Wiring Line Level Thermostat into HUH724ST

[Aegwyn11]

I've got a HUH724ST (manual says Marley, but looks like its marketed under several brand names). Is anyone familiar with this unit? Its the bigger brother to th HUH524TA (7kw instead of 5kw). I'm trying to wire a line level thermostat (Honeywell RLV310A) to control the heater. I pretty much just wired it so that the Honeywell is wired in the same place the stock knob was (knob is removed from the circuit). The thermostat seems to work fine until it tries to kick the heater on...as soon as it does it seems to 'reboot' (goes blank for a second, then comes back on). No clue whats going on.

Anyone familiar with this heater? I can try to post the wiring diagram if necessary. It seems like the manual doesn't exist on the internet. Its similar to the G73 everyone on here seems to talk about, but it has a 240V relay for the main lines and the thermostat is wired in so that it is what triggers the relay.

 

[Aegwyn11]

Okay, looks like I found the issue. The RLV310A has a comment in the first part of the instructions that it will not work with inductive loads, such as a contactor or relay. Argh. I had no idea there were resistive loads and inductive loads for line level thermostats. And when I called Marley to ask (before buying the thermostat), they said -any- line level thermostat would work.

In any case, does anyone have any experience with the Honeywell TL7235A? Its a digital line level thermostat, but appears to be a totally different design than the RLV310A. It doesn't explicitly forbid inductive loads like the RLV310A does, but I can't find any specs saying that it supports them either. It would actually be a little nicer if it would work as it'll go down to 32 degrees F (part of the purpose of this heater is to keep the garage above freezing through the South Dakota winter).

 

[Aegwyn11]

So in the world of line level thermostats, it looks like there are two basic types...one could make the analogy of one to a simple on/off light switch and the other to a dimmer switch. The one I have is of the latter, analogous to a dimmer switch. This won't work to control a relay...you need the simple on/off type.

The issue is I can't find a simple on/off type that goes lower than 50 degrees F, which would GREATLY increase operating costs over being able to set it at 40 degrees F most of the time.

From best I can tell, it looks like low level thermostats are all of the simple on/off type for whatever they're controlling.

My thought is to convert the whole thing to use a low voltage thermostat. If I put a 10x transformer on the input power (converting 240v to 24v), I could then use that to feed a normal low level thermostat. The output of the low level thermostat would then feed another 10x transformer (converting 24v back to 240v), which would tie to the 240V control coil on relay in the heater.

Does that make sense? Any reason it wouldn't work?

 

[Stuart in MN]

That unit heater typically comes with a built in thermostat. Is it missing from your heater, or do you just want to run it from a remote thermostat instead? The built in one goes down to 45 degrees, I don't think you want to try to set it much lower than that. If you do there are low temperature thermostats out there - go to google, enter in 'garage thermostat' and then check the 'shopping results' at the top of the page.

Berko/Marley sells their own line of remote thermostats, you could take a look to see what they have. http://www.marleymep.com/en/berko/products/heating/unit-heaters/huh524ta.aspx