Flow the life blood of your heat pump

There is a joke in the world of heat pumpery, here goes:

There are only 3 things that matter in heat pumps, the first and most important thing is flow, once you have got that sorted next you need to concentrate on the flow and after that look at the flow again. Hilarious huh? but scarily true.

Heating engineers appear to find the whole idea of flow rate completely alien, its something they have never had to bother with. In their defence its never been an issue so why learn it. Think about your house, all you care about is if the radiators are hot, you have no way to measure the flow rate to your heating and you dont care. If a room is not getting up to temperature you just wind up the boiler temperature using the very easy to operate controls and the increase in temperature means the rooms all get warm. see example below of superb and easy to use controls, see if you can work out how to turn up the boiler temperature?

The really clever bit is you have no idea whatsoever what the effect on boiler efficiency is because no one measures it, you just trust that the boiler manufacturers are right and your boiler is 90 % efficient. but that's a story for another day.

In heat pumps flow is all that matters, we send out water at a lower temperature because the higher the temperature the more the heat pump costs to run, its just like your car, faster is more expensive. You need to supply much more water to the radiators if the temperature is lower than you do if the water is really hot to keep the rooms warm. Typically in a heat pump system we need to pump the water round at twice the speed as you do with a boiler.

What no one tells you is, if the flow rate falls below the optimum the efficiency of your heat pump falls really dramatically, the output also falls and the run cost goes UP, UP, UP. so if you have a heat pump you too should be obsessed with flow rate. You will never reach that published SCOP or efficiency figure if the flow rate is not high enough.

If you dont know what flow rate you need its pretty simple, you need 2-2 and a half litres per minute flow for every kW your machine can give. Example: a 16kW unit needs a minimum of 32 litres a minute and a maximum of 40 litres a minute. If you have less flow than this you are spending more money than you need too. Think of it as the equivalent of driving your car with the hand brake on a couple of notches.

This leaves us with a problem, if you want to move water down a pipe there is a speed range which works best, if you go too fast the water makes a hell of a noise, it sounds like a tap running, not ideal in a heating system. To get the speed down we increase the pipe size, very roughly as you increase the size from 15mm to 22m or 22mm to 28mm the speed of the water halves and the noise disappears, clever huh? that's why we use big pipes on heat pumps.

When you go to college to learn this stuff the teachers love to talk to you in ridiculous units which mean nothing, its designed to make sure you leave knowing very little, its hilarious. All they need to tell you is you cant move the water at more than 3 miles per hour, or walking speed, if you do its noisy. If you are a geek and think this is too simple do the maths its a bloody good rule of thumb.

But the problem doesn't end there, you need bigger pumps and every bend, valve, fitting , filter etc in the pipe work slows the water down and creates a resistance. Its just like when you stand on a hose pipe when your wife/husband /kids are washing the car, bloody hilarious and a good training in fluid flow and resistance.

Heat pumps usually have some way to detect how fast the water is moving, in the old days we used a paddle switch but now we use a flow meter like this.

Somewhere inside your unit you can see what this meter is reading. Think of this thing as your systems speedometer. BUT the heat pump manufacturers hide this data from you, it is without doubt the single most important piece of information you and the end user needs to know about the unit so the software lurdos hide it. I have no idea why they do this but they all need a bloody good kicking, this info should be displayed on the front screen all the time.

As im an arse I decided to see how difficult it is to get this reading out of all the heat pumps i could find, and rate them in a table of disgrace, here are my findings.

Samsung gen 6, 21 button presses including a password, to get to flow rate in litres/sec .

Hitachi 13 button presses including a password to get to flow rate in m^3 per hour (what on earth does a metre cubed per hour actually look like?)

Midea 12 button presses, no password, to get to the flow rate in m^3 /hr (see above)

With some help from heat pump friends it appears Daikin also have this feature hidden behind 12 button presses and a password and when I tried to work out how it was done on a Mitsubishi I lost the will to live after 2 days so I dont know the answer.

In brief my conclusion is that all of the units need a kick in the arse, 12 button presses is appalling, can you imagine getting in your car and finding the speedo was buried in the boot or hidden in a sub menu, "sorry officer I have no idea how fast I was going, the software on this car was written by an idiot".

So we put a mechanical flow meter on all out heat pump packs like this, anyone can read it anytime.

Except on Midea that is, On a Midea we use the Freedom heat pumps app, as Paul and I wrote it the app flow rate shows all the time on your phone.

Oh and RED, well the RED unit controls the flow rates for the whole house and you get a phone call off the guys at RED if the flow rate ever drops.