Great that you're thinking about this before the installation... let's run the numbers.

For a 3.5 kW unit, plug it into the formula. Aiming for an efficient delta T of 5C:

Required flow = 3.5 ÷ (5 × 0.0698) = 10 litres per minute

That's a very modest flow requirement. At 10 L/min, even 22mm pipework would probably handle it comfortably... so 25mm with a 20mm internal diameter is absolutely fine for a run of 10 metres. Pressure drop and flow velocity will both be well within sensible limits.

The 32mm proposal isn't wrong... it's just oversized for a unit of this output, and on a short 10 metre run the difference in heat loss is negligible either way.

To my understanding, where oversizing external pipework becomes a real consideration is on much longer runs with larger units, where the additional thermal mass of water sitting in wider pipe can actually slow system response. Not really a concern at 3.5 kW over 10 metres IMO, but would be interested to hear what others think.

Does anyone think it would be helpful to have this as a simple calculator on the site?

@editor I think any tool that might help one or more would-be heat pump system purchasers might be grateful - you can’t have too much useful information available and where better than on RHH?! Toodles.

Posted by: @editor

↑

Does anyone think it would be helpful to have this as a simple calculator on the site?

Not sure what 'this' refers to but to the general question, yes, definitely.  Perhaps branded as a sense check.

It needs a bit of thought and can't give a definitive answer, but can 'red flag' the likely baddies.  Happy to contribute to a review of the algorithm.

I can imagine a series of sense check tools here to reduce flag the potential cowboys.

Posted by: @trebor12345

↑

I watched Mars' video yesterday on his heat pump install "What Does a Sub-Par Heat Pump Installation Actually Look Like?"

 

Got me thinking about pipe sizing from, the outdoor unit to the cylinder inside, for my new 3.5kw vaillant arotherm being installed in a few weeks time. 

The proposal was for 32mm PEX/PE-RT/Multilayer Pipe, has oxygen barrier. The run is 10m in total length.

In this case 32mm (26 mm ID) might be too large, and 25mm (20 mm ID) might be a better size. 

Can I have your thoughts please?

 

The 32mm PEX/PE MLCP is not 26mm at the connection points, it is 22mm, I do not know the brand they are using but they are often simular to each other, these underground pipes are often an insert type double rubber O ring then compression clamp, on 32mm pipe its 26mm ID with a 2mm walled internal connection plug making the connection likely a 28mm / 1" compression to copper but reduces down to an actual 22mm inside the MLCP at connection on each end so 2 bottle necks. This 32mm selection is correct for 10m run.

If they use the 25mm pipe with a 20mm ID the bottle necks will be 16mm at both ends and under sized for the run. I would only use 25mm MLCP for mains water line, never for any ASHP regardless of the run 1m or 20m. I dont think there is a large difference in price either, maybe 10%.

Like with some other minor mistakes made, this is something that catches out even good ASHP companies sometimes, MLCP is not a good solution for any heating if you do not understand the way the system works. We have unfortunately had to quote another 2 customers a full rip and replace for systems less than 4 years old, so badly installed and designed, you can see who ever did the work, they know what they were doing with the actual plumbing, pipes are neat, solder joints look professional and then you see 2No Y plan 3 ports (both mid position) and 3 No 2 ports with 4 pumps, Bosch ASHP has almost had it, pipework is installed back to front, so flow is on return and return on flow, 2 pumps running against each other and so on... its a mess.

One of these homes is 70% MLCP and speed fit, not great TBH, we as a company have to be carful, do we, A shove the biggest pump, mod what we can and hope for the best (£3,000) or do we B change everything including the ASHP properly and offer a full warranty (£9,000) big difference, both will get the system working, one will last a couple years and cost 50% more to run the latter will work for 15+ years and come with a warranty. Question is, who has 9k 4 years later to throw at their heating system. 

70% of retro-retro fit jobs we quote are only 2-6 years old and are new builds, new builds are often in poorer shape than Grant works as they have no over-site at all, they don't even need to be MCS or audited outside of building control, and when building control pop round they are often building or construction experts not heating, ventilation or fire so they sign it off if there is some heat and hot water. Sometimes the builders are excellent and pick up the Tab, sometimes not and sometimes the owner cant even reach them but the years of unreliable hot water and the lack of winter heating year after year they eventually call us for help.

