combining CI rads with fintube baseboard

Eastman

What are the best practices for combing existing over radiated CI rads with fintube baseboard?

Jamie Hall

I hope this is hot water? Since they have very different heating and cooling characteristics, I'd want them on separate loops -- or at least home runs with balancing valves.

Eastman

Yes, hot water.

NY_Rob

If they're on the same zone you will find the CI rads will literally suck all the BTU's out of the CH water till they warm up to SW temp leaving very little for the fin-tube to work with. The workaround is to reverse your flow at the boiler connection (if needed) so the fin-tube gets the hot water first and the CI gets what's left.
I speak from experience on this one...

BTW- wait till you see the DT on the CI zone.... your mod-con will love that zone!!!

Eastman

What do you mean by "...reverse your flow at the boiler connection (if needed) so the fin-tube gets the hot water first and the CI gets what's left."?

Gordy

Make sure the fintube gets the supply water first then the rads.

I would zone them separately also. Add some trvs. Which may also benefit the fintube.

Is this a continuation of direct pipe for a modcon?

Eastman

No, not really.

When is it worth mixing a new temperature?

Gordy

Why mix anything? Let the ci rads have what’s left over from the fintube, and the mod/con after that.

NY_Rob

Depending on how much CI there is, you can have a prolonged period of 30-40F DT with CI and low flow rates. If you start with 120F SWT (assume mod-con boiler), you can get 80F water exiting the cast iron then going in to the fin-tube. 80F water into fin-tube gives you no heat, so you pipe the system to get the hot supply water into the fin-tube first where you only drop maybe 10-12F so now you still have fairly hot water entering the CI rads which will give you plenty of heat out of them and it will pull plenty of BTU's out of the return water for a great DT at the boiler.

If you're lucky, your flow will be going the right direction already, if not- a little re-piping at the boiler is all you need to reverse the in to out flow to that CI/fin-tube zone.

Eastman

Yeah, but the supply temperature for the rads will fluctuate depending on whether the fintube is in operation or not. Doesn't that bother you?

Gordy

Of course this is a simplistic theory based on wanting the coldest return temp to a condensing boiler.


The thought process is the same however when dealing with different temperature requirements of different types of emitters.

In the case of the fintube having lower deltas than the ci rads. It’s more predictable what water temp the fintube will see, and more useful btus to the fintube when they are seeing the first supply temp. All that mass of ci rads is a huge buffer tank. Slow to heat, and slow to cool.

So to answer your initial post more clearly. One needs to know more about the systems expectations.

Eastman

It's hypothetically hypothetical. Would you concede that a slab should have a separate mix temp from fintube? What metrics dictate this decision?

Gordy

Lol, okay well is all three emitters in the mix now? Or is it now fintube, and slab?

My old rental was slab on grade with ci baseboard. Ci boiler with three zones. One zone slab, and two zones ci baseboard. One of which had a thin tube rad at the end of the loop which heated a garage. 60’s construction. Yes the slab had a mixed temp. But then we are not talking about a mod/con trying to get the lowest return temp to increase efficiency.


Yes You really need to mix a slab supply temp to protect the panel from over temp damage, and temp overshoot.

I suppose with a mod/con, and the fintube was sized ample enough to heat the load, and the end temp was low enough to supply the slab to meet the requirements then it could be done as a single zone. However that would take near an act of god for everything to fall in place.

Eastman

Someone asked me about temperature mixing, and I thought, "I got this." But honestly, I don't know how to decide when it's actually necessary. Or when is it worth using a floating action valve versus a thermostatic? Budget? Personality?

Gordy

Any type of radiant would require a mixing valve if high temp emitters are part of the system, and a ci boiler is used. Even if a mod/con is used.

If the emitters are all radiant, and the panels all require the same supply temp,and you are using a condensing boiler then no. If you have different types of panels which require different temps you may need more than one.