To answer this properly you need a heat loss calculation.  Then you can work out how much flow each radiator needs which will tell you what diameter pipe (internal diameter) is required.  Until you have this its somewhat impossible to tell for certain (but see below).  Did your pervious quote include a heat loss calculation?  Have you actually established that re-piping is necessary.  Some installers love to do it but its not always necessary.  If you post your previous quote perhaps there are some deductions that can be made.  

A few more questions which might help point in the right direction, answer those you can!

• Whats your largest current radiator and what type (eg 21, 22).  How many do you have?
• Does the microbore all come back to a manifold and if not how is it connected?
• What diameter are the primaries - the pipes leaving the boiler
• Whats your house and construction/age/standard of insulation
• Whats your annual heating gas consumption and pattern of heating

You should be able to do 750W comfortably through 8mm id pipe without impacting efficiency so if no rad is  over this size no problem.  Even if they are you may well be able to get away with it with a bit of finessing of pumps/DT etc., or just replump one section.  Personally I detest throwing out perfectly good kit and a full replumb is pretty disruptive.

@jamespa Thanks for taking the time to answer.  I wasn't looking to check whether or not I needed to replace the 8mm microbore.  As you say that would require full heat loss calculation plus much more detailed work checking existing pipe runs.  Access to the pipes is difficult and I only want to start lifting carpet and cutting holes in my floor to check how far the primary pipework runs when I've decided to proceed even if it transpires that I'll need a full repipe.  If I discover I can achieve good efficiency without repiping then that's a bonus.  Additionally as the system is over 30 years old I would be concerned about possible future blockages as corrosion builds up over subsequent years.  Whilst I've had the rads cleaned out, I was warned off power flushing microbore as it can force sludge from one part of the system into the microbore. 

So at the moment I'm trying to run my regular boiler at as low a temperature as I can to lengthen its lifespan whilst I get a better understanding of the ASHP world and inistallers near me build experience. One of the new (or at least new to me) areas was the use of MLCP in place of copper pipe.  So I was trying to get an understanding of how it is used and what are its advanates and disadvantages.

Posted by: @iotum

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.  So I was trying to get an understanding of how it is used and what are its advantages and disadvantages.

OK, TBH I dont know enough about MLCP to comment with any authority.  The simple message so far as I am aware is: Nothing inherently wrong with plastic pipe but it has a smaller internal diameter and is rougher than copper pipe so for any given external diameter you get less water through.  This may or may not matter depending on your design.   Easy to route round gentle bends so particularly good in a new build where everything is 'open'.  Not recyclable whereas copper is.  Others can hopefully give a more authoritative answer.

Posted by: @iotum

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Additionally as the system is over 30 years old I would be concerned about possible future blockages as corrosion builds up over subsequent years.  Whilst I've had the rads cleaned out, I was warned off power flushing microbore as it can force sludge from one part of the system into the microbore. 

Dont get this.  If you shut down sections sequentially so there is only one route through at any given time that cant happen.  Something of a PITA but less so than taking up all the floorboards!

I note your floor area is 120sq m but its single storey.  Assuming you have decent loft insulation and the walls aren't all solid then I would doubt its 7kW and, if it is, you may be better investing in insulation upgrades instead of re-piping!  If you know your annual gas consumption in kWh then divide it by 2000h, and separately divide it by 3000h.  The loss is highly likely to be between these two figures.  eg 18000kWh annual consumption is likely to translate to somewhere between 6 and 9kW.  As a concrete example my house was 18-20kWh, measured (not calculated) loss is 7kW.

Posted by: @iotum

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So at the moment I'm trying to run my regular boiler at as low a temperature as I can to lengthen its lifespan

Thats a very good idea anyway as it lengthens run times and means its more likely to condense, which both reduces consumption and improves comfort!"

Posted by: @iotum

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I had a quote a couple of years ago for fitting a 7kW ASHP and complete copper repipe which came to over £20k for a single storey semi detached cottages (123 sq metres)

Sounds a lot, is this after grant or is the real price £25K? 

When you come to proceeding do shop around and do fell free to ask for help here.  There are good installers and some who are still using practices from 5-10 years ago.  Under no circumstances allow anyone to fit a buffer, low loss header, or plate heat exchanger between heat pump and emitters, there are unnecessary particularly in a property of your size, they compromise efficiency if not properly set up (which they rarely are) and make fault diagnosis difficult.  A 2 port volumiser is OK however and may be needed.  You want exactly zero external controls and certainly no 'smart thermostats' like Hive etc.  TRVs are OK if they must and can be useful to limit (but not control) temperature in rooms with high thermal gain or cooking, BUT you will be setting most of them to max or even removing the heads so they have no effect.  This is pretty much the polar opposite to what we have learned (in many cases wrongly) about controlling boilers!)  The controller for the heat pump needs to be sensibly sited in a representative room NOT in a cupboard.  Some have wireless controllers which you can put where you want which is probably the best.

Posted by: @jamespa

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Dont get this.  If you shut down sections sequentially so there is only one route through at any given time that cant happen.  Something of a PITA but less so than taking up all the floorboards!

