Unvented Hot water loop from woodburner in a boat

peoplenotrobots! · Monday at 5:50 PM

Hi All,

Firstly, as I'm sure you'll notice, I'm not a plumber, I'm an electrical engineer. I am fully aware that there is a lot I don't know, and I want to make absolutely sure my design is correct before I continue.

I am refitting a narrowboat and will have a multi source heating system using a backboiler on a log burner, and a hydronic diesel heater.
There will be a two coil hot water tank for the domestic hot water and the coils will be connected to the log burner, and the central heating loop.

I have made a very rough draft diagram of my plans. This is just a quick sketch and can be changed. I have left out many things such as pumps and non return valves, I just wanted to get a rough idea of system layout.

My biggest concern is how to avoid overheating the wood burner water. In the sketch I have an expansion tank and a PRV, but I don't think this will work very well as the water may end up super hot and all vent out as steam, leaving empty pipes which will continue to overheat.

The norm seems to be a vented setup, which I am open to, but I am still thinking that this could lead to total water loss in the loop as it all turns to steam and vents.

I will be adding thermostat controlled pumps if this makes a difference?

Here is my draft plan. I have another slightly different one which configures the central heating and diesel heater slightly differently, but that part isn't relevant to this post. I'm also aware that the manifold won't be suitable for UFH, but that's also not relevant at the moment.

Thank you in advance!

Rtplumb · Monday at 6:25 PM

peoplenotrobots! said:
Hi All,

Firstly, as I'm sure you'll notice, I'm not a plumber, I'm an electrical engineer. I am fully aware that there is a lot I don't know, and I want to make absolutely sure my design is correct before I continue.

I am refitting a narrowboat and will have a multi source heating system using a backboiler on a log burner, and a hydronic diesel heater.
There will be a two coil hot water tank for the domestic hot water and the coils will be connected to the log burner, and the central heating loop.

I have made a very rough draft diagram of my plans. This is just a quick sketch and can be changed. I have left out many things such as pumps and non return valves, I just wanted to get a rough idea of system layout.

My biggest concern is how to avoid overheating the wood burner water. In the sketch I have an expansion tank and a PRV, but I don't think this will work very well as the water may end up super hot and all vent out as steam, leaving empty pipes which will continue to overheat.

The norm seems to be a vented setup, which I am open to, but I am still thinking that this could lead to total water loss in the loop as it all turns to steam and vents.

I will be adding thermostat controlled pumps if this makes a difference?

Here is my draft plan. I have another slightly different one which configures the central heating and diesel heater slightly differently, but that part isn't relevant to this post. I'm also aware that the manifold won't be suitable for UFH, but that's also not relevant at the moment.

Thank you in advance!

View attachment 96520

Personally I would do away with the log burner doubling up as a water heater and just use the diesel heater to do everything (hot water and heating). Use the log burner to keep the area it's in warm.

Put water movement sensors on the hot and cold taps supply and feed the cable back to the main tank pump. That way the pump will run when the taps are opened.

Zone valve the heating and hot water feed from the diesel heater to the coil (three port valve) and use a cylinder stat and room stat to control the heating and hot water separately.

The expansion vessel and PRV can be taken from the the hot tap feed to the top of the cylinder, also fit a Temp/PR valve to the cylinder and join up both PRVs on the same discharge pipe. You might want to fit a combi valve to keep everything at 3 bar (on the hot water side). Put the cold feed from the tank into the combi valve not the bottom of the cylinder. Then feed from the combi valve to the bottom of the cylinder.

You'll also need a cold feed to the heating side.

I'm sure I haven't covered everything but it's a start.

peoplenotrobots! · Monday at 6:29 PM

Rtplumb said:
Personally I would do away with the log burner doubling up as a water heater and just use the diesel heater to do everything (hot water and heating). Use the log burner to keep the area it's in warm.

That would be a lot easier, to be honest, but in the winter the fire will be on most of the time and it would be nice to have hot water readily available for free