Hydronic separator

Snowmelt

Getting back to the cast iron boiler. I have two small zones maybe 8,000 btu total of radiant floor. Bathroom & kitchen, I have a coleffi hydronic separator, if I use that with the cast iron boiler would that help mix the low water temperature going back to the boiler.

GroundUp

Yes. But it's a hydraulic separator

hot_rod

Are you trying to assure boiler return temperature?

One of three conditions is present in a hydraulic separator, low loss header or P?S piping.

When the circulators are running.

IF both the boiler loop and heating loop are flowing the same gpm, temperature will be the same going to the loads. return will be based on delta T the system is operating under.

If the heating loop is flowing more gpm than the boiler loop, the mixing will occur at the supply to the loads. So boiler output temperature will be blended down to the loads.

If the boiler loop is flowing more gpm than the heating loop, a mixed temperature will be at the return to the boiler. So boiler return will be higher than system return.

If you know flow rates and temperatures you can define those temperatures.

Same thing happens with closely spaced tees in a P/S piping.

To be absolutely assured the boiler has adequate return temperature you need a device that can react to temperature, like a 3 way mix valve.

If the boiler side is always flowing more gpm than the heating loop you will have a blended return, insert your numbers to be sure it will be adequate.

When dumping radiant return temperature to a non condensing boiler a lot depends on the mass of the radiant. A concrete slab (high mass) could keep the boiler in cold conditions for long periods. A small dry radiant would probably not be an issue with an over sized boiler. Only one way to know for sure.

Snowmelt

Yes hot rod I’m trying to make sure no condensation go back to boiler, I think I’m going to try closely spaced tees and see wha happens .

hot_rod

If the loads are small like that and the mass is low, this is what will be going on. Exactly the same regardless if you use a sep or build your own closely spaced tees. It's the "universal" mixed temperature formula that applies in either case.

This example shows the result when the boiler loop is flowing more gpm than the distribution loop.

Insert your actual numbers to predict what happens at the boiler return.

Tinman

A motorized 4 way mixing valve would be my suggestion for protecting against flue gas condensation on a cast iron boiler.

hot_rod

An opinion from this 2001 article by Siggy regarding low temperature zones on a cast boiler. The rule of thumb he indicated is IF the load is 20% or less of the boiler output.

You mentioned an 8,000 but/hr low temperature load, what size is the boiler?

https://www.pmmag.com/articles/84495-a-little-floor-warming-please-john-siegenthaler

JohnNY

If your goal is to protect the boiler from low temperatures, you'd be better off installing an old-school bypass before your secondary connections on your primary loop. Separators and closely-spaced tees are not reliable as return water temp stabilizers, as Hot Rod explained.
A mixing valve is best.

Tinman

https://www.pmengineer.com/articles/87534-injection-versus-4-way-valves

And another point of view from Siggy. There’s options here but I agree a separator wouldn’t be my first choice.

hot_rod

So many correct ways to pipe a boiler. Most important is knowing why you choose the method. Every job has it's own set of requirements specific to the building.

On the water side, flow, air and dirt, elimination, and return temperature protection top the list for "must haves"