Adding a OWB to existing oil boiler

[tawilson1152]

Hi, first post and I wish I'd found this forum a few months ago.
I've got a Crown Royal owb on the way and looking for advice on piping it into my existing boiler. Attached is a sketch showing my existing and proposed setup.
The heat exchanger is a something I salvaged from a gas conversion job, 2" bungs, 50 plate, but the tag is worn off. I would like to set it up as a primary/secondary loop but as I didn't pipe my original boiler that way this is how I would like to do it without redoing all the existing. Will it work? Thanks.

 

[ewdudley]

Hi Tom. Welcome and thanks for starting right in with a schematic.

From what I've learned here, what you've drawn is called a 'figure eight' connection to an existing boiler, and it's at least one right way to do it.

The flow from the heat exchanger (HX) goes clockwise. The zone pumps direct some or all of the HX flow to the loads. If HX flow is more than what goes to the loads then the excess flows through the oil boiler (OB). If the HX flow is less than what the load pumps require then some flow will come through the OB to make up the difference.

When there is no load flow all the HX flow goes clockwise through the OB, which maintains the OB DHW coil temperature, if you have one.

The only downside is that the OB stays hot all the time, which could be a problem if there is much air flow through the boiler and up the flue when the OB is not running, which shouldn't be a problem with more modern oil guns. Also will lose some heat through the OB jacket, which always used to happen when the OB ran, but nonetheless the jacket insulation could be improved if desired.

Just need a valve, manual or aquastat controlled, to stop flow through the HX when the wood boiler is offline. Or maybe not, with those fat pipes going to the OB, whatever parallel flow there would be through the HX when the HX pump is not running might very well not amount to enough to worry about.

Don't need a crossover.

Also, how long are the 1" PEX runs from the WB to the HX and from the HX to the manifolds? Depending on your temperature drop across the loads, the lengths of the PEX, and temperature drop across the HX, there may be one or more bottlenecks in your heat flow.

--ewd

 

[tawilson1152]

Thanks for the time, Eliot. I had planned a manual bypass around the hx on the wb side, just forgot to draw it. I don't have a problem with the ob staying hot and have already looked into a motorized flue damper. I was concerned about the zone pumps and the primary loop pump working against each other.

 

[ewdudley]

I had planned a manual bypass around the hx on the wb side, just forgot to draw it.

I meant a valve on the load side of the HX. If the HX pump is not running and you're heating with the OB, the load pumps will draw some flow though the OB and some flow through the HX, even though the HX pump is not running and even if the HX pump has an IFC. But it may not be a problem because the majority of the flow will likely be through the OB because of its lower resistance, and therefore you may get plenty of heat from the OB even without a valve to prevent parallel flow through the HX.

I was concerned about the zone pumps and the primary loop pump working against each other.

Looks good to me as long as the HX pump on the OB/load side is pumping clockwise.

Cheers --ewd