A short list of design themes that are guiding the work we are doing with the combination of solar thermal and radiant heat gained from many years of trial and error and a fair bit of study.

1. Absolutely stop mixing potable water and radiant floor water.

I can't really lay any incidents of Legionnaires related pneumonia on domestic - radiant loops but the odds just don't justify the costs, which turn out to be minimal. I’ve had to bleed out potable floor loops at the beginning of the fall to clear air bubbles and the color of the water that comes out after being stagnant in the pipes all summer is concerning. Sediment drops out of the well water into the loops as the concrete absorbs the heat and during the summer this sediment can grow bacteria, not what you want to have spraying on you in the shower. I’ve also had to repair two systems that had leaks in the slab floor loops, with a potable loop these leaks can cause flooding, with a closed loop they just cause pressure drop and air in the pumps.

Running the floor loop through a flat plate heat exchanger with the spin pump and using the warming pump to add BTUs to that heat exchanger isolates the floor from the house and lets us drop the pressure to about 20 psi and reduce flood risk while also keeping the minerals from building up in the floor loops. Low mass “staple up” type systems don’t have the delayed heating problem we see in slabs so don’t need the spin pump primary-secondary pumping scheme. I’m partial to flat plate heat exchangers (sit in a sauna for a while and then blow on your hand, heat transfers better with turbulence and a flat plate heat exchanger maximizes turbulence on both hot and cold sides of the heat exchange) but on staple-up type low mass radiant floors we may still use coil-in-tank heat exchangers on a single pump system.

Some the slab systems are prone to overheating problems caused by adding hot water to the slab when the indoor thermostat calls for heat and then allowing that hot water to rest in the slab when the thermostat shuts the pump off allowing that heat to rise into the home too rapidly. Some radiant installers have addressed this by using one small pump to circulate the water in the floor 24 hrs a day while a second pump adds heat when called for by the thermostat. When the warming pump shuts down the spin pump keeps the heat flowing out through the concrete so it doesn’t just stop and go straight up into the room above but spreads out and is released more slowly, the down side is that the spin pump can draw as much as 60 watts 24 hours a day burning almost a dollar a week in electricity. Spinning the water in the slab can have the benefit of evening out the temperature throughout the home when the thermostat is not calling for heat increasing the virtual area of the solar inertia of a slab in direct sunlight or in the vicinity of a wood stove. It also allows me to deliberately design a slab with “hot spots” in the mud room, master bath, under the dining room table and in the “monopoly playing zone” in front of the fireplace by sending the heat to the floor in these areas first and allowing it to circulate out to the rest of the house secondarily.

Many installers use an outdoor re-set mixing valve that blends water returning from the slab with water from the heat exchanger, and responds to the outside temperature by reducing the temperature in the water entering the slab in response to the outside temperature so that as the day warms up in the morning the inside thermostat can continue to call for heat but the outside temperature sensor will dial back the slab temp to prevent over-heating while a cold snap arriving in the evening will cause the mixing valve to react by increasing the temperature of the water entering the slab to anticipate the increased energy loss of the building envelope. This adds about $800 to the cost of the system and reduces the run time of the spin pump.

2. Stick with drain-back solar thermal systems

When we look to solar to provide supplemental heat for radiant floor we immediately come up against the fact that most panels are designed to provide a majority of hot water only and really don't have much left over for floor heating during the late fall to early spring time when we would really like to avoid using the back up heating system.

Some designers have increased the size of the storage tank thinking that they can store heat during the summer for use in the winter but have run into problems with the big (1,500 gallon+) tanks relating to durability, failure of the tank liners at high temperatures and escaping humidity associated with open-top tanks as opposed to pressurized tanks.

If we have the room for the panels, we can increase the daily heating potential quite a bit by adding panels without increasing the size and cost of the tank and the rest of the infrastructure. The problem here is when the family cannot use the BTUs generated by the panels during a summer heat wave or vacation. With an anti-freeze type solar collector this can result in a condition where the heat accumulating in the panels can boil the propylene glycol back into the expansion tank and leave a coating on the inside of the water channels inside the collector panels. Since these channels are quite small (the typical 4'x8' panel holds one gallon of heat exchange fluid) any accumulation of residue can impede efficiency of the system.

