Tankless coil not getting priority
I have:
- Weil-McLain WTGO-5 with tankless coil, no indirect tank
- HydroStat 3250-Plus
- Argo ARM relay board
- 3 heating zones (one is the basement and rarely used)
- There was previously an indirect tank but it started leaking and was removed; the circulator and priority zone wiring are disconnected
Short version of the problem: when the heat is on, the shower is cold.
The HydroStat recently went kaput and was replaced with the same model. The installer wired up T-T to X-X, set the Zone/Indirect switch to Z, set the limits to 160/190, and that's it. Now on cold days when one or more of the thermostats is calling and the circulators are running, the domestic coil isn't getting enough heat.
The HydroStat manual has wiring diagrams for giving priority to an indirect, but I can't find a diagram that matches this situation (tankless priority without indirect). I tried removing the ZC/ZR jumper on the ARM and connecting ZC/ZR from the HydroStat to the ARM, and switching the HydroStat Zone/Indirect switch to I. Everything works but it's still the same situation, cold showers when the house is calling for heat.
How can I get it to prioritize the coil and not run the circulators when the coil is below temp?
Comments
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After pondering it a bit, the problem is that there is no way the Hydrostat can know that there is a HW demand from the coil. You'd need another aquastat mounted to the incoming cold water line. Run the the ground wire(s) from the circulator(s) through this aquastat. When it opens (due to 50F fluid), the circulators stop. It would recover fairly quickly at the end of the shower above a setpoint of 120F and restart the circulators. I'd probably set it at 120F because the boiler will never be below this value if it is operating.
Unfortunately, this solution is not going to prevent a lukewarm shower if the boiler temperature is at 140F at the moment the aquastat opens.
The tribulations of a tankless coil…………………………..
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Sigh, thank you. I think you're right that there's no way to measure demand on the coil. But I did make one more change this afternoon… I disconnected T-T, leaving only ZC-ZR connected, and set the switch back to Z. Now whenever the stat temp drops below the low limit, it cuts off the circulators. That should let all of the heat go to the coil until the boiler temp is back up above 160. I haven't tried it yet but I'm afraid the shower temp will still fall uncomfortably low before the boiler has a chance to recover. But at least it should come back up, and preempt the heating zones in the meantime.
It also causes some extra cycling, because if the zones call with the boiler already at temp (and no demand on the domestic), it will quickly drop that heat into the house, and then stop circulating until the boiler comes back up. I don't think this will be a huge deal though.
Let me know if you can think of any other problems with this approach, otherwise I'll try it for a few days. This could be the motivation I finally need to switch to a heat pump water heater while there are still rebates & tax incentives…
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If you disconnect T-T, I'm unsure how the zones can call the circulators. Of course the boiler can maintain 180/160 forever but what stops the pumps………….only when they fall below 160? It might get a bit warm in there………………..
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your Hydrostat will maintain the minimum temperature needed for the tankless coil to function. it should disconnect the heating circulators when there is a hot water demand. There are other things to check.
what size tankless coil?
did you use a mixing valve?
and if you did, did you set the temperature?
what temperature is leaving the coil?
what is your low limit set for?
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your Hydrostat will maintain the minimum temperature needed for the tankless coil to function
Unfortunately incorrect. When multiple zones call, the Hydrostat has no chance to maintain minimum temperature. The radiation controls that.
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When using a zone panel when the boiler has a tankless coil, ZR,ZC must be used, not XX to TT. This is a line voltage circuit but it's the only way.
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No, there's an Argo relay board. What I have now (with only ZR-ZC and no T-T) seems like exactly what I wanted. The Hydrostat always tries to keep the boiler temp between 160-190. Each zone's circulator only runs if the Argo gets the signal from the thermostat and then engages the pump relay. When the boiler temp goes below 160, the Hydrostat breaks the circuit between ZR and ZC, and the Argo doesn't allow any of the zone relays to engage. So in theory, heating demand can never cause the boiler to go below 160, and there will always be hot water available for showers.
I haven't taken a shower since making this change but in practice, I don't think it's going to work that cleanly. I was watching it today and when the ZR-ZC cuts off at 160, yeah the circulators stop and the burner kicks on, but the boiler temp continues to drop as the heat dissipates, going down to about 130 before the burner has warmed up enough to start raising it. Granted it was a very cold day today. But if you're in the shower when the heat comes on, I predict you'll get a few minutes of lukewarm water and then eventually it will get hot again.
This must be what led the previous owners to add the indirect tank, which was installed later than the boiler but failed first and had to be removed.
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Many tankless coil systems were traditionally set at a low limit of 180° and a high limit of 200° F. Assuming the tankless coil is clean (not limed up) and there is a low flow showerhead, that should work if the aquastat has good thermal contact and is working properly, wired to act as a circulator/low limit control to maintain boiler temperature. For safety, current codes require a thermostatic mixing valve with this arrangement to reduce the danger of scalding.
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Bburd0 -
Thanks, I do have a thermostatic mixing valve, put in when the tank was removed. That's a good suggestion, if the temp is dropping too low after the stat cuts out, I'll try it with higher limits.
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It appears you have it setup as best as you can. I am a bit curious why the boiler drops 30F AFTER the circulators stop. The only way that is possible is if the Hydrostat and the temp gauge are in two completely different locations on the boiler and it takes some time to equalize once the circs stop. 30F seems to be a massive differential inside the boiler.
As you noted, you won't be pleased if the boiler drops to 130F at the point where a shower is demanded. Remember the shower will demand about 100K (80F rise with 2.5 GPM) so you only have about 45K remaining to lift the boiler temperature and all that cast iron. It's going to take a bit of time.
The higher limits will help but the fuel consumption will climb due to maintaining the LL of 180……….indefinitely.
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