Boiler Flow/Return differential and cycling

[Duncan]

I understand from googling that a radiator should have about a 12deg C differential between flow and return.

Does the same logic apply to the boiler flow and return.

If a boiler circuit attemps to reduce cycling it may hold off untill the return temp is 12 deg less than the flow before firing.

In this case for maximum efficiency should the continue to run untill the either of the following is acheived.

a) fire boiler untill flow temp reaches desired target (i.e 80deg etc) regardless of return temp (i.e also allow to return at say 80deg) b) room stat is satisfied

OR

a) flow temp reaches desired target (80deg etc)/boiler stat b) return temp is less than 12 deg below flow (i.e if boiler is heating to flow to say 80deg, at at 60deg the temp of the return is say 55deg should the boiler switch off anyway? c) room stat is satisfied

I suspect the second is preferable, as this would take into account that not much heat is being lost in the system (i.e warmer house) and compensate accordingly.

Any thoughts which is best? or your own theory!

[IMM]

On a non-condesning boiler, yes.

Yes.

Depends on the boiler control system. Some may, some may not.

Many non-condensing boilers have modulating burners. These tend to modulate to maintain a flow setpoint, say 80C. When it approaches 80C the burner modulates down. When the temperature goes over 80C it then switches off.

On balancing you are supposed to set the pump speed to maintain a 10C differential. See FAQ.

Condensing boiler maximise efficiency by having the return temperature as low as possible. "Most" have what is known as load compensation control. A typical operation would be: The burner modulates to what the buildings heat requirements are. It does this by sensing the flow and return temps. It also attempts to maintain the 20-22C differential required on condensing boilers. You still need to balance the system to get the 20C; see FAQ. The control system on start up would be flat out. It should be balanced for 20C anyhow. When the differential narrows the system assumes the house is warming up. Then it drops the flow temp to maintain the 20C differential, which in turn drops the return temp improving efficiency. If the differential still keep narrowing it still keeps dropping the flow temp, until it get to the lowest burner level and then switches on and goes onto cycling.

Some boilers have modulating pumps and burners to maintain the differential and the lowest return temp required. These are difficult to balance as the boilers actually modulate the pump speed. Some are said to be self balancing.

I hope this helps.

[Christian McArdle]

Given that the return and flow temperatures at the boiler will match each other perfectly a short time after the burner turns off, it wouldn't be possible to use it as a way of turning the boiler back on. At the least, you could only use it to turn off and then start a timer to have another go. However, things have moved on now, with modulating burners and microprocessor control. The algorithms now used are much more complex than either scenario you suggest.

In this scenario, the closeness of the return/flow temperature is probably related to the fact that the boiler hasn't properly heated the water up yet. For example, if the boiler is heating a hot water cylinder currently at 55C, but required to be 75C, the return temperature will only be 55C. If the boiler is only producing 60C, the problem is that the boiler is set to produce too little heat, not too much. In this case, the boiler should ramp up, not turn off.

Christian.