We use drain-back systems due to their ability to tolerate excessive panel area to tank size by draining the panel when the tank bottom sensor reaches 170 degrees in the summer in the same way that they drain back when the panel is cooler than the storage tank. We locate the drain back tank and heat exchanger as close to the panels as possible, it does not need to be adjacent to the storage tank and it benefits from having the least rise possible from the heat exchanger to the panels.

3. Never heat the solar tank with the radiant floor.

This system uses a tempering tank as a sort of thermal switch board to interface between the solar tank, the floor and the condensing gas water heater. We use a tempering tank and re-charge pump whenever we use a demand water heater to eliminate the “cold water sandwich” as well as to eliminate the pressure restriction of the heat exchanger inside the demand water heater and to improve the delivery rate of hot water to multiple users beyond the top flow rate of the DHW (a Rinnai can deliver 8.5 GPM with a flow restriction equal to 28' of head depending on the temperature of the in-coming water, we use a Taco 009 bronze circulator or eq, that can deliver this flow rate at this head restriction and avoid running the DHW at a low burner modulation which can lead to a sooty build-up on the interior of the DHW heat exchanger.) Bypassing the demand water heater also allows us to deliver hot water flow rates below the threshold of the heater to accommodate low flow shower heads. Avoiding the need for a extra low BTU flame modulation allows us to use a demand water heater with a higher bottom end (and lower top end ) so I can use a Quietside 120 ODW 94% efficient condensing gas water heater which sells for under $1,100 and vents with common three inch PVC pipe.

While it does convert a tankless water heater into a tank style system it gets rid of the un-insulated combustion tube found in the core of most 60% efficient gas water heaters and keeps us from using electricity to heat water, which in our area means burning coal at 30% efficiency. We adapt small electric water heaters for use as tempering tanks by removing the electric elements and short circuiting the thermostats that would have controlled those elements which converts the thermostats to switches which we use to control the pumps and gives us 1” threaded ports at convenient locations on the side of the tank.

If we simply connect solar water into the system as a pre-heat tank we will only flow water through the DHW when lack of sun or use of the hot water allows the tempering tank to drop below 120 degrees. But when we add a radiant floor system we want to flow the water from the tempering tank to the solar tank when there is heat available in that tank but divert the flow to the DHW when the solar tank is cool to avoid using the DHW to heat the solar storage tank as it's easier to heat cold water than hot.

In this design the thermostat in the bottom of the tempering tank turns on the re-charge pump and also sends power to the thermostat in the top of the solar tank which sends power to a 120 volt motorized three-way valve so that if solar water over 130 degrees is available the switch is open (element off) and the water flows from the tempering tank through the solar tank to re-heat the tempering tank. When the solar tank is below 130 the switch closes (trying to turn the element on) which energizes the three way switch and the reheat loop is diverted through the demand water heater. All cold water enters the system in the solar tank so any domestic water use advances hot water from the top of the solar tank to the tempering tank which helps keep the tempering tank warm to avoid use of the re-charge pump.

4. Minimize the use of low voltage controllers

We need to use low-voltage differential temperature controllers to provide the differential temperature controller “brains” for drain-back and propylene glycol systems but they seem to be vulnerable to power surges so the fewer of this type of controllers we can have in the system the more rugged they'll be in the long run.

Use molded end line sets, cut ends of grounded extension cords, to connect pumps and switches so swapping out pumps and tanks won’t require a visit from the electrician.

5. Design around easily available electric water heaters

Units with 1” threaded heating elements give you extra ports for pumps and hot water outlets. The price is usually right for 80 gallon tanks and the sources of supply are generally just around the corner. If you need more storage you can just add more tanks in series.

Coil-in-tank heat exchangers are generally not as efficient, flexible or economical as flat plate heat exchangers. Marathon high performance tanks and rubber tub-style open top tanks are readily available but there is much to be said for grabbing the biggest tank they have in stock locally and working with that.

