Posted by: @roel

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plumber also claimed the buffer tank was necessary to prevent short cycling when heating DHW.

No it isn't. Make sure he has installed a DHW cylinder with a 3m2 coil. If they haven't ask why not.

Posted by: @roel

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We have UFH throughout seperated into 8 zones controlled by separate room thermostats with manifold valves

That's why you need a buffer, get rid of the thermostats and manifold actuators you will not need a buffer.

Posted by: @roel

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Samsung Gen6 12kW ASHP

Sounds big for a new build, unless it's a massive house?

@johnmo 
Agree. It's a 180m2 bungalow with modern 150mm cavity insulation.
When questioned the developer, he said 'well, just to be sure it will handle it'... 

The reason the other heat pumps failed may be that the houses have zones, NOT good with heat