A thermostat really triggers a relay, couldnt you feed the thermostat to an input for the elk and then link to the heating system from there? This way the elk has an understanding of what the thermostat is requesting, and it can also apply some control to the heat.
It is clearly not as good as hooking it to a serial expander, but it might be an option for the short term.
A good question might be to ask how they want to use it with the elk, as that might give more options (they may not be looking for something real complicated).
Maybe I'm oversimplifying, but maybe this gives you an option.
It feeds 110V 15A to the Thermostat then 110V 15A down to the mat. From the instructions I am guessing the tempature probe is in the thermostat. My idea is to use a HAI RC-80 Single Stage that would control a 110V 15A Relay. I also need to build GFI into it some how and comfirm with the Manufacturer that this will not be an issue.
I just put an electric radiant floor heating system in my bathroom a little over a week ago. I used the same thermostat that was shown in the link you provided.
I thought of intergrating it with my M1 but I didnt see the benifet. The thermostat has a temperature probe that comes with it that is low voltage and embeds into the mortar under the tile. The thermostat is more that a relay and a timer. It will actually adjust the voltage going to the mat. When it first turns on it goes full output to the mat. After it reachs the set temperature it shuts down (so far the ELK could do all of this). Once it is at temperature it will cycle on and off at various levels to maintain the set temperature. That is someting that the ELK may not be able to do. It also has a built in GFCI. Also the thermostat learns how long it will take to warm the floor to the desired temperature I think so it adusts its turn on time etc. There should be a manual online.
I would ask your customer what they would like the ELK to do that the thermostat cant. By the time you buy everything you would need to intergrate with the M1 you could just buy the thermostat and it would be a lot easier.
BTW the heated floor is great! You dont notice its heated but you do notice that its not cold. Installation was a piece of cake. It added about an hour to the entire bathroom renovation project. The contractor didnt even charge me for the time (I bought all of my materials and paid for labor only and the floor was a maybe when I signed the contract).
The way it sounds there really is no integration possible. They are wanting thermostat access from the M1 and their UI. This way when they turn on the alarm it will bring down the temp with the rest of the house and bring it up when they get home.
I have both the manual and installation guides here on my desk an I do not see anything regarding a dry contact connection. But from looking at a small picture it seems there is a terminal with positions 1 and 2 that are not used. What I could do is add a temp sensor in the room and hookup an Elk output to the contact and that should be enough.
A serially controlled wall thermostat like the HAI, RCS, or Aprilaire has an output that could be fed into a 24VAC coil of a contactor relay to control the mats. This way you will have setback temperature control using the M1 when the security system is armed.
The output of the thermostat in the link that is above is 120 Vac I beleive since its the same thermostat that I have based on the picture (its made for many differnt companies). You could have a relay with 120 Vac contacts open once the M1 is armed away and thus breaking the circuit to the mats. You could also have it drop out if there was a fire alarm condition etc. thru the rules (I will do that with my furnace, hot water heater etc some day soon).
I plan on doing that for the floor heater when a get around to it (aka "round tuit" which I constantly put as needed on the "honey do list"). I mounted my thermostat on a 4" square box with a single gang mud ring. just enough room to get sneak a decent size relay into the box.
Double-check on that NuHeat thermostat. I did a NuHeat installation a couple years ago, and it had a dry contact for setback.
They also sell a cheaper thermostat (actually a Lutron dimmer) that has no brains, i.e. the current amount only varies when the dimmer setting is changed with no relationship to the actual temperature. Using this approach with a 115V relay is another option.