Pacowy wrote:We used a DHW coil in an EFM 900 and it was awesome. In your case, I agree you're not going to get 6 gpm out of it in real time. The actual number depends on the inlet water temp and the rise needed. Using typical values you probably could get a max of 4.0-4.5 gpm; more than a modern showerhead and less than many older ones. You also start with the btu's in 42 gallons of hot boiler water; between that inventory and the real-time capability it seems like a good producer for most normal DHW needs.
To prevent the heating load from giving you a cold shower (and stressing the boiler), my limited understanding of controls would be that it is important that the heating circulator shuts off when the boiler temp goes below the low limit setting. To ensure satisfactory DHW under those circumstances, you might need to adjust the low limit. For us 160 was a good setting but "your results may vary".
Agree that a tempering valve of some type is essential.
I think I may plumb it like hotblast did, going from the boiler to the electric hot water heater that way I will never have any issue? I will put a boiler bypass in though just incase.