LsFarm wrote: why would the water temp be 160*??
As far as thermal shock,
I just don't see a need for the complication. My system is just as I describe, but the boiler is remote, and the return water goes through a water/water heat exchanger instead of through the boiler itself. Maybe this is comparing apples and oranges, but I don't think so.
You suggested running the solid fuel at 40* higher temperature than the gas appliance, I would assume that would be the very least it would have to operate at, probably more. Maybe.
I am not talking about thermal shock, I'm talking about the rate of loss. With the single loop the mass of heated water is reduced in half. The boiler will see a faster drop in temperature due to that fact. Rermember the loop doubles the amount water available at heating temperature to service the demand.
With your single loop the gas boiler does not see heat unless a zone calls for it, a long draw of domestic water will fire the gas burner. There is no doubt in my mind that this will happen. If I spent $3-4000 to heat with coal, I would not be happy if I heard the gas burner running a couple of times a day or even in the course of a week. There is just something unnerving about that.
I don't see the additional loop as complicated. The only difference in the two designs is really one part and the flow pattern. The pump, you are already have one in the system. Do you think it would be more prone to failure? Is it the cost? Any good plumber worth his salt is going to insist on installing the checks between the two boilers in either configuration to stop stanby losses when the coal boiler is out of service. The lift checks are as reliable as the pipe they are installed in. Those parts are about $80. So that should not be any different. What is different is that there is an additional pump. That is $60, maybe $80 dollars. I'm not sure where he lives, but that is probably a 1/4 to 1/2 a ton of coal for most of us. I would think the cost of the pump will be paid back the first year in savings from two things. One, the firing rate will be easier to tune to the load and second, the gas boiler will never fire. It will also reduce the risk of overfiring the coal. I see savings there. The cost of running the pump is insignificant relative to the fuel savings and ease of operation.
There is one more point here. In the event of the zone circulator failing, the coal boiler pump should carry the load. It may have a lag, but I believe it will heat the house reasonably well. The single loop will not. Backup is worth something, yes/no?
It is automatic, no operator interaction. Works when your home sleeping or visiting grandma for two days. No valves to play with, no stack to remove, no electrical connections between the two. Plenty of reserve DHW and the gas/oil never starts unless the coal fails. I think that is pretty simple too.
All that said, send the beer to Greg. I don't drink.
I am just amazed that the guy that flies jet airplanes for a living finds my loop and pump on a heating system complicated.