6. Take control of pipe turbulence

Pipes are like rivers. When the water flows around an elbow there are eddies. An elbow at the top of the hot water tank starts the flow off with turbulence in it which mixes hot and cold together so you get warm water at the faucet long before you get hot. Gary Klein has done a lot of the research on residential hot water distribution. He advises that we eliminate all elbows and bull head tees in hot water lines to minimize turbulence and speed hot water delivery to the faucet.

However, when water re-enters a tank from a heat exchange loop an elbow close to the tank will add turbulence to help gentle the stirring effect of the water flow entering the tank. In the illustration we are returning the water from the demand water heater to the tempering tank through a 1” pipe nipple screwed into the tank where the electric element has been removed. This would have a 1” brass elbow on it with a 1”x ¾” bushing and a ¾” pex adapter. The water leaves the ¾” pipe with momentum but the combination of the elbow with the larger pipe diameter gives it a very turbulent and gentle flow into the top of the tank. The problem we were having was that a sudden input of water into the tank could “roll the tank” causing the hot water at the top to roll down to the bottom and bring cold water to the top where it would cool the water leaving the tank towards the owners shower. This was bad.

7. Eliminate check valves

They just seem to get jammed at half-open position and can be devilish to diagnose, even the expensive 300 psi brass gate checks with the stainless hinge pins seem to be prone to this.

8. Look out for thermo-siphon prone lay-outs

Locate the radiant floor heat exchanger below the tempering tank to keep it from thermo-siphoning heat out of the tank when the heat is off. Use a heat trap at the out-flow to the house.

9. Look out for un-intended pressure differentials

When one pipe with a pump on it is connected with a tee to another pipe with a pump on it you run the risk of creating a relative point of high pressure at that tee that could drive water through the idle pump creating flow in a place where flow is not desired. Break pump loops apart rather than rely on check valves to control this. Check valves fail at precisely this sort of low flow situation.

10. Dump the “boards”

It's important to let your systems express what they are doing by the way the pipe layouts are visually clear to a future service tech as to which part controls what process.

Mount the pumps and valves on threaded pipe directly on the tanks and heat exchangers.
Mounting all your components on a piece of plywood and then piping it across the room to the tanks and pumps just makes things more confusing and makes future service more complex. Don't layer the pipes up into a three dimensional matrix either (boy have I been guilty of this over the years).

11. Mount pumps in a vertical flow orientation

Bubbles are the death of pumps. Bubbles want to go up. Let that happen.

12. Locate air elimination devices at the point of lowest relative pressure

Bubbles drop out of closed systems between the restriction of the heat exchanger or floor manifold and the intake of the pump which is the place to locate the bleeder valve and the purge valve.

13. Use counter-spiral radiant pipe lay-out

Starting with a doubled pipe in the middle of the floor and then spiraling out towards the manifold will get the flow to the center of the room quickly and return back so that every other pipe is running in the opposite direction and is inversely distant from the heat source. Starting at one side of the room and running back and forth to the other side means that one side is closer to the heat source than the other.

14. Use equal length loops within any radiant manifold

Pipe runs have resistance proportional to their length. If all the runs on a manifold are the same length they will all have similar resistance and share the flow equally. If one of the pipes is shorter than the rest it will have less resistance and get more flow. Break 500 foot rolls in halves or thirds to make equal loops.

15. Break out pipe subassemblies for service

Let’s face it, it's a whole lot easier to chase leaks if thoughtfully placed isolation flanges and unions with ball valves allow major components to be serviced or replaced without draining and dismantling the whole system.

16. Lay off the copper

Except where the pipes are likely to get hotter than 130 degrees.
If PEX will get the job done then go ahead and use it. Threaded brass fittings are expensive but they can support the weight of pumps and valves and earn their keep.

17. Don't pan individual components, pan the whole area

Round commercial drain pans do meet code, but what is the point of “meeting code” when you have water on the floor. We make pans out of EPDM roofing or PVC shower membranes and just line the entire area with a two inch drain to daylight with no trap.

These and other details at http://www.chandlerdesignbuild.com/index.php?id=construction&t=Green%20Building%20Construction%20Details