control recommendations

Tekmar makes a wide range of controls. They are not compatible with Nest and have their own sensors. There are generally 3 types of settings needed....low temp, high temp and DHW. The low temp is usually coupled with a 3 way mixing valve and motor actuator when a high temp (radiators) zone is present. The controls can be purchased to work with pumps or zone valves. They are not inexpensive and the "near-boiler" piping must be done correctly, per their schematics. Some boilers have their own integral control that comes with the boiler and does all of the above (Viessmann)

I called WM tech support about a year ago and they said that Opentherm was not enabled on the boilers that had the Openterm terminals, so you may want to call them and confirm that. Opentherm is still and across the pond thing as far as I know. If they have enabled Opentherm, there are transducers out there that may be able do the conversion from 0-10v.

Since the Aquabalance has outdoor reset, you may be better off with a buffer tank and a simple 4 zone relay and maybe a mixing control/valve for you lower temp zones.

On the Nest -- as has been said before, at least once or twice -- it is a nice widget for forced air heat, which is what most people have, and it heavily advertised. However... it is not designed for or meant for use with hot water or steam systems, which lose efficiency with large setbacks, and it certainly was never intended for use with radiant floors, which can't have setbacks lasting less than several days.

Therefore there is no reason for Tekmar -- or anyone else making equipment for hot water or steam or radiant heating -- to consider it in the design of their equipment. And no reason for anyone with such a heating system to install a Nest to control it.

Sorry to be a bit harsh on the Nest.

On the "waste" heat: if your radiant floor system is properly designed and controlled, the secondary pump will operate at the correct speed to provide 120 degree water to it, or whatever else the floor temperature controller requires. The boiler, in the meantime, will supply -- either modulating or turning on and off -- enough heat to satisfy the circuit. It's all a matter of selecting and setting up and using the correct controls for the system...

rhl said:

Jamie, i'm not an expert, but this latter statements don't seem reasonable. Why have higher temperature water in the primary loop than is needed in the secondary loops? That just seems inefficient.

It's a matter of mixtures. The secondary loop (if the system if piped properly) takes the hotter water from the primary loop and mixes in a proportion of the return from the secondary to achieve the desired feed temperature for the secondary. Depending on the heat output of the boiler, this may or may not take all the flow from the boiler; if it doesn't take all the flow then the boiler may cycle on and off (or modulate combustion) to maintain hot enough water coming into the system to give the desired result.

There is an amazing variety of control strategies to accomplish all of this!

It's worth remembering, too, that temperature and heat are not the same thing at all. The heat input to your slab, for instance, is determined by the temperature drop between the feed and the return and the flow rate. The heat output from the slab into your conditioned space is determined by the temperature of the slab and the area of the slab. One of the keys to radiant design (there are several!) is that one wants to keep the slab temperature uniform, or reasonably so, across its area -- and the easiest way to do that is with a relatively low input temperature and a moderately high flow rate.