Fixed speed, constant pressure, proportional pressure, ΔT control. 15-50, 15-60, 25-80, 32-100.

Pumps are a lot.

How on earth do you know what size to choose and what to set it at?

For decades pumps were simple: three settings, usually left on maximum. 

Modern pumps vary their output as the system changes and report through a screen or app.

They're impressive, but they're not a fix for a bad system. Misread that and you'll be misdiagnosing forever.

Let's sort out what the pump controls and what it doesn't.

What the pump can't do

The pump feels like the boss, because it has the power. It shifts the water.

But it doesn't decide how much heat the house needs, how much water must move or how hard that journey will be.

The system decides that. The pump can only respond.

So what are all those settings doing?

Different systems need different pump 'modes'. These determine whether the pump stays on one setting or varies its pressure as valves and zones open and close, or as ΔT changes.

• Fixed speed: suits a steady requirement, like a cylinder circuit. One setting, though the pressure it produces still depends on the system.
• Constant pressure: suits similar circuits on a manifold, including underfloor. The pump varies its output to keep its push steady as circuits open and close.
• Proportional pressure: suits radiator systems with TRVs. The pump reduces its push as valves close and increases it as they open.
• ΔT control: varies the output to keep the gap between flow and return near its target.

In the right system, these modes reduce noise and electricity use. What they can't do is overcome restrictive pipework, poor balancing or a blockage.

Whatever the mode, it's the three rules of the system that control success.

The pump isn't always innocent

Pumps fail. Some can't meet the system's needs. Settings can be wrong.

But check the system before you adjust, upsize or add a second pump.

If 25-80 is your default for bigger homes, you're wasting money

It's easy to see a large house or lots of radiators and reach for a 25-80, or a second pump. 

But pumps aren't sized by house size or radiator count. They're sized by the three rules.

Most homes sit comfortably within a standard 15-50 or 15-60. 

Default to a 25-80 and you're paying more to create excess flow, noise and reduced efficiency.

You won't get away with it on a heat pump

On a boiler, max pressure got the heat around restrictive pipework or poor balancing, but the wear and inefficiency were still there.

Heat pumps move far more water to deliver the same heat, so those hidden problems stop hiding and seriously affect performance.

Know the system. Then pick the pump.

There's a right pump for the job and ideally it's the one in the boiler or heat pump. 

But working that out starts with the three rules. You can't guess.

Never been taught the rules? 

We teach all this and more, online or in the classroom. 

But you can start with the free ebook: 

Rule the system, before it rules you. 

Our quick, practical guide to the logic behind every heating system.