I have a house that is 100% radiant floor heated, DIY, Closed Loop, with Primary and Secondary Loops, with a Metering Valve and PLC; and would not recommend anything else. It really is the most efficient way to heat a home. It is the most expensive set-up, but then again, you only buy a system once, but you pay for fuel EVERY year, so keeping that fuel cost down, makes the most sense.
The effecieny of the whole system is NOT using a hot water heater as a heating unit, and instead going with a purpose built "boiler". I put that in quotes because technically they are boilers due to the presure (12 PSI) but do not actually turn the water to steam. You get that effeciency because with a hot water heater, they lack the controls, and a person can often get short-cyling. A hot water heater is just not designed for the demands of a radiant floor heating system. They are made to provide hot water, at a given temperature, at a moments notice. A hydronic heating system demands varying degrees of hot water, constantly, so the application is drastically different. A hot water heater will work, just not nearly as well as a purpose-built boiler.
With a boiler/primary and secondary loop system coupled with an adequate PLC, the boiler has enough
controls so a tempering valve/metering valve is not needed, unless you want to add a high temperature, secondary heat source. You can add a tempering valve, but that never made sense to me; why spend money on fuel to heat up water, just to inject cold water to lower its temperature at the source? I thnk it is far better to go with a metering valve and keep your heat in the primary loop so no energy is lost by cooling
it down, but that is more of a powerful argument for a primary/secondary loop system then in using tanks in the system too. And that is also where the effeciency comes in, because now you have the PLC controlling both the boiler, and the metering valve to get the least amount of fuel consumption, and yet heat the house. That is where William;s thoughts come into play; heating water, and controlling it, are really two different things, yet MUST work in unison to get the most per BTU consumed.
How well does it work?
Our zone valves are the dumbest part of the system and never come into play. That is because the PLC works so well with the boiler, the metering valve, the sensor outside, the sensor in the floor, the sensors on the returning water coming out of the loops, that once every minute it predicts the heat loss of the building, what the actual heat loss is, and makes up for it, that our house stays within 1 degree of what the thermostat set temperature. That is pretty good; from closet to livingroom, every square foot of the house is warm with no noisy air blowing on and off, no dust blowing around, and no duct work. The only reason the zone valves are there is in case of over-temperature, but the system is smart enough so it does not do that. Even the thermostats per each zone do not really do much. They tell the PLC if there is a need for heat, and then the PLC takes over. But even there I added some effeciency by ensuring at least two zones call for heat before the system fires up. This further presents short-cycling, and really unifies the house.
The only real drawback to my system is, because the zone valves never come into play (because the system self-adjusts the water temperature circulating throuh the floor based on outside temperature), it means my heating system never shuts off. So I have (3) circulators running from October to May, which causes my electricity bill to go up by $10 per month. Considering what I am saving in fuel, I can gladly live with that.
I built this sytem 13 years ago, and it still is state of the art
In fact if the government ever gets its act together and does digitalized weather forecasting for residences and not just commercial locations, my heating system could get further effeciency because the PLC would then know what is GOING to happen, instead of reacting to outside weather conditions minute per minute as they happen. I mean if it knew in 12 hours there was going to be a warm front blowing in, it could start backing off the heat output to reduce fuel consumption even more. But the opposite would help too. If a cold front was moving in, it could ramp up heat output so that it did not go into over-ride. (I set my system up so that when it goes to 0 degrees outside temperature, it overides the outside temperatire modulation, and just pumps 100 degree water through my floor. Normally the water in my floor loops operate between 76-90 degrees). If it knew it was going to get very cold, it could run the boiler at 60% instead of 50% before the cold dnap hit so that the boiler did not have to run at 100% when the cold dnap hit. Of course
the PLC would then tell the metering valve to put that extra 10% in the floor so that when the cold snap hit, the floor had already been addionally been heated.
It all seems pretty complex, but it is actually pretty simple once you get to understand the components. It took them explaining it to me (3) times before I understood why this was a much, much more effcient system.
(Probably the readability and spelling on this reply sucks, but it is 1:37 AM and this is pretty hard to explain, though a rather simple system).