I think the concern was about crud being forced from the manifold into the microbore. Also difficult to shut down any sections as all the rads run on a single zone valve. 

Posted by: @jamespa

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If you know your annual gas consumption in kWh then divide it by 2000h, and separately divide it by 3000h

Yes I've done that and I get a lower range of 4 - 6 kW. Also I've been monitoring my gas usage on specifically cold days and using the spreadsheet provided by Protons for Breakfast.  It's averaging at about 5kW although I think it might be under reading a bit as the kitchen and utility can struggle to get to temperature but that's because the rads are not big enough.  I did my own heat loss calc and it came in at 7.5 kW.  So the jury is still out on the actual heat loss.  The house is a mix of 90s timber frame and old stone cottage with internal timber frame.  I've insulated as well as I can given the age of the building whilst also being aware of maintaining ventilation to avoid damp build up. 

Posted by: @jamespa

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Sounds a lot, is this after grant or is the real price £25K? 

That was before grant.  It included a buffer tank so that put me off as I'm aware of the reduction in efficiency is would cause.

Thanks for your help.

@iotum, this is a really good question and you’re right to be cautious rather than just chasing whatever’s cheapest or fashionable.

Like @jamespa, I didn’t know enough about MLCP to give a confident, first-hand answer, so I reached out to installers and engineers in my own professional network who actually use this stuff day in, day out. 

The short version is this: MLCP is not new, not experimental and not inherently risky, but it does need to be designed and installed properly, especially for heat pumps.

On track record, there was strong agreement. Craig Gilhome said he’s been using MLCP for over 20 years in both new build and retrofit work. Jean Fourie and Craig Brookes both highlighted how common it is in commercial and renovation projects. This isn’t some new “microbore 2.0” experiment... it’s a well-established pipe system with a long history behind it.

On why installers like it, @damon, @saenergy and Ryan Sharp all pointed to the same advantages: long continuous runs with far fewer joints, which is a big win in retrofits and awkward properties. Pre-insulated MLCP for primaries came up repeatedly too... cheaper than copper plus lagging, quicker to install and far tidier.

Simon Smith (@saenergy) said one recent retrofit would have been “a mare” in copper, but MLCP went in easily because of the pipe routes.

Water quality and system longevity also came up a lot. @damon, Alexander Rovacsek and David Humm all highlighted the permanent oxygen barrier from the aluminium layer, which is particularly important for heat pump systems. David Humm mentioned running over 150 metres of MLCP in his own heat pump install with only a single joint (excluding radiator terminations), using manifolds to dial in accurate flow rates around the system.

Where everyone was very clear is that design matters more than the pipe material.

Jean Fourie, Ryan Sharp and Alex Winters all warned that MLCP is not a like-for-like swap with copper. The internal bore is smaller, so you generally need to go up a nominal size compared to copper. Get that wrong and pressure drop becomes an issue.

Fittings and elbows are the other big gotcha. Ryan Sharp described MLCP elbows as having very high resistance... in some cases equivalent to several metres of straight pipe. Because of that, experienced installers like @damon and Ryan Sharp avoid elbows wherever possible, favour swept bends or physically bend the pipe, even at large diameters. If MLCP is installed like copper, with lots of tight elbows everywhere, performance will suffer.

Alex Winters made the point that while MLCP can be cheaper on materials, it can become more expensive if fittings pile up or extra labour is needed... again, coming back to design and installer competence rather than the pipe itself.

On tooling, a couple of people noted that MLCP does require specific press tools, which some domestic plumbers don’t have. That’s not really your problem as a homeowner, but it does mean you want an installer who genuinely uses MLCP regularly, not someone “having a go” for the first time.

There was also a clear distinction made by several contributors (including Alexander Rovacsek and Ethan Wadsworth) between heating and potable water. Many are perfectly happy running heating systems entirely in MLCP, but still prefer copper for drinking water out of conservatism and familiarity, rather than because MLCP can’t do the job.

Bringing this back to your situation: you’re right that 8mm microbore is far from ideal for a heat pump. A full copper repipe isn’t the only credible solution. MLCP is a perfectly valid alternative if it’s correctly sized and installed by someone who understands flow rates and pressure drop. The real risk here isn’t the pipe... it’s poor design and lazy installation. 

If you were nervous about MLCP five or ten years ago, that would have been understandable. In 2026, with this many experienced installers using it successfully, I wouldn’t see MLCP as the risky option based on what the guys have related. I’d be far more concerned about who is designing and fitting it.

I wonder if anyone else on the forums has piping done in MLCP.

Hope that helps.

Thanks Mars and thank you for reaching out to so many people on my behalf. I really appreciate the effort.  

I think my house fits the definition of an "awkward property". I've tried to figure out how to make some of the pipe runs whilst satisfying aesthtic needs, efficiency and ease of install and it will be challenging.  So it's good to know there are options that could make the job easier and I'm now a bit more informed which is always a good thing before starting off.  

Posted by: @jamespa

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You should be able to do 750W comfortably through 8mm id pipe without impacting efficiency so if no rad is  over this size no problem.  Even if they are you may well be able to get away with it with a bit of finessing of pumps/DT etc., or just replump one section.  Personally I detest throwing out perfectly good kit and a full replumb is pretty disruptive.

