Adding radiant zone to two-pipe direct return

farnja · September 2017

Hi Everyone - This forum has been extremely helpful for me as a total novice. Thanks for all the contributions.

I recently purchased an old house with a two pipe, direct return, hot water heat system and we're making some changes that will require new heating solutions.

I would like to:
1. Add a radiant floor zone.
2. Remove or move an existing radiator.

I am pretty confident with the design of the radiant module (or will be once I iron out integration) but would like to make sure I am on the right track with how to integrate into our existing plumbing.

Current Layout: ( I apologise, I'm sure some of my symbols are incorrect).
- Orange radiator to be moved/removed.
- Red circles are available ports to tie in new zones.

Based on my research, it seems a major concern with mixing radiant flooring and other types of radiant heat is protecting the boiler from cold return temperatures. It seems that primary-secondary pumping is the best solution for this.

In addition, I've gotten feedback from pros that trying to move a radiator and then tie it back into the existing system is very difficult. The existing cast iron and steel pipes are 100 years old; who knows how they will react. They've suggested removing the radiator and capping the existing lines. If I still need the radiator's output, they suggest that adding something via a separate zone is much cleaner.

Given the above...
Proposed Layout:

A few questions:
1. Am I on the right track? Can I keep it simpler? Looks like I'll be adding several hundred feet of PEX, 3 circulators, and a whole bunch of valves, joints, etc that I haven't included - let alone the manifold(s) and other radiant material.
2. Does the location of pump B matter? I know I am not "pumping away", but given pump A is already installed, putting pump B in a similar location means that my pressure changes will be consistent in each loop.
3. As long as my boiler can produce enough heated GPM, does the second loop (green) affect the flow of the first loop, or is it largely constant given the pump output and the head-loss for the loop? FWIW, I think there's probably 6-8 GPM going through the original loop.
4. Should I really add a whole new zone for another radiator or go through moving it and then adding it back into the existing system? Given the heat-loss calcs I've done, I *don't* think I'll need it, but I may.
5. Do I need to modify the existing loop to turn it into a secondary loop off of the new (green) primary loop?

A few more statistics on the system:
Boiler : Peerless WBV-04, 160,000 BTUh
Original (Black) lLoop Heat loss: ~60,000 BTUh
New (green) loop heat loss: ~15,000 BTUh
Main pipes (thick black) : 1-1/4" Steel (I think), cast iron joints.
Radiator pipes: 1" Steel (I think), cast iron joints.

Thank you in advance - please let me know if there's any more information you need.

Ironman · September 2017

What you need to consider is that the radiant floor will require a separate water temp zone with much cooler water than what the rad's require. You will either need a SMART mixing valve or variable speed injection mixing with outdoor reset. Either will provide boiler protection and require their own piping arrangement.

How do you know 15k btus is the heat loss for the radiant? Have you done a scientific heat loss calc like ACCA Manual J?

farnja · September 2017

Thanks Ironman.

To get the heat loss I ran my info through the slant/fin application. Is there any downside to running more GPM than necessary in the primary loop (green) assuming the secondary (blue) are sized correctly?

Yes, I am planning on some sort of thermostatic mixing valve to ensure that the radiant floor loop is operating at the lowered temperature.

Ironman · September 2017

Not a thermostatic (dumb) valve, but a Smart valve or VSIM. Look at Taco's ISeries valve or Tekmar 356. Which evere one you choose will determine the piping.

To get a true primary/secondary setup, the direct return would be piped off of the primary loop. The way it's now drawn, there's no boiler protection from the CI rad's on the direct return loop.

farnja · September 2017

Ahh, I see. Just to make sure I get it, If I had a thermostat controlling the secondary pump could I get away with a simple thermo mixing valve?

Also, I know with the current loop it’s not true primary/secondary, I guess I’m asking if it’s worth the effort to try and change it?

Ironman · September 2017

Is the RF a slab, staple up or an over-pour?

farnja · September 2017

It will be radiant panels (sunboard, warmboard, roth) under backer board and ceramic tile. With what I spec'd out to make sure it'll work, I'd need about 115-120* water w/ 12" spacing to keep the floor at 85*.

Ironman · September 2017

A thermostatic valve would be OK, but you may have thermal expansion noise from the floor. It also won't provide any boiler protection. A smart valve is a better option and not all that more $$ when you look at the total picture.

Going with pri/sec piping is the better option. In that scenario, you would pipe the green loop from supply to return of the boiler with a circulator in it right after the expansion tank connection. Then the first thing Tee'd off would be the existing rad's, then the radiant after that. You want the highest temp loop first off of the primary and the lowest temp loop last.

hot_rod · September 2017

I don't agree that a thermostatic valves are dumb

farnja · September 2017

Thanks hot rod and Ironman.

Given the boiler's been running as-is for 10 years on the current loop and the only thing I'll have changed is removing a radiator (should have even less delta-t on the loop), I'm a little hesitant to do all the work required to make that a secondary loop off of a new primary loop. I can add the new primary loop without touching the existing plumbing.

I suppose I could wait and see? Create the new loop off of the existing available connections to run the radiant floor as a secondary loop. Run enough GPMs through to protect the boiler from low-water return and mix the little hot water needed for the floors with a mixing valve. If there are issues, then remove the existing loop and splice it into the new primary to create true primary/secondary?

Ironman · September 2017

hot rod said:
I don't agree that a thermostatic valves are dumb

Bob,
"Dumb" as opposed to a "SMART" valve. In other words, it has no logic.

I'm not saying that someone is dumb for using one, because I have on many occasions.

hot_rod · September 2017

But thermostatic do have the ability to respond to temperature changes and to react.
They can protect boilers and provide accurate temperature mixing.
A TRV is another example of a smart control valve option.

Dumb valves are manual operated valves like 3 way ball valves or tekmar valves without actuators attached

I agree intelligent valves include microprocessor that can offer more adjusting options

Maybe we agree that dumb valves are manual operated only.
Smart are thermostatically controlled
Intelligent are microprocessor driven valves.

EMV electronic mixing valves are starting to replace thermostatic bring intelligence to DHW temperature control.

Gordy · September 2017

The home I use to own had a taco panel-trol mixing valve for the radiant from 1952. Still running today. Actually 2 of them the other was in the detached apartment.

A mixing valve that has lasted 65 years....... still going. no wonder they quit making them

farnja · September 2017

Thanks for all your help, guys!

Adding radiant zone to two-pipe direct return

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