Under floor water heating - result of overflow

[TSN79]

Several apartments have under floor heating (water) with a common boiler room and a central pump. There is no pump in the apartments. Each pipe run on each apartment's manifold has its own flow-indicator. The owner of one apartment is worried that if the neighbor turns off its heating, then more water will be forced through her manifold - increasing the usage and cost (they only have volumetric meters). They don't have any valves installed to prevent overflow to each apartment.

Some argue that even if this is the case, this will only result in the apartment being heated a bit quicker, and so the manifold actuator will close a bit earlier than it otherwise would - and there will be no difference in final cost. I can understand the reasoning, but I'm unsure if it's that easy. If she receives 20% more water, does that mean that the apartment will reach its set room temperature 20% earlier?

 

[Lnewqban]

What system controls the indoor temperature for each apartment?
What happens to the main flow and pump when all or most apartments have reached the temperature set point and are satisfied?

 

[TSN79]

What system controls the indoor temperature for each apartment?
What happens to the main flow and pump when all or most apartments have reached the temperature set point and are satisfied?

The temperature is controlled by room thermostats that open and close actuators (on/off) on the manifold. I'm not sure how the pump is controlled. My best guess is that it keeps a constant pressure - so I do believe it reduces rpm as head loss increases when most actuators close.

 

[Lnewqban]

The temperature is controlled by room thermostats that open and close actuators (on/off) on the manifold. I'm not sure how the pump is controlled. My best guess is that it keeps a constant pressure - so I do believe it reduces rpm as head loss increases when most actuators close.

That is a common setting; pump may use a variable frequency drive, or a by-pass loop.
In that case, the inlet pressure at that each would be more or less constant, regardless hot water demand from other apartments.

 

[TSN79]

That is a common setting; pump may use a variable frequency drive, or a by-pass loop.
In that case, the inlet pressure at that each would be more or less constant, regardless hot water demand from other apartments.

So the pump setting might eliminate the whole issue? Worst case is that the pump is constant speed only. If an apartment was to get some overflow - would the reasoning about faster heating hold at all?

 

[russ_watters]

So the pump setting might eliminate the whole issue? Worst case is that the pump is constant speed only. If an apartment was to get some overflow - would the reasoning about faster heating hold at all?

If the pump is constant speed, then the operating point will move along the pump curve. It may be slightly less efficient with lower flow, but that's a tiny fraction of the system energy; the vast majority is in the actual heating of the water. Your logic is correct that if the apartment isn't overheating it is getting the same total amount of heat (again, to a pretty good first approximation).

 

[Averagesupernova]

I would think the most accurate way to do this would be that the system attempts to maintain a constant pressure. If not, one apartment could be overcharged if they were the only one calling for heat. There would likely be enough flow in that one apartment that the heat exchanger in said apartment would not extract the heat at the same rate as with a lower flow. To me, in order for the system to work, there needs to be a constant system pressure.

 

[Lnewqban]

So the pump setting might eliminate the whole issue? Worst case is that the pump is constant speed only. If an apartment was to get some overflow - would the reasoning about faster heating hold at all?

Yes, and yes, but with increased noise and operational costs.
In order to make systems more efficient, the flow supplied by the pump(s) is reduced via slowing down rpm's.

Copied from
https://www.pmengineer.com/articles/84208-the-dos-donts-of-hydronic-system-design

"Do #4. Install gP bypass valve on parallel zoned systems. The ability to extensively zone heating distribution has long been heralded as a major advantage of hydronic systems. At first, the concept appears so simple: Just install an electrically operated valve for each zone and open a given valve when its zone needs heat. Too often overlooked is the increased pressure differential across the circulator each time a zone valve closes. Left unchecked, high differential pressures can lead to flow noises, overheated pumps, flow leakage in other (closed) zone valves, and even erosion of piping materials.

The solution in small to medium size systems is to install a differential pressure bypass valve across the supply and return mains of the distribution system. In effect, this valve "caps" the upper differential pressure across the mains.

On larger systems, circulator speed can be adjusted using a variable frequency drive (VFD) to maintain differential pressure within prescribed limits. In addition to regulating pressure, this approach reduces energy consumption of the pump under partial load conditions. Look for variable speed distribution pumps to be available for smaller residential and light commercial applications in the near future."

You can find more info by using "control of hydronic loops" in any search engine.

 

[russ_watters]

If not, one apartment could be overcharged if they were the only one calling for heat. There would likely be enough flow in that one apartment that the heat exchanger in said apartment would not extract the heat at the same rate as with a lower flow.

I think it is unlikely, but it would indeed be a bad metering system design if it was only measuring flow and not heat. The system needs to measure flow and inlet/outlet temperature.

 

[jrmichler]

Look for variable speed distribution pumps to be available for smaller residential and light commercial applications in the near future.

The future was here over 10 years ago. Photo of the Grundfos 15-55F/LC hydronic pump in my house:

It adjusts speed according to the number of open zone valves. The display alternates between GPM and watts. It shows 3 GPM at 24 watts with one zone open, and 8-9 GPM at 35 watts with all three zones open. The pump is self contained. It apparently calculates flow and head from power and RPM, then sets speed to maintain the desired head. This particular model has been around long enough that it is obsolete.