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I have two heat-pumps controlled by a tekmar 262 boiler control. The heat pumps heat water in a storage tank, which is then circulated through the house. The storage tank is designed for hydronic heating and contains a regular electric heating element which, if powered, can serve as a backup or partial backup.

Normally, the 240VAC circuit to the storage tank element is turned off. When the water in the storage tank is not hot enough and there is heat demand, the tekmar control turns on one heat pump. After some extended period of time, if that is not sufficient, the control also turns on the second heat pump.

In the event one of the heat pumps is down and it is very cold so that one heat pump is not enough, I would like to be able to switch on the circuit breaker for the storage tank heating element so it can kick in when there is demand.

My problem is the tekmar 262 only has two control relays. I'm thinking of connecting some kind of relay to one of the heat pump relays, and wiring that relay to activate the heat pump and the storage tank heating element.

Proposed wiring diagram: Wiring Diagram

Questions:

  1. Is there a better way to do this?
  2. Suggestions for a relay? It seems like a difficult combination, since the heat pump is activated by a low voltage circuit on a PCB in the heat pump, and the heating element is 240VAC 30A.

I understand this is not a perfect solution, as it depends on which heat-pump is down. Ideally, the one which is down would be the one used for this circuit, and it would also be the one which fires second. But for now I would like to get the basic control relay figured out.

Gary Aitken
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  • Is replacing the tekmar 262 with a more capable control an option here? – ThreePhaseEel Jan 23 '23 at 02:56
  • Use 24 Volt AC relay coil, the heat pump is not 30 Amp, maybe 5 amp – Ruskes Jan 23 '23 at 03:00
  • Does it randomize which heat pump to use? Or does it put load on one heat pump most of the time? Are you on a demand-peak pricing energy tariff? – Harper - Reinstate Monica Jan 23 '23 at 03:25
  • @ThreePhaseEel The newer replacement for the tekmar 262 is the 406, but it still is limited in terms on relays. – Gary Aitken Jan 23 '23 at 03:47
  • @Ruskes The 30A circuit is the backup water heater element – Gary Aitken Jan 23 '23 at 03:48
  • @Harper The control can be set to randomize or to honor a specific firing order. I normally have it set to randomize, but if one goes out I set it to always fire the other one. We are now on a demand peak pricing scheme, 7-10am and 5-8 (I think) pm. Why do you ask? – Gary Aitken Jan 23 '23 at 03:52
  • What do you mean "goes out"? A heat pump can't possibly be failing *that* often. My line of thinking is that only on rare rate plans do the power company care what your momentary peak loads are. If they don't care, then you should simply gang the two heat pumps on one control channel so they always run together, and use the now-empty control channel for the resistive heat. I don't see any advantage to sequencing them, unless you have a weird rate plan. Your water tank might not be your biggest energy storage vessel. Watch this. https://www.youtube.com/watch?v=0f9GpMWdvWI – Harper - Reinstate Monica Jan 23 '23 at 04:07

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