I'm sorry, but it won't.

You can't advise on a job which you can't see because there are so many variables. The fact of the matter is if 8mm plastic was changed for something which flows better then efficiency will go up, electric use will go down.

This is one reason so many installs are going wrong, they didn't go to the expense of replacing the pipework.

@editor Lots of good info in that post. Have you or anyone else done a post or video examining and explaining fluid dynamics, how it's effected by bends, fittings, pipe diameter etc?

Posted by: @deltona

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You can't advise on a job which you can't see because there are so many variables.

I agree, I wouldn't and I didn't.  That doesn't however mean that one cant give general information based on reasonable assumptions, otherwise this forum becomes entirely pointless.

Posted by: @deltona

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The fact of the matter is if 8mm plastic was changed for something which flows better then efficiency will go up, electric use will go down.

... er why would it make a material difference to electric use so long as its within sensible limits and you can maintain sufficient flow to get an acceptably low deltaT?  If you push this argument to its logical extreme lets re-plumb everything in 35mm right up to the radiators and aim for a deltaT close to zero. 

The cost and disruption of a complete re-plumb if it isn't necessary is going to overwhelm by a massive amount the cost of a few extra watts of water pump power, if it can be done that way!  

Posted by: @deltona

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This is one reason so many installs are going wrong, they didn't go to the expense of replacing the pipework.

The main reasons we see here are:

• Unnecessary buffer
• Massively oversized heat pump (very occasionally undersized)
• Heat pump incorrectly commissioned/operated.

My personal experience, which of course may not be typical, was that several installers wanted to replace pipework in my house unnecessarily, NOT the other way round.  Of course there are times when pipework needs to be replaced and in that case it should be replaced, but the design needs to be done properly before making assumptions that existing pipework isn't adequate.

Posted by: @jamespa

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I agree, I wouldn't and I didn't.  That doesn't however mean that one cant give general information based on reasonable assumptions, otherwise this forum becomes entirely pointless.

You did, unless it's been edited out it's up there in print.

You are indeed 'giving general information based on reasonable assumptions' thus making 'this this forum entirely pointless'. It then becomes Facebook.

If you read this thread you'll see the response of experts. That's what we want to be reading.

Posted by: @jamespa

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plastic pipe is rougher than copper pipe so for any given external diameter you get less water through.

Is this fact or fiction? If fact then where is the irrefutable evidence due to testing? Having worked with it I can tell you it's smoother than copper inside.

Posted by: @jamespa

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... er why would it make a material difference to electric use so long as its within sensible limits and you can maintain sufficient flow to get an acceptably low deltaT?

You don't know that it is which is why there is no 8mm/750w rule, it's rubbish. You can have a very poorly fitted 8mm system or a very good one. The difference between the two is huge.

You've then gone on to answer your own question with:

Posted by: @jamespa

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 extra watts of water pump power

The difference between a good flowing system and a poor one can be double the amount of pump electric usage and number of pumps.

Posted by: @jamespa

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If you push this argument to its logical extreme lets re-plumb everything in 35mm right up to the radiators and aim for a deltaT close to zero. 

Which is just silly and wouldn't work either. There is an ideal pipe diameter, go either side of it too much and the system loses efficiency.

ASHP work on the principle of everything being perfect to be efficient. If anything is less than perfect then the energy use will rise accordingly.

Posted by: @jamespa

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My personal experience, which of course may not be typical, was that several installers wanted to replace pipework in my house unnecessarily

But it isn't fact, it's just your unprofessional opinion. If you had it done and enjoyed a higher SCOP then it would be useful information.

@deltona 

I did draft a long point by point response but on reflection it isn't worthwhile.  Suffice it to say that I find some of your wording quite offensive and out of kilter with the tone of this forum. 

Instead I will just make two statements:

a) 750W is from Heat Geek, based on 0.9m/s and DT5, both of which are quire reasonable.   Of course it may not be possible to get 0.9m/s if the pipework is too long or badly installed, but if it is then 750W is what you will move at DT5/0.9m/s.  

b) The thrust of my posts here, which I think I made abundantly clear, was that given the disruption and cost that re-piping causes., existing microbore should not necessarily be dismissed without doing the design,   Even it requires an extra water pump to achieve sufficient flow rate, the additional 35-50W consumption may pale into insignificance compared to the re-piping cost and disruption.  That should be a choice for the householder once a proper design has determined the options.

I will also ask two questions of you:

1. You said that "there is an ideal pipe diameter, go either side of it too much and the system loses efficiency."  Can you please explain the mechanism whereby an oversized pipe makes the system less efficient

2. From the tone of your responses I get the feel that you may believe that microbore can never work in a heat pump system.  Is that your belief and if so on what basis?

Finally, I am quite happy with my own system thank you.  The proof of the pudding is in the eating, and I certainly would not have wanted the unnecessary pipework replacements that some of the more gung-ho installers were suggesting unless they were absolutely essential, which evidently they were not.  It's my house so its my choice!