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I want to connect two QMark MUH108 10kw electric garage heaters to a single thermostat. According to the spec sheet, this unit uses 24 V control.

It was previously asked about a low temp thermostat for an electric garage heater and the Honeywell CT50K1028/E was mentioned (which is your basic low voltage thermostat) that goes down to 35 F.

The M600S from Marley was suggested as a simultaneous switching thermostat. The temp range for it is 50deg F - 80deg F. I only want to keep the garage above freezing (at a cabin in the mtns) so 50 deg F is too high for the low end range.

Are there other options like the M600S with lower temp range or is it possible (or advisable) to connect each heater to a Honeywell CT50K1028/E?

closed as off-topic by ThreePhaseEel, mmathis, Tyson, Daniel Griscom, Machavity Feb 12 '18 at 15:53

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    What about using the thermostat to controll contactors (heavy duty relays). – Ed Beal Feb 6 '18 at 21:59
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This is a hard problem

Heaters of the 10,000 watt size use a variety of ways to support external thermostats. Most of them use internal contactors, but most of them assume they are the only heater under external control. Their control methods are dangerously incompatible with paralleling.

The control is simple enough - they provide two wires and you shunt them together to call for heat - but each heater needs its own "switch".

Worse, they are 240V and your selection of 240v thermostats is already quite limited. The selection of true multi-pole 'stats is far narrower, as most "2-pole" stats actually only switch thermostatically on one leg, the other is just an on/off switch. Your fairly specific requirements will make for hard shopping.

There may also be heaters with no thermostatic control at all, and need to be hard switched by a contactor. The actual load is 41.7 amps, plus a fan, pushing it near 48 amps.

So my goal here is to come up with an affordable, off the shelf, legal control method that will work wih a variety of heaters and 'stats, and be understandable to any HVAC technician.

My answer is 24V control voltage just like household HVAC uses. It's easy to wire with inexpensive off the shelf components. Transformers are $13, you can use almost any thermostat from a $15 Honeywell to a a Nest, and it's easy to wire under the low-voltage rules. Don't haul line voltage any farther than you have to.

Paradoxically, low voltage is the only way to switch this much current.

The basic setup is a 24V transformer typically 40 VA (watt), readily available for about $13, that mounts in a junction box lid. Designate the two 24V wires R and C. Then to the thermostat of your choice, which takes wires called R and W (and C if a smart stat). Then on to each furnace location, and designate the two wires going to the furnace W and C. Here is how you hook up each type of furnace.

A furnace with onboard contactor

Generally these types of furnace support an external thermostat by providing one pair of wires. These are to be shunted (shorted) together to call for heat. Some of them only need to operate their internal contactor itself (less than 1 amp) and others operate the fan also (less than 7 amps).

You use any common relay with a 24V coil. The coil goes to R and W from your thermostat wiring. The contacts go to the two wires the furnace provides.c. Appropriate relays are kinds that mount in a junction box "knockout" (the furnace will surely provide several of these) such as RiB or Aube relays. An internal relay could also be mounted, such as a $12 air conditioning contactor would more than suffice.

Contactors

This is the word for a "big relay". If you are controlling a heater that does not have its own internal contactor, (such as paralleled Cadet heaters) you will need to provide your own contactor.

And the place to look for sanely priced 50A-tier contactors is air conditioning supply, as larger air conditioners have 40-50A rated motors. These contactors are actually made to work from thermostats.

Heat pump, ideally ground sourced

At typical electric rates, it will cost you $2.40/hour to run these heaters. If they are on 20% of the time, that is almost $400/month. I don't know who pays for your power or if you have a special rate plan. But the payback time for a ground-sourced heat pump would only be a few years. At which point the cost of opeation would be nearly nil compared to this.

A heat-pump does not require fat air ducts running everywhere, the units can use separate "registers" connected by Freon lines - this is called a "mini-split".

The reason to consider ground-sourcing is that if the heat pump air-sources, it will become too cold to operate some days of the year, and will "fall back" on emergency heat, which is resistive electric heating exactly like this here. In fact, with the contactors discussed, these very heaters would suffice as emergency heat.

Any heat-pump solution also provides air conditioning.

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There may be, depending on the current you need. It's unclear because you can't get 20 kW through that Marley anyway.

A quick search on my preferred industrial supply house shows the Johnson A72CE-1C, Raychem EC-TS, and a store brand unit (Dayton 6EDY8). These are all line-voltage, minimum 24A thermostats that go down to 30 F.

The Dayton unit is 30A double-pole, not sure if it's combined or separate.

  • Not that we're about shopping here, but none of those units are rated for the 50A per unit OP needs to switch. (10kw x125% / 240V per unit). – Harper - Reinstate Monica Feb 7 '18 at 0:32
  • This is out of my comfort zone here but does the thermostat need to be rated at 50A? I thought it only needed 24A since it just signaled the heater to run or not. The heater needs the 50A rating. – BobP Feb 7 '18 at 19:53
  • These heaters require 41.6 amps each x2. I get where this product is tempting since it is at-hand, but you tell me. Is 41.6 + 41.6 < 24? – Harper - Reinstate Monica Feb 7 '18 at 23:41
  • @BobP You have to tell us what model the heaters are at least. I skimmed a few spec sheets and it seems they typically use 24 V control with an internal contactor at the 10 kW level. – user71659 Feb 8 '18 at 0:45
  • ...and I'm skimming others and find they have an internal contactor which operates on 240V, requiring a 240V tier thermostat, but putting negligible current through them (some only contactor current, others contactor+fan). Because of their wiring method, the contacts cannot be paralleled across multiple heaters. Lordy, this is a mess, especially if he ever intermixes different heater brands, and perhaps he already is. – Harper - Reinstate Monica Feb 8 '18 at 1:04

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