Hello Frank, I haven't heard of that system on an open fireplace before, only on a solid fuel stove (which I have). I can't imagine you'd get much heat transfer to the hot water with an open fire as most of the heat would go up the chimney. I advise to install a stove, as you can restrict the flu opening with a butterfly plate, and therefore increase the heat transfer to the hot water jacket on the rear of the stove. You can also adjust the air flow through the stove to alter your burn time / heat output. As far as the pump and radiators go, that really depends on how many radiators you're trying to heat, and the temperature settings on your radiators. If you have a lot of radiators (I have 16) it'll take a long time to heat those furthest away in the heating circuit. You'd need to be burning hot (coal for instance) for a long time before you feel the effects. I have my pump (for feeding the radiators) set to switch on when the water in the pipe reaches a certain temperature. Until that temperature is reached the stove sends hot water up to the tank for the taps. Hopefully something I've said there is useful to you.

Where in Ireland are you

The back of beyond 😂

Not the primary from the stove.... but check out NRG awareness. Then look at their NRG link.... using a plate heat exchanger you can go from open to sealed. But primary from stove is on open.