This is the spot.. Above the big fire-box. Front-to-back & between the weld nubs.
It does have pretty good insulation. Testing with the 250w pad (using 120VAC)
showed some
heat gain. So it was over-coming the losses.
I plan to do a lot more insulation on the pipes coming out the back,
I just have to make really sure that it has zero water leaks first..
If I can get near 500w into the water, I'm sure the positive gain will be excellent. (fingers crossed)
If they aren't, I'll be doing some super-insulation on the whole system..
I' know there's a sizable loss from the floor contact, since I can measure
floor-heat from the boiler. It falls off as you move away from the boiler area.
I plan to inject insulating foam underneath, to keep the heat off the pad.
In the summer, the hot water mostly stays near the top. And, there is almost no circulation inside the water jacket. (Since the pumps stay off).
If the results are unsatisfactory (after final insulation), I'll start thinking
about just getting a small tank, that I can heat without incurring a lot of loss,
and shut down the Tarm all summer..