What to do with microbore pipework on heat pumps 

October 24, 2022

Consumer Advice

What is Microbore and Why is it a Problem?

For heat pumps to work efficiently we need flow, and this does 2 things. It will mean the heat pump’s plate heat exchanger will work at maximum efficiency AND we can use the lowest possible flow temperature, maximising heat pump SCOP. Any restrictions such as microbore pipework could drag our SCOPS right back down again.

Microbore pipework is any pipework smaller than 15mm diameter which comes in 6,8,10,and 12mm, which restrict flow. 

If you have plastic pipework, that's much worse as it also has inserts that further restrict flow. Generally the smaller the pipe the greater the problem.

But microbore does not mean you need to repipe your house necessarily, if you're either well insulated, or an attached house or flat (which is the same as being well insulated), have a small, say 6kW load or below, your property will be absolutely fine on microbore, especially if you have 12mm pipe. 

Don't get me wrong, microbore is not the best, if it's kicked or knocked it can easily restrict flow and cause a raft of problems that will essentially end up with your heat pump having to run hotter or cycle more. But it can work and work very efficiently and might save the agony and disruption of ripping apart your entire house.

To find out if your pipework is too small use this table and our ‘Does My Pipework Need Upgrading for a Heat Pump?’ video and article to check if it needs upgrading! 

microbore technical specifications table

If you are within these approximate limits you may be OK, but the other issue with microbore is volume, to get around this here’s our first tip.

Tip Number 1: Use a Good Control Strategy or a Volumiser

reduce flow temperatures diagram

If your issue is not flow, but volume, there are 2 simple solutions. You can install what's known as a volumiser on the return pipe to elongate heat pump cycles and give volume for defrost cycles. 

OR, and my preferred approach if i had to choose, use a good control strategy, for those in the know this means a more ‘open loop’ strategy, but we are going to do a video and infographic on this soon based on the advice we give our customers, so lookout for that. 


Tip Number 2: Increase Insulation

insulation of pipes tip

If your problem is less about volume and more about resistance to flow, insulate. If you halve your heat load you halve your required flow rate. Halve your flow rate and you quarter your resistance to flow! This may mean your microbore could now be effectively oversized! 

The other benefit here is that if you halve your heating load your radiators also become bigger relative to the demand and your heat pump efficiency can jump from a standard 350% efficiency to 500% efficiency!

So, if you get a bill of say 3 grand to repipe your system, perhaps ask if that money is better spent on insulation. As with a heat pump this investment will compound for years to come!

If after insulation measurements you find that microbore still wont work, try tip number 3!

Tip number 3: Use Hydraulic Separation!

hydraulic separation diagram

If you are above say 7.5kW, you're likely to need hydraulic separation anyway. Hydraulic separation is when a second circulation pump is installed and the two circulation pumps (the pumps that pump the water around the system) are decoupled by joining the flow and return pipes together so they essentially have their own separate circuits. Think low-loss headers, closed coupled tees and buffers. 

This way the radiators can run at a slightly lower flow rate than the heat pump if required,   the heat pump will be kept happy and all radiators will heat up.

The downside is that this will mean the heat pump will have to work at a slightly higher temperature than the radiators, which will cause a slight efficiency drop. This is called distortion and is covered in our video “Why NOT To Use a Low Loss Header”.

Tip Number 4: Simply Repipe The System

pipework replacement diagram

This will mean you’re less likely to get blockages and no one can accidentally sabotage your heating system by knocking a pipe. 

It also means you will naturally have more volume in your system. And if you do have hydraulic separation you can much more closely match the flow rates and maximise SCOP.

A quality heating engineer will understand these options. If you're a heating engineer and don't understand, don't worry I've 100% been there too! Just check out our heating system design course. 

See ya.

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