Radiant floor storage tank

Mo9013 · August 18

I have 2 zones in my house. One is cast rads in my basement and radiant floor in my main house. Because of this I have short cycling when the main house is running. To fix this I want to use a indirect water heater as a storage tank for the floor zone. The question is should I pipe it as a buffer tank or as a separate zone? The reason I'm using a indirect for a storage tank is that I have one laying around. I just want to get some opinions on what I should do.

EdTheHeaterMan · August 18

Having one laying around is the best reason to use it. What is the model and brand name and how many gallons? That is so we can look up what tappings are in that tank to help design the best use. @hot_rod would be the best person to answer this one.

hot_rod · August 18

let’s start with some numbers.

What is the btu/ hr load of the zones? What type and size boiler?

Are you running two temperatures?

A closed loop system?

What are the cycle lengths?

Rule of thumb you want at least a 10 minute run cycle when the boiler fires.

Mo9013 · August 19

indirect ms40. I was thinking the 3/4 inlet and outlets are to small for the flow rate for the floor heat but I was hopping the shortness of the 3/4 runs to a 1" zone header with mixing valve would negate it. So running 130 deg water to a 80 deg mixing loop with 10 deg dif would have more flow going around then pulling from the tank.

boiler GB 142-24 set to L5=40,000 Btu for gas but I'm running LP

floor zone is <20,000 Btu. 2 zones main living area is 14,000 and bedrooms are 6,000. Keep bedrooms at a lower temp. Was considering combining to one 10 port manifold

basement is <10,000 Btu

Boiler has outdoor reset ranging between 120 and 160 deg. Using mixing valve low temp on floor zone.

closed loop

basement loop runtime is about 35 to 45 min

floor is about 10 to 20 min above 40 deg outside temp and 5 to 10 min below 40 deg outside temps

to know my times I put blu-tooth thermometers on all the supply to find out what was going on with my system.

hot_rod · August 19

Do you need to run 160F? It seems radiators in he basement could run lower SWT?

That boiler turns down to 22,000 BTU/hr, so it is not grossly oversized. Assuming the total load is 25- 30,000?

Your bedrooms at 6,000 would be the smallest load.

Is the boiler piped with the factory low loss header?

How are you generating hot water?

A simple option.

Basically the boiler sees the buffer tank as its You just turned it into a high water volume boiler.

Boiler pipes to direct to coil. tank operates 120—160 off ODR.

Hi temperature comes off header at tank. 2 pipe buffer.

Low temperature via mix valve at top connections

just increase the tank 3/4 coil connection to 1" right at the tank, same at the boiler so you have a 1' primary loop.

With a 25- 30,000 load you only need to move 2.5- 3 gym. 3/4 & 1" pipe is plenty for that.

Mo9013 · August 20

No I don't but never changed is since the radiators never get over 140 degrees during the run time. This is one of my recordings for the radiators.

house temp rad.pdf

It is piped with the lo loss header from the factory.

I'm Using a indirect for hot water so I am already using the indirect loop.

At this point the primary loop off the header is 2 1/4 pipe branching to 3 1" loops. 2 are for floor. One 5 loop and the other is a 6 loop both have a mixing valve and circ. the other is the basement loop.

When the floor loops installed the bedrooms had zone valves for each room that now are open and the loop runs off the master bedroom. Still had short cycling. So I went a little further and now I have both loops running at the same time to help with the short cycling.

Your drawing is what I was thinking of doing. I was even thinking of taking off the low loss header. My thinking was the lo loss header and a indirect as a buffer would work the same way.

I was going to ask the pipping part after I got opinions on which way to go. especially if I went with a buffer. Never did one before but did the research so have an idea but still would like more info on the oversized header.

hot_rod · August 20

I agree you could lose the low loss header. The second indirect and it's piping become the hydraulic separation.

I=1/4" piping from the bpoler to buffer.

Indirect comes off as a parallel zone, same boiler controil as you have now.

If the boiler accepts another supply sensor, that goes in or on the buffer indirect.

If not a Ranco electronic setpoint call the boiler on and maintains the tank temperature vis ODR.

Keep the hi temp take off close to the tank, 1-1/4" as that piping is the llh.

You can zone however you want as long as every zone calls on the specific pump. Either low temp or high temp.

The boiler loop pump, shown at the boiler, is controlled from the boiler, it only runs when the tank needs to be heated.

The DHW idiect pump only runs when that tank needs input.

Screenshot 2026-08-20 at 10.08.59 AM.png

Mo9013 · August 23

I was going to use a temp control that would go to my zone controls to work the buffer zone. Thinking about setting the cut on/off 100 and 140 deg.

Would I be able to put unions between the tank and the tees on the buffer or will that make it too long to work properly? Always try and make things come apart easy if I have the chance.

Won't I need check valves on the DHW and High temp zone to stop back feeding when not running?

hot_rod · August 23

You have a zone control box running the boiler? The XX terminals going to boiler TT?

If you want the buffer maintained for instant heat, just connect TT to a setpoint controller.

If you connect thru the relay box, the tank only warms when there is a heat call.

Yes you could include unions.

You want a check in or at all pumps, I didn't show that or iso valves at each pump.

Radiant floor storage tank

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