Wood boiler HX piping.

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New Member
Nov 4, 2023
Upper Michigan

Hi all, needing assistance with my wood boiler’s HX piping I have received alot of help thus far from other resources (thank you Hot-rod and ground up) however not out of the woods yet, attached is drawing of the most recent configuration, this is the 3rd attempt here…. On this one it was explained to put the HX CST just before he combis main return CST. I’m getting some flow or more flow than previous configurations however the issue I’m having is there 150 degrees coming into the HX from outside and 150 degrees on supply side of the HX which is awesome and all however, I’m not seeing anything better than 110-115 degrees at the floor pex manifold on the other side of the combi.

So others have said don’t shoot directly into the boiler as I was before, but rather into the main loops return which is what I have now, unless I’m miss understanding this concept?

There are three pumps in this configuration. One coming from the HX to the boiler loop, one internal on the combi and one moving from combi to pex manifold.

It seems as though I’m losing a mass amount of heat in the 3 or so feet of copper loop and or inside the combi, I’ll have 150 degree water on the stand by but once the call for heat comes and the pumps run I’ll only see 110-15 at best going into the floor but I still have 150 at the plate and that’s at best. The only thing I can think of at this point is adding more air Seperation in the HX loop going to the boiler loop? Or which I really hate to another complete re pipe… thank you for any advice with this.
Are the floor loops in concrete ? If they are, 110 - 115 is about max I'd want going in there. I doubt you'd want much more than that for wood floors too. Temperatures higher than that will cause the concrete to start spalling.

A picture of your current setup is worth a thousand words. The picture you posted above is a little hard to follow.
Hi. The floor loops are half inch pex retro stapled up under hardwood with fin plates and insulated. I agree on temps for the floor so it’s not a 911 emergency here however, I’m still remodeling the house and the system is to provide 180 to the finnished basment radiators then mixed down to 120 for the floors on the main level. With the way it’s performing there’s no way to get that 180 down stairs as it takes everything it has to get the floor loops up to the 115-120 mark.
I’ll try to take some actual pics tonight however it’s gonna be hard to make out as this is alot of copper crammed into a small space.

I have confirmed that water temp coming into and out of the plate is at the same temp which is 150, but by the time it gets over to the floor manifold I’ve lost 30 degrees?
I can't make sense of the diagram. But if at all possible I would try to replace the radiation that requires 180 with something that requires a lot less. That is the number one critical fault with distribution - needing high supply temps.
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I think I'm starting to wrap my head around it. A question on the control if there was a call for heat and no fire I assume the circulator turns on and the combi maintains temp. If there's a call for heat and there is a fire is it the same scenario as above but the boiler heat exchanger circuit acts as a preheat for the combi?

It would help to show the heat exchanger circulator as well as any check valves and arrows to show the direction of flow in the heat exchanger piping.

From the sounds of it your main loop should be 150° up to the mixing valve for the in floor. And the return from the mixing valve should be less than 120° and if the combi is boosting it to 180° then you should have 150° to the return of the combi, then the combi outputting 180 ° out to the mixing valve.