that the Fuel Mizer temp sensor is placed on the supply side of the boiler. It has a non-condensing setting to not go below 140, but it kind of seems like any delta T puts the return water temp in condensing territory. But what I loved about the system was that the fuel mizer runs the pump with any zone closed end switches when it isn't firing the boiler, and I get all the circulating water dumping mild heat. Home comfort was a fantastic improvement.

System worked great, but I now find myself with a cracked boiler section, and am in the process of replacing. I don't know if the crack was caused by condensation, or if a 40 year old boiler could be showing signs of wear :) I had planned to have the boiler replaced all along, and had expected to just provide it a TT signal and let my installed controls do all the work.

With all that backstory, here is the question:
Replacement boiler is a non-condensing unit that has a circulator delay feature under 125 deg. On start up, and if return water falls below 125, circulator is interrupted until temp meets goal. So my gut wants to use that to reduce chance of condensation on start up and low running temp based on OR, but then I give up all the free recirculation. The only signal the boiler will get from my controls is a closed TT, so in my mind it will only continue to run the circ if its internal aquastat gets met/exceeded, but it won't know anything about the thermostats going to the 3 zones controlled by the zone controller.

So do I use that and ignore the Fuel mizer except for OR temp control, which would essentially just shut down the boiler when temps hit 140? I worry about a lot of short cycling. Short cycling would likely have me abandon outdoor reset all together and manually adjust aquastat settings for time of year, and live with slower DHW recovery.

Or do I leave the pump on the Fuel mizer and ignore the low temperature circulator delay? If I do this, I would likely move the temp sensor to the return side of the system, so I shut off if return water hits 140 as a low? that worries my a little, because I really don't know my system delta T.

Or do I wire in a 24V relay, and try to blend both? I am thinking that I could wire the boiler's circ pump circuit to a normally open set of contacts, the Fuel Mizer's to a normally closed set, and let the same TT signal that fires the boiler pull in these set of contacts. I don't know how often this relay would have to toggle, but don't think the two different sources and millisecond switch would impact the pump.

I will discuss this with my contractor doing the boiler install, but his preference right along has been a mod con, so whenever I ask a controls question I get essentially "this wouldn't be an issue on a high efficiency boiler" answer. So I am looking for thoughts.

thank you


  • RollCNY
    RollCNY Member Posts: 4
    And just in case anyone wants any details on the bulk of the system:

    Two heating loops, both 60' of emitters, copper finned tube. Two story house, one zone above the other, boiler in basement. All plumbing 3/4", including zone valves.

    DHW is plumbed 1", also on zone valve. Temp set at 145, plumbed with heat trap loops and incoming expansion tank. For all practical porpoises, I have not encountered the ability to run this out of hot water.

    In six months of run, the DHW has never timed out to the point it turns on heating loops, so in all cases the system has been either heat or HW, but never both.

    Current boiler and new boiler are both significantly oversized. I am moving smaller with replacement, but not as small as I would go if left to my own mathematic devises. They are not sized at Heat + DHW load, and the new will actually be sized under DHW load.

  • MikeAmann
    MikeAmann Member Posts: 422
    edited March 21
    I would do as you suggested: >>> Or do I leave the pump on the Fuel mizer and ignore the low temperature circulator delay? If I do this, I would likely move the temp sensor to the return side of the system, so I shut off if return water hits 140 as a low? that worries my a little, because I really don't know my system delta T.

    If your new controller is a Hydrostat, then it has a DHW priority switch that might help your situation.
    It makes it really easy to add a second circulator just for the IWH.

    Why that choice? Here is what I just wrote on another post: I have a Hydrostat also, and I wasn't happy that it took ECON mode hours to finally ramp up to the HI Limit with a very long call for heat. My old setup worked the opposite way - run burner up to the HI Limit and then economize on the way down. Because of my work hours, I usually turn the heat down to 55 F when I am not at the house. I used a cheap China programmable timer relay with the contacts connected to ZR ZC and set it for 45 minutes. A CFH via ZR ZC overrides ECON and automatically fires the burner up to the HI Limit. The Hydrostat automatically abandons an indirect's CFH after 45 min and reverts back to heating the house. You might consider doing the same. All your wife would have to do it hit the button to activate the timer while she is getting ready for her shower. You could even use a remote transmitter, like a car alarm.

    You like the way your old controls were doing the job. Don't change this to start. The new controls do much more, but they also do things differently, and that might not be what you are after.

    If you downsize the boiler too far, then the DHW might not keep up with demand. Think about it - does any of, or even both heating loops combined equal 40 gallons?
  • RollCNY
    RollCNY Member Posts: 4
    thanks for the feedback, especially since it is opposite my gut. I was leaning to trusting the circ delay on the boiler.

    I don't have full details on the boiler controls yet. I don't know if Hydrostat is a brand or type of device (sorry). Details I have on the new boiler (Dunkirk DXL-K series) show aquastat low voltage input, and they show a dedicated output for a DHW circulator. I am assuming that they have a priority to it, but don't know any of the logic, don't know if adjusts the boiler's aquastat setting like the Fuel Mizer does. Documentation that I have access to now isn't clear on that.

    New boiler is only about 10% smaller than current, and will have auto damper (as opposed to no damper), and do away with standing pilot for electronic ignition, so I largely expect it to be a wash. I take your point on going too small being most noticeable on DHW, so I gave up on my 40% smaller version. Too much of a pipe dream
  • MikeAmann
    MikeAmann Member Posts: 422
    edited March 21
    Well, that would depend on how great your DHW needs are.
    As far as sizing the boiler for heating, the way the industry is doing it now is to size so that the boiler runs constantly on the coldest day of the year. Any larger being a waste.

    Back to the Hydrostat - you can choose how it fires the burner >>> activate on TT, activate on ZR ZC, or both. I have you thinking now, don't I? But use you current setup as is to get started. You can always change things later. Basically the new controllers are PLCs, and you don't need to change any wiring or add relays, etc. You just enter programming mode and change a setting. Simple.
  • RollCNY
    RollCNY Member Posts: 4
    edited March 21
    Thanks! I did some googling and all indications are that the unit comes with a HydroStat 3200-Plus, and looking at the manual for that I should be able to tweak to exactly what I want. I was doing manual searches on the boiler and not finding enough information to hatch a plan.