Hot water boilers have a pressure reducing valve in the fill line, reducing house pressure to 6-8psi. The 30 psi relief valve is just for overpressure from heat expansion, there usually is an expansion tank on a boiler too.
A hot water tank has a Watts valve, that will blow off with either over heat, or overpressure, or both. The water that comes out is dangerously hot sometimes it's steam, that is why building code says a hot water heater PRV must have a pipe on the outlet down to within a few inches of the floor.
BCinRI, I think you will find that buying an extra tank costs more than a circulating pump. And the simplest way is to hook the loop into your existing tank with a circulating pump. A gravity or thermosiphon sounds neat, but it circulates slow, and will have a very slow recovery rate. The system will probably thermosiphon through the circulator pump anyway, so you don't have to run the pump 24/7.
Another thing to think about is, if you have a preheated tank, or tempering tank feeding your electric water heater, the water heater's stored water is not going to stay hot unless the electric element is powered.
IF you don't use any hot water for 8 hours, like at night, then the water in the electric heater will cool off, even though the tempering tank is getting really hot from the coal stove all night long. You need to keep the stored water in the electric heater hot too.
I'd hook up to the electric heater either with a circulating pump, or thermosiphon. This keeps the water hot that you will use first. If you desire to have more capacity, then an additional tempering tank can be added at any time.
I'd recommend a simple system first, and if it doesn't work well enough, then add an additional tank. What is your hot water use pattern or habit? Do you have many people taking showers in the morning, needing more capacity?
In my system, I have a hot water heat exchanger in the cold water line feeding my propane hot water tank. The heat exchanger heats the incoming water to about 150*. The propane tank's thermostat is set to keep the water at 110*. At night, just before going to bed, I will run hot water into a bathtub, leaving it there overnight. This does two things. The volume of water in the bathtub is replaced in the propane heater by 150* water, so the water will not likely drop to the 110* threshold and trigger the propane valve overnight. And the second thing is the tub of hot water helps humidify the house.
Last edited by LsFarm
on Fri Dec 08, 2006 8:37 pm, edited 1 time in total.