I have been looking at various combinations using some of the componets that various ones have mentioned. First of all it seems like I need a water heater of some sort that puts out more heat than does a typical 10 gallon tank type water heater. I thought for a bit that using a marine electric water heater with a heat exchange coil built into it would be a good way to isolate potable water from heating fluid. I say fluid because the heating part of the system could be some form of coolant if it was isolated from the drinking water. I thought about using just one heating pump with two solenoid valves that would select what part of the system got water flow when the pump and tankless water heater were running. One valve would let the heating fluid flow through the heating loops and the other would let the heating fluid flow through the heat exchanger in the electric water heater. I thought that I could tap into the heat sensor of the electric water heater for a signal to tell the system when to pump water to that part of the system. In investigating the various configurations I found out some interesting things.
From what I can tell solenoid valves that can handle hot water run close to $200 each. See the following:
Asco-Valve | 2 Way Valve | 8221 Series Slow Closing Solenoid Valves
Adding two such valves is way more expensive than just adding a second hot water pump to pump fluid to the hot water tank.
While the marine electric hot water heaters do not cost much more than a flat plate heat exchanger would there still is the space and weight issue of having a tank full of hot water - as well as the expense. Also it seems to me that having to maintain a tank full of hot water somehow defeats the intent of having a tank-less on-demand water heater to begin with.
I don't really like the idea of having fans involved in my heating system both for noise and electrical consumption reasons. I would rather supplement the heating loops in the floor with radiant panels mounted somewhere on the walls or ceiling. I am leaning in the direction of figuring out how to make something that fits the need my self rather than buying something. I might use some sort of radiant heating fins attached to PEX tubing for this. Perhaps this type of fin could be made to look attractive enough in an exposed location.
Ultra-Fin Radiant Floor Heating System
Maybe there is some type of radiator that I can scavenge from some other type of system. I also happen to have some flexible copper tubing left over from an old project that I might be able to use. Any thoughts on how to build an inexpensive radiant surface for mounting on the the wall or ceiling would be appreciated.
I seem to be gravitating back toward my original idea of having an open loop system where the domestic hot water is taped off of the same plumbing that is used for heating. The system is very simple and the number of components is reduced. If I use an outdoor tank-less water heater that has freeze protection and consider letting my heating system run at least some while on the road when it is cold I should be able to avoid needing coolant in my system. The unanswered questions at this point are:
1.) Which tankless LP water heater to use? This issue also involves figuring out a way to get around the potential problem of having the water heater shut down if the input water temperature is too high. The front runner right now is the following unit:
Eccotemp L10 high capacity battery and propane powered tankless water heater
I have been told though that it might shut off if the input water temp reaches about 105 degrees.
2.) Where to find a suitable mixing valve to temper the heat of the output for the domestic hot water part. That is an item that I have not yet found and priced.
3.) Identify an appropriate type of thermostat and pump controller for running the system. I might need to have some sort of temperature sensors on the radiant surfaces - at least the floor. That is unless the wall mounted radiant surfaces can run at the same temp as the floor. What I am concerned about is keeping the floor from getting too hot.
4.) Determining what type of pressure and temperature gauges to use in the system. I would love to find some sort of monitoring panel to which I could attach electronic sensors such that I could centralize control and monitoring of the system. I need to find something that is cost effective though.
5.) Find a specific expansion tank to put in the system.
Any further input would be appreciated. Once I get all the details sorted out I want to start another thread to detail the progress of the installation and testing.
By the way the hot water pump that I ordered just arrived today and it is a nice little unit. One very interesting feature is that it can run on voltages from 8 to 24 I think. Also it has a flow rate adjuster. The brush less motor has an estimated lifespan in excess of 50,000 hours at 12 volts. The company that I bought it from was very helpful on the phone and shipping was very quick too. Take a look:
East Coast Solar :: Laing D5 VARIO Bronze PV-Direct Circulating Pump $169.90