I currently have a forced hot water heating system with 3 zones, all of which have the circulator pump on the supply side of the line. I need to install a 4th zone and am being told that putting the circulator on the return side is more efficient.

What are the pros and cons of the circulator's location? Are there issues having some on one side and some on the other?


Water pumps pump water, not air, so putting it on supply side makes water allways available for circulator to pump. If you put that circulator on other side it could be hard to fill system with water and avoid air pockets. And basicly you can't pump air with water pump as it makes them break.

  • Actually, putting it on the return side can create pockets of air, if you have a strong enough pump. – Brad Gilbert Oct 28 '11 at 5:12

I'm making a bit of a guess here but I would speculate that there are two factors at play:

1) Hot water going through the circulation pump is more "active", and thus could shorten the lifespan of the pump (active meaning more likely to cause corrosion); and 2) When you push water through pipes, the path of least resistance will get the greatest water flow. When you PULL the water through the pipes, you'll create suction on all the flow paths, which I guess would mean the water flows more evenly through them.

All of this is sheer speculation on my part, however.


The location of the pump would all depend on ehat kind of a heating system you have. If you have multiple zones and the zone valves are on the supply, then the pump can go either on the return or the supply. If you have multiple zones but with circulating pumps the i would put them on the supply. However, if you do have multiple zones but with circulating pumps and the pumps are on the return, then you would need to have zone valves on the supply so that one of the pumps only heats up the area where the thermostat is calling for heat.


Fluid dynamics is a field of engineering that is very intense and complex. However simply put, it is a game of pressure differentials and not a path of least resistance or "last river" concept. Pumps "pump" because they create a lower pressure on their suction side, and a higher pressure on their discharge side, and flow is created. The higher pressure will always seek the lower pressure. Every time the flow meets resistance, ie pipe friction, turns, coils, heat exchangers, the pressure gets a little lower on the other side of that resistance.

Now depending on how you pipe your boilers, if you put your pumps on the supply side, my personal preference and the preference of reputable heating contractors, you keep the higher pressure at the air removal portion of your piping and you have a greater chance of getting the air out of your system.

The other school of thought is to have the highest pressure of the system at the boiler as it is the greatest resistance to flow. Also most boilers come with some kind of rudimentary air scoop in their exchanger, and if that is all you plan to use to remove air, then the return is a good option. I also wonder if its sheer laziness as the boiler usually comes out of the box with the pumps mounted on the return side.

To sum up, it sounds like you had a good heating contractor design your piping system and I would stick with keeping it on the supply side!!!



Flow Dynamics dictate that you should have it on the feed not the return also try adding so glycol to the system as it will extend the life of your circulator

  • Also you don't need the flow check's they are obsolete there is a ciculator with an internal check or you can buy the check and insert it yourself just remember to get a new flange gasket – Bruce Miller Sep 22 '18 at 16:52

From a water flow point of view, a circulating pump can be placed in the boiler feed or the boiler return. But there are other considerations.

Cavitation is where the hot water turns to steam briefly. Water boils at a lower temperature when the pressure is lower. Older heating systems have boiler feeds at higher water temperatures. The pump creates a pressure difference with lower pressure on its inlet. If the overall head of water above the pump is small enough there is a risk of cavitation that is more noticeable at higher flow settings. Cavitation is noisy too.

Sludge accumulates over time. The heat exchanger in the boiler will shed iron oxides (rust) as the oxygen in the water is released over time. Over decades, this rust settles in lower pressure zones. Think like a Dyson vacuum cleaner where dust drops out of the air at a low pressure point.. The fine particles of rust will accumulate in pipes, valves and in pumps where pressure is lower..A pipe will become narrower as the sludge builds up. The reduces the pressure on the pump inlet further still, increasing the chance of cavitation.

Electric motors in circulating pumps often need to use a capacitor to create a phase shift that starts the motor turning in the desired direction. The capacitor is an electrical component that operates at mains voltage whenever the pump is active. Temperature cycling can reduce the working life before failure. Again they are robust components that should last decades. But they do fail. Lower temperatures create less thermal stress.

Motor bearings are often water lubricated. A circulating pump needs to be mounted such that the spindle tends to sit in the main bearing by gravity. A slight incline is all that is needed. Physical restrictions may force you to install the pump in a particular place and orientation. Clearing airlocks may also affect location. An air bleed valve can sort that out if a problem.

Taking all this together I prefer to put the pump in the boiler return which is at a lower temperature and with less sludge.

On more modern systems the water temperature is lower. I have seen reports that water temperatures for new systems will be reduced to 55"C Which is lower than the temperature at the boiler return on my old system.


This is an old question, and I will try to give the old best answer. The point at which water is connected to the heating system is known as the neutral point. This should be on the return to the boiler, and be the nearest connection before the return from the system enters the boiler.

Some systems have open vents, which are run to above a small (feed and expansion tank), typically in an attic in a domestic, or even a commercial building.

Such vents should be the first connection off the flow pipe from the boiler.

The best place for the pump (s) is just after this vent.

Many modern systems are pressurised and do not have open vents.

The best place to connect the water feed is still on the return at the boiler.

The best place for the pump(s) is still on the flow side.

This keeps the bulk of the system under positive pressure from a combination of the pressure imposed by the system water connection, and then when the pump is running, the pressure provided by the pump.

The only area where the pump pressure in the system is negative is within the boiler.

The pressure imposed by the water connection should be great enough, and positive, to ensure that even in the boiler the combined pressure will be positive.

The full result is that you do not get boiling due to negative pressure at the pump suction (entry point), and do not get air drawn into the system through the almost microscopically thin gaps around valves, and other connections in the system.

  • Note that the neutral point may not be strictly at the return to the boiler in systems using hydraulic separators. – ThreePhaseEel Feb 23 at 1:18

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