Welcome to the forum @penval.

The key thing to understand with heat pumps is that you’re no longer “blasting heat” into the house like an oil boiler would. You’re moving heat gently and continuously through water at a lower temperature. The entire system is designed around shifting a steady volume of warm water through emitters, rather than short bursts of very hot water.

Because of that, performance comes down to two things: how much heat the radiators can emit at lower temperatures and whether the system can move enough water to carry that heat around the house. In simple terms, the heat being delivered is a function of flow rate and temperature difference, so if you drop the temperature (which you do with a heat pump), you need sufficient flow to compensate.

This is where microbore becomes a consideration… not automatically a problem, but something that needs proper assessment. Smaller pipes can restrict flow, especially if runs are long or the system layout isn’t ideal. That can limit how much heat you can move, regardless of how good the heat pump itself is.

That said, ripping everything out is often a knee-jerk reaction. If the pipework is well laid out, circuits aren’t overly long and the system can achieve the required flow rates, microbore can and does work with heat pumps. The deciding factor isn’t the pipe diameter on its own, it’s whether the system can deliver the required flow at a reasonable pressure without causing issues like noise, imbalance or excessive pump strain.

A decent, competent installer will be able to the calcs and see what’s possible.

Radiator sizing is the other half of the equation. As flow temperatures drop, radiator output drops significantly, so they need to be sized correctly for low-temperature operation. If that’s done properly, you reduce the demand on the system and make everything easier to run efficiently.

A good installer should be measuring and calculating this, not guessing. That means checking heat loss room by room, confirming required emitter outputs at lower flow temperatures and then verifying the system can achieve the necessary flow rates through the existing pipework.

So the “right” approach is neither blindly ripping it out nor blindly keeping it… it’s assessing whether your current system can move the heat the heat pump will produce. If it can, keep it. If it can’t, then targeted upgrades are far better than a full repipe.

Posted by: @penval

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Installing ashp in early 2000 homes. We have a 2010 self build which we bought in 2013. Insulation is good but down stairs has a floating floor. We’re going to move from oil to ashp. We’re getting mixed messages about the existing system which is microbore pipes. Some advise ripping it out and re piping. It’s going to be messy. Others say that with modern ashp as long as the radiators are sized right it doesn’t matter. What is the right approach?

I am soon going to install and review an Adia Thermal connected to a Vaillant, microbore drops 10mm, no rad changes and no DHW change, they claim providing the rads are somewhere near correct size (they can be slightly under) they claim that this system works fine on microbore, manages and matches the ASHP, tariffs and pump speeds to minimise losses while TRV controlling every room. These are new to market but have a good length of testing so should be good.

I will be posting the installation, process and review on performance during May and onwards. It may be a solution for you? 

@ashp-bobba bang on, and I can't reveal too much but there's another product that might be entering the market later this year that promises to commission and actively balance systems. These are all innovations that should hopefully start to make heat pumps work that little bit better.

@editor Nice, its about time, we need some professional add ons that help on that side of things to make these systems a bit more cost effective.

I can se I will get excited and be added an ASHP system to my shed to try that one out, I am running out of buildings to put test rigs in lol...

Oh I know I could offer trial sites at cost and try all this new stuff out 🙂

Hello @Penval

unfortunately, now the winter is gone and you missed that opportunity to mimic heat pump operation with your present oil burner to test the hydraulic system on usefulness. You can try anyway but results will less clear at mild outside temperatures.

Before you go on have pen and paper at hand to document any status quo before you start shifting levers (young people however will take pictures with their smart phones).
Procedure how to operate a heat pump is all over town already; turn the flow circulation pump to full blast and open any thermostatic radiator valves (TRVs) fully. Now control the system only from the weather compensation (WC) with a minimum possible flow temperature to keep rooms warm enough. Make notes of outdoor temperature, flow temperature and all return temperatures of the particular radiators. There might be rooms too cold by trend and some too warm .... keep records.
When there are lockshield valves ('RLVs')in the return pipes of the radiators, you might try manipulating them by throttling 'fast' or encouraging 'slow' emitters. Take double care then to remember their original setting. Have in mind that valves come with a 70/30-30/70 characteristic: at 30% opening the flow will be 70% already, the last 70% opening add 30% flow only. When thin tubing is involved, the flow is limited anyway and the whole action may be in the first quarter-turn of the valve. Mechanics use to think in 'spanner flats' (sixty degree angle) and 'half spanner flats' (thirty).

When you find out after a while that, let's say, flow temperature (FT) is around 40°C when it's outside 10°C, you may conclude that it will be 60°C at 0°C and find the latter to be scarce.
With cooler FTs it will be more promising, at even warmer flow temp. level you will want to get drastic.
Also return temperature will play a role; generally low levels indicate little flow; when little flow can increased e.g. by a stronger circulation pump, this means that for a constant heat supply FT can be lowered to please the heat-pump-to-come.
When overall results come out indistinct, you may consider to deliberately choose a too small heat pump .... boosting the living room with an oil filled radiator at some ten days per year may be more comfortable than laying waste to your lovely decoration.
When selecting a heat pump type, look at which temperatures which power is supplied -- two examples saying '10 kW' might be very different in detail. Also take care of the minimum flow requirement, e.g. 1000 l per hour will be 'ouch', 400 would be 'aaah'.
Last-generation R290 (propan) pumps are said to supply high flow tempeatures while not sacrificing too much efficiency, but when comparing figures, I find no big differences. Anyway figures and reality may be different matters ('the difference between theory and practice is bigger in practice than in theory'). However many propan pumps are limited in their minimum power generation to roughly 35% of nominal value, which seems to come from lubrication issues relating to the refrigerant. R32 pumps can get to lower compressor rpms in average, at least with less trouble or risks.

The rented 45-m²-two-bedroom-ground-floor-flat I'm sitting in at the moment, comes with a 30 kW Worcester compact gas boiler and, I think, nominal 6 kW of panel radiators installed with microbore piping. The Worcester manual says 'DHW minimum heat input 7.3 kW', and on full blast (85°C FT) and all TRVs fully open, the boiler heats without cycling, so I assume that it puts plusminus 7 kW into the system. When the wife moved in here, of all things the living room radiators came slow, which I could tweak easily by turning the lockshield valves.
When 7 kW heat transport take place at some 60°C temp. difference between room and radiators, it may be like 2000 W at 20°C, so fourtysomething centigrade FT.
The flat is made of cavity walls, already blown out, with nice new windows featuring low-E-glass, so I assume that the overall heat loss will be like 60 W/K and 2000 W cover nearly everything even when the tenant in the first floor will be absent with his heating turned off.
It is difficult to get climate data here (compare e.g. https://www.stiebel-eltron.de/toolbox/klimadaten/ and cry bitterly) but I found some official website saying that we can expect half an ice day per year -- I think, as long as there's outdoor plumbing downtown, there's not much of frost to expect.
So, bottom line, the said flat here would qualify for a heat pump easily, just as an example.

Now it's your turn, and later, tell us what came out!