The sad part of all the above is, we go in, pipe it correctly, commission it properly, install a top brand rather than an unheard of or low brand ( I am being kind here) and they are sorted for years. This is just a skill gap on the installers side and a habitual need to get the cheapest willing to have a go engineer in to maximise profits on the sale of the new build, (not all home builders are bad, sometimes they don't know the system is wrong so it hurts them to when they have to pay)

It starts with things like MLCP sizing and ends with 5.4kW of UFH loop installed to serve a 9kW home and then the new owner finds out that once they have 60k worth of flooring laid and realise after 2 years the ASHP is not rubbish its a lack of actual UFH pipe in the floor. For every story you hear about how bad a system is I have 5 worse, I hold on to these as I don't spread the news on the very bad installs, we rescue them to maintain the dignity and for as low cost as we can, we feel their pain, we tell everyone when installed correctly they are the best form of heating on the market today, we prove this over and over. 

We have installed a lot of A2A and A2W ASHP's and they are a superior comfort compared to other forms of heating its just about this experience gap in the market.

On a lighter note, our new Adia Thermal with a 5kW Villant bolted on the back is running well on whole house micro bore pipe drops in dot and dab walls, slightly under sized rads, original cylinder (0.7m3 coil) and roughly 4.3 SCOP for 3 weeks running, heats the tank 210ltrs from 35-55 in 55 mins each time so far. Not bad at all. ( still testing very early days).

Posted by: @ashp-bobba

↑

On a lighter note, our new Adia Thermal with a 5kW Villant bolted on the back is running well on whole house micro bore pipe drops in dot and dab walls, slightly under sized rads, original cylinder (0.7m3 coil) and roughly 4.3 SCOP for 3 weeks running, heats the tank 210ltrs from 35-55 in 55 mins each time so far. Not bad at all. ( still testing very early days)

Now thats the sort of post that could break the mould, for the better!

I think I mentioned that I am on the Hertfordshire Retrofit Strategy group (as a consumer, I have no 'professional interest) and yesterday the team set itself the task of coming up with an offer for 'boiler failed in winter' scenario.  I must say I thought this was brave, clearly this scenario needs to be tackled eventually, but I wasn't personally advocating this task at such an early stage. 

One person, obviously from the industry, made the comment 'our heat pump lead times are 6 weeks, I doubt many will wait that long' to which my response, only partially tongue in cheek, was 'well that's a problem then, the local plumber will fit a new gas boiler next week - something needs to change, and its not the local plumber!

The experiment you are doing is, I would suggest, directly relevant to the 'boiler failed' scenario.  I would be willing to bet that you can now estimate heat loss of a house (within acceptable bands and probably as reliably as the spreadsheet method) from floor area, a rough idea of its construction, EPC and consumption records, or a subset of these.  With that information, the Adia kit, and an approach of not changing the cylinder, a high proportion of houses could receive a heat pump as soon as the plumber is available, with further upgrades when its convenient, not as a distress purchase.

I look forward to hearing the long term test results!

Posted by: @ashp-bobba

↑

Like with some other minor mistakes made, this is something that catches out even good ASHP companies sometimes, MLCP is not a good solution for any heating if you do not understand the way the system works.

Are you saying the MLCP (aluminum core with inner and outer layers of polyethylene) is not the right material to be used for the water pipe run from the monblock to the storage tank?

Still on topic so hopefully not too much of a hijack... we've just published our first pipe sizing calculator, specifically for the primary external run between the outdoor unit and the house. Based on the mod feedback on this first calc, we'll be following it up with an internal distribution calculator that should give some useful insight into branching circuits and whether microbore is actually viable for a given output.

Would love to know what you think... and if anything looks wrong, shout.

https://renewableheatinghub.co.uk/is-poor-flow-throttling-your-heat-pump/

@editor I ran the calculator for my 3.5kw with a run of 10m.

It said 32mm PEX ID 26mm resulted in a pressure prop of 28Pa and this was looking good.

For 25 mm PEX ID 20mm the pressure drop was 216Pa  with a yellow caution.  But still said that this was Looking Good in the text. Is the 216 Pa drop OK?

I also emailed Valliant this morning to ask them with the same question above.  They came back and said "Generally, 28mm pipe is recommended for a single fan Arotherm Plus."

So it looks like 32mm with an ID of 26mm is OK.