Don't do it. The thermostat you are currently using is NOT for hydronic floor heat. Hydronic floor heat thermostats should calculate the thermal mass of the floor, and cycle the pumps on 3-4 times an hour for a couple of minutes at a time to maintain consistent temperature in the room. If you use a thermostat that doesn't do this, you have a situation where the thermostat might be set for 70, but the temp will drop to 68 in the room before it turns on, and when it gets to 70 it will shut off, but continue to radiate heat until the room is 72 or higher. Then it will drop back to 68 and start all over.
You end up with huge temperature swings, and it's incredibly wasteful. You probably want this thermostat:
http://energyonlinestore.com/tstatradiantb...4v2a-p-146.html
It doesn't say in the description that it does thermal mass calcs, but it does, I have two of them right now. It does it *without* a floor temp sensor. They have other ones that have floor temp sensors built in also. This thermostat will integrate with your ELK to be able to switch between day/night presets, it just has a set of contacts on the back that you hook to an ELK relay.
If you're looking to replace your pump controller/relays, then I posted something on the Home Automation forum here a couple of weeks ago. Search for "120v" and look for it in the title of the thread, trying to watch the kids right now and don't have a ton of time to find it.
Bottom line, don't use the ELK to do full-control of the hydronic floor heat. And, you should replace your current thermostat as it's costing you money and giving you much less consistent temps than you should be getting.