Stream Boiler w tankless HW heater; can I turn off HW and leave it dry?

bwt

Hi all. I've got an old house with steam heat. Until recently, our oil-fired Weil-McLain boiler (SGO series, I think) supplied both steam heat and HW via the Weil-McLain's tankless components. A few months ago, I had the plumber install an electric HW heater w 50 gal tank to supply a new tub. This isolated the HW supply for the house and the intent was to use the boiler for steam only. The HW pipes are still connected to the boiler but shut off so the boiler's tankless component is dry. The pressure relief valve on the tankless HW is left open.

The other day I turned the boiler on for the first time (its getting cold). I noticed that a little steam came out of the open pressure valve. This got me worried -- and I may be over thinking this -- but the tankless components wouldn't normally get more than 135 deg F with HW in them. But now that the tankless is dry it will get much hotter (I assume) when the boiler is running for heat. Will the hotter temps damage the tankless or, via conduction in connected copper lines, damage the (now unused) heater control or mixing valve?

I'd appreciate any thoughts.

EBEBRATT-Ed

If you continue getting water or steam then

1. the tankless coil could be leaking
2. a valve could be leaking water back into the tankless coil

Or you could just have a little bit of residual water in the tankless coil that will steam out and dry up.

Run the boiler and see what happens. If the water dries up and you don't get any more your ok.

If not call the plumber back and have him check it out.

Jamie Hall

This ^^^ is really all that is actually needed -- but I would prefer to see the tankless system completely disconnected, but not with the lines capped. If there is a good tight shutoff valve on the cold water supply line you show, and one on the mixing valve outlet, this could be done simply by loosening the two unions you show on the lines to the mixing valve from the tankless coil, without actually disconnecting anything.

bwt

Ed, Jamie - Thank you. That helps.