coalkirk wrote:You are correct as far as you went but you're leaving water temperature out of the equation. If he's got 4 zones going to town, there needs to be a sufficient volume of hot water coming from the coal boiler via another circulator or his gas boiler water temp will drop to the point where it's not recovering fast enough. A 3/4" line from his coal boiler will not keep with demand. I'm not an engineer and don't even play one on TV so if I'm wrong or missing something here, I know someone will let me know.

First, what size reducing bushing is in his PRV if 3/4" is such a crisis?

One zone or four, the flow rate remains the same. The gas boiler won't lose any heat, the hottest water from the coal boiler goes there first, then to the zone where it loses its heat. If the pipe is 1 1/4" or 3/4" the pump will still move the same 10 gallons of water in a minute. The only change would be the velocity of the water through the pipe, NOT THE BOILER, THE PIPE. The large pipe, the water would move slower. The smaller pipe, the water would move a little faster. It is still 10 GPM leaving the coal boiler. 10 GPM entering the gas boiler. And when a zone calls for heat it is moving at 10 GPM. Exactly what he has now.

The pump he has, whatever its rating is currently heating his house with a gas boiler the size of two vegetable crates. I'll assume it has about 20 gallons of capacity. I don't care if the pipe is 3/4" or 6", the water will only flow at the pumps flow rating. Now he adds a coal boiler with say 25 gallons of capacity. The pump on the coal boiler should be the same size as his circulator. When up to temp, the coal circulator starts......... and runs at his current flow rate in a loop. On a call for heat, his zone circulator starts and draws water from the top of the boiler to the zone. The coal boiler continues to pump heat into the gas boiler. Instead of returning to the coal boiler, now it goes to the zone. The cold return runs into the coal boiler. The rate of flow in the system remains exactly the same as it did, whatever it is. Since the volume of heated water is almost double, the shock from the cold return is reduced considerably.

If the VF3000 can't heat at the flow rates his gas boiler currently runs at, I would think he spent a fortune for nothing.

You are correct about pipe sizes as an engineering practice. However engineers will always use the theoretical scenarios. Steam coils in pairs always have a separate trap because its practice. They work fine with one. The additional cost is carried by the consumer, the profit for the additional parts and labor goes into profit. Still works the same, just costs more money.