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


  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.

  • Is replacing the tekmar 262 with a more capable control an option here? Jan 23, 2023 at 2:56
  • Use 24 Volt AC relay coil, the heat pump is not 30 Amp, maybe 5 amp
    – Traveler
    Jan 23, 2023 at 3:00
  • 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? Jan 23, 2023 at 3:25
  • @ThreePhaseEel The newer replacement for the tekmar 262 is the 406, but it still is limited in terms on relays. Jan 23, 2023 at 3:47
  • @Ruskes The 30A circuit is the backup water heater element Jan 23, 2023 at 3:48


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