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Discussion Starter · #1 ·
I am trying to understand the principle behind setting up primary-secondary boiler loop with short common pipe for instant, condensing boiler.

I somewhat get the point about hydralic isolation and pressure drop. In the modern condensing boiler, the primary loop will ensure water return temperature is high, nearly equal to supply. Isn’t this the opposite of what condensing boiler wants?

2nd point, if we want short common pipe, why not use some kind of 4-way cross, factory manufactured copper connector. two lines will connect primary loop and two lines for secondary. the shared section is nearly the diameter of the pipe.

Thanks
Gene
 

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The primary/boiler loop keeps required water flow through the boiler. It doesn't maintain water temp at the boiler.

Manufacturers don't make a 4 way cross since they don't know if you will have room for it in the configuration they would make it.
 

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If you have only a single zone, then it doesn't matter if you have a primary loop or not. When you have multiple zones, it ensures the boiler has the same constant water flow that it requires, no more, no less, regardless of what the zones are doing. (both can cause damage to the boiler.)
 

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Discussion Starter · #4 ·
I have a setup with domestic hot water and radiant heat floors; three zones one for each room in the apt. There is a single floor pump and 3 actuators.

I can’t speak for all boilers but Navien combi boiler model has separate supply for DHW (shower, sinks) and another supply/return for radiant heat. Isn’t this equivalent to having secondary loop built right into the boiler?
 

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Discussion Starter · #5 ·
The primary/boiler loop keeps required water flow through the boiler. It doesn't maintain water temp at the boiler.

Manufacturers don't make a 4 way cross since they don't know if you will have room for it in the configuration they would make it.
I see. I read that for condensing water boiler, return temperature should be much lower than 140F. although it is not for flow, primary loop seems to be short circuiting the boiler supply and return and indeed raising the temperature. sorry for my confusion.. just trying to understand the design.
 

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I see. I read that for condensing water boiler, return temperature should be much lower than 140F. although it is not for flow, primary loop seems to be short circuiting the boiler supply and return and indeed raising the temperature. sorry for my confusion.. just trying to understand the design.
No. If there's proper flow through the whole loop (usually more than what the boiler wants) then it won't be short cycling at all. The supply from the boiler is mixed with the excess flow through the rest of the loop.
 

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The primary/boiler loop keeps required water flow through the boiler. It doesn't maintain water temp at the boiler.

Manufacturers don't make a 4 way cross since they don't know if you will have room for it in the configuration they would make it.
Can't you use mixing valves with primary-secondary loop setup to keep the return water temp up on non-condensing boilers yet keep the circulating temp lower?
 

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I thought that's used a lot in commercial buildings with non-condensing boilers to cut heat output in mild weather but protect the boiler.
 

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It will reduce heat output, but the boiler temp still rises. Has to over come the cold return water.
 

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I thought that's used a lot in commercial buildings with non-condensing boilers to cut heat output in mild weather but protect the boiler.
We use them but they are very expensive so barely used in other companies unless the engineer specifically demands it.

Inlet of the boiler is no less than 140°f while the loop can be lower. (I set it as low as 95°f for fan coils and 110°f for rads, at 60°f outdoor temp)
 
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