Adding radiant area to existing multi-radiator zone - am I missing something?

grant_andrew

Hi Everyone,

I'm in the process of planning a basement renovation and zeroing in on my approach to heating both my existing and the new space. On my first floor I have 7 CI rads on one zone, which cover about 1200 sq. ft. I plan to repipe the system with pex-al-pex homeruns to each radiator, so I can balance the flow to each radiator and tuck the tubing into the joist bays.

I will be adding a baseboard radiator in the basement, which will be run off of a new zone, using a zone-circ approach, which is straight forward enough.

When we renovated our living space a couple of years ago, we relocated the kitchen radiator to a new bedroom, and went to a more open layout. One of the consequences from this change is that the kitchen is a little cooler than the rest of the house. It's not too uncomfortable, but it's not ideal. I'm thinking that this is the ideal time to add radiant heat under the kitchen, since I'll have access to the joist bays under the kitchen and will be re-plumbing the system.

Since our living space is currently one on zone, and the thermostat is relatively close to the kitchen, I'd really like to add the supplemental kitchen heat to my existing zone instead of adding a new one, and unless I'm missing something, I think it's pretty straightforward.

Could I just tap into the radiator return manifold to supply the "hot" water to my radiant mixing valve, and then have the return from the radiant loop tie into the return header at the boiler? My thought is that by adjusting the setting on the mixing valve, and/or the water flow through the radiant loop, I can tune the system to throw off just enough heat to make the kitchen more comfortable, and be roughly the same temp as the rest of the house. In this setup, the radiant floor and the CI rads would all be heated simultaneously.

My thought is that by using the CI radiator return as the radiant supply, the mixing valve won't have to use as much cold water to lower the radiant temperature, conserving water and energy. And by introducing the radiant return upstream of the zone 1 return, it will mix the two loops prior to returning them to the boiler, so the boiler never sees just the cooler water returning from the radiant loop.

The other, maybe simpler option, could be to just add a hydronic toekick heater in the kitchen, but where's the fun in that?

I've attached a proposed schematic of the system, as well as a 1st floor layout. The kitchen is in the top left, with the proposed radiant area shown w/ a red loop. The other radiators are shown in red throughout the house. Thanks in advance for any support!

grant_andrew

Was just thinking about this some more and realized that as-drawn I don't think I'd get any flow through the mixing valve, because there is no significant delta P across the radiant floor, as the inlet and outlet are both connected to the return to the boiler.

If i put a circ between the radiant loop and the mixing valve, good to go? I think I would just tie it in so it runs off the same tstat as the radiator loop.

The new schematic is below. Thoughts?

EBEBRATT-Ed

Radiant heat is usually 120 degree water maximum so you certainly need a mixing valve and probably an additional circulator probably two of them. I would use a pump for the radiant zone , an injection pump and a mixing valve

hot_rod

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

Ironman

@grant_andrew
Take a close look at the piping diagrams in Siggy's article that hot rod posted. You're gonna need to make some changes in yours so that its in line with one of those. Particularly, the circulator must draw out of the mixing valve and the cold supply line has no connection to the radiant loop. It's strictly for filling the system and maintaining pressure. It's not part of a circuit.

grant_andrew

Thanks for that links. I've read through dozens of articles on that website but never came across that one!

All of those suggestions show the radiant loop on the supply. Is there any reason I can't use the radiator return as the supply to the radiant loop?

Ironman - did you see the second image I posted, showing the closed loop for the radiant with the circulator? I think that checks all the boxes, but any reason it can't be on the return as I've shown? My thought is that I can have the radiant circ start with the zone 1 circ and adjust the setting of the tmv or the flow valve to dial in the temperature response to be in line with the ci rads.

Ironman

grant_andrew said:

Thanks for that links. I've read through dozens of articles on that website but never came across that one!

All of those suggestions show the radiant loop on the supply. Is there any reason I can't use the radiator return as the supply to the radiant loop?

Ironman - did you see the second image I posted, showing the closed loop for the radiant with the circulator? I think that checks all the boxes, but any reason it can't be on the return as I've shown? My thought is that I can have the radiant circ start with the zone 1 circ and adjust the setting of the tmv or the flow valve to dial in the temperature response to be in line with the ci rads.

That would work. Just make sure that the Tees are kept less than 12" apart.

grant_andrew

6 months later, I'm finally getting to this project!

My original plan was to supply the mixing valve to my radiant floor with water returning from my existing radiator return manifold using close tees, with the logic that the water should be hot enough to heat up the floor, since my boiler's lowest setpoint using outdoor reset is 140, and thus the lowest return temperature would be 120, once the boiler is up to temperature. The layout is shown below:




However, it just dawned on me that when my boiler has a cold start and is getting up to temperature, it may be a while before my radiant loops sees warm water, and the floor temperature might lag behind the radiators.

To combat this, I could use supply water from the boiler on the hot port of the mixing valve, using the spare port on my radiator supply manifold. If I did this, could I connect the radiant floor return to the mixing valve cold port and the radiator return manifold? Are there pressure issues I'm not seeing? This proposed layout is below:

grant_andrew

Just wanted to bump for some potential feedback. The short version:

I'm adding staple-up radiant in my kitchen, off of an existing zone of CI radiators, just to bump the kitchen temperature a bit and warm our feet.

I was planning on tapping into the return from the radiator manifold, but now I'm wondering if that water will be too cool to be effective, since I have outdoor reset and the CI rads take a bit to get up to temperature.

My other idea was that I could use the spare supply and return ports on my manifold that serves the radiators. A pic of my setup is below, with the two options circled in yellow - let me know if you have any suggestions!

hot_rod

What temperature is the manifold running at?

If it runs high, say over 120F or so you might consider the UltraFin method. It is more of a suspended tube joist bay system, can run 180 without over-heating the floor surface.

Easy to install also.

grant_andrew

I have outdoor reset, so it fluctuates between 140 and 180. I was planning on using the return water from the radiant loop as the cold inlet to a mixing valve.

hot_rod

But the radiant would only run when another zone is calling? If you use that manifold port, with and actuator, the radiant could be its own "controlled' zone.

The return from the 140- 180 operation could be 130- 170 at some point as the delta T drops. That would be excessive for a plate type radiant system.

You piping modifications would go away if all the systems could run one temperature, no mixing devices to pipe in.

Paul Pollets

The Oventrop UniBox is designed to add a radiant zone without the need for an extra pump, and tapping in to the radiator stubouts. It presumes the boiler is on constant circulation.