Loop VS Reverse- Return

JG_2

I have an finished attic with heatloss of 19K.
I have and Old American orignal wood now Oil, Steam Boiler with an Howtwater Coil with 3/4 tappins. This coil is used as a single loop for the basement heating. My desing cosist of using pex and running to the attic using reverse-return to the 4 small rooms. And controlling each basebord with Thermostatic valves and constant circulation. Beside Cost and insulation will I have any problems that I should watch for using reverse-return instead of a single loop.
Thanks for you replies

Brad White

Reverse Return

is a nice way to go if you can. I have that in my own house in fact.

In theory, it self-balances. This is because any circuit through any emitter will have the same pressure drop, same length of path more or less. Some runouts may have more elbows so there goes that theory, but not by much.

Where reverse return falls short in this regard is if the emitters (coils, radiators, whatever they are) do not have the same pressure drops, you do not get true self-balancing.

But you know what?

Who cares! The differences are academic so long as the emitters are at least similar. In other words, don't put a 14 row serpentine coil in a set with large tube cast iron radiators, that would be kind of dumb anyway.

I think a more accurate description of the benefit would be, "reverse return will achieve the same potential differential pressure across the mains at any given emitter".

If you have TRV's on all of them, you cannot ask for better "at the room" control.

If you use single-loop on the other hand (series loop?) you will lose individual control at each radiator. If you have a monoflow or diverter tee system, that small pipe size has a limitation as to how many tees you can install in any circuit. As with series loop, there is a "decay" of temperature along the path, supply to return. Your last radiators should have some extra surface to compensate for lower water temperatures.

I would go reverse-return and get the same HW temperature to all radiators at about the same time.

JG_3

Loop Vs Reverse-Return

Thank you, Mr. White, I allways enjoy your responses. You are a great Teacher.
On a reverse-return, is there a rule on how far you could branch off to an emitter (baseboard) before pressure and Delta drop becomes an issue?

How you ever written a book or are you planning to?

Brad White_2

You are way too kind, JG...

Thanks for the encouragement! I do teach and in fact today am off to conduct a seminar at my local (Boston) Building Materials Cooperative. No books planned, the good ones have already been written by Messrs Holohan, Siegenthaler and Silberstein, et. al. But thank you for the kind words.


To your question, if properly calculated, your longest set of run-outs (branches to a radiator off a main), would be factored in and would define your total pump head.


By "longest set of run-outs", I mean the total resistance of valves, fittings/elbows and pipe length, not just pipe length itself. (Ten feet with ten elbows has more resistance than forty feet with no elbows, by illustration.)

If one of those is particularly egregious, (say 10 feet of head loss versus 10 feet on the rest of the system) we would have to have a more serious discussion! Going up a pipe size would certainly help at the same flow rate.

In actual practice, most run-outs work out very similarly, not enough difference to make a difference in flow.

Besides, flow is pretty forgiving. If you cut flow by 50 percent, you will still get about 90 percent of capacity. All you do when halving the flow is to depress the average water temperature by half the original delta-T. A 180-160 degree circuit with a 170 degree average water temperature becomes a 180-140 degree circuit with a 160 degree average water temperature. Not a bad bargain!

In short, absent any really wild piping (and why the heck are you doing THAT?), I would not worry about it.

Have fun!

Brad