I should have included more in the title of the posting, since it really concerns water heaters.

What brand is the boiler? Hot water coils can be made for any boiler. Tunneson is one company that will make an exact replacement coil for your boiler. You need to supply the old coil plate and the coil, so they can reproduce it.

"Could I install 2 TPRs instead of one to obtain redundancy?" You could. One installed near the stove coil, the other on the tank. Note that T&PR valves have a plastic stem for a thermal sensor which should be in the hottest water area. You may need to configure some fittings to attain that in your piping if the stove coil doesn't have a port. My own setup has a tempering tank prior to the water heater, which has a 75PSI relief. No check valves in the system. "Are there better quality TPRs around?" I'm assuming the valve meets governing code req's, and is likely a Watts. They make a reliable valve, but if you have acidic or mineral laden water, best to verify operation periodically or install new. "Is it possible to obtain a TPR with a regulation on the temperature to have it open at a lower temperature?" Industrially, yes, but I don't know of an easy homeowner solution. Modifying the valve requires knowledge and a test bench and absolutely waives insurance. If your system develops sufficient temperature to pop the relief, you can use a circulator pump, thermostatically controlled, to bring the temperature down by mixing water strata in the system or by infusing colder water from a tank or the cold side (supply). I avoided electrically controlled or operated components as we consistently lose power at the coldest times of year. My design is such that constant firing can't exceed the pop off point (unless there is no water usage, period.-but then there would be no inhabitants and no need of heat). The mixing valve is a very good idea. A solution to these safety concerns would be to make the system atmospheric. Re-pressurise at the reservoir outlet. Sure, you can visually monitor the temps, but then you are tied to the area (which may not be a problem). "Having a potentially higher water temperature than with an electrically heated tank I will also end up with higher pressure in my system. How can I account for this difference in the design of the system? I suppose with an expansion tank." Your existing bladder tank will continue to do the same thing as long as there are no check valves between it and your distribution. Although high pressure cycling isn't a desirable thing, safety ratings for components are (generally!) in excess of 100PSI. The danger of water becoming steam is of prime importance.

So, that's a good thought, but the "whole house filter" is just a particle filter, no charcoal in it. Also then should have then been in the cold inlet as well. We personally installed the bathroom, the plumbing within the bathroom is all new copper pipe from our reno 5 yrs ago. There was never any house filter until I installed it at the well entry to prevent junk clogging up the new fixtures!! When we opened up the fixture, the hot orifice was filled with it- so probably a full 1/2 measuring teaspoon (that was quite a lot from that tiny place). Plucked some out but then had to tap out the rest from the mixing channel. The pic is just a few of the bits that I scraped up from the sink after cleaning it. This post was edited by pepperidge_farm on Sun, Oct 5, 14 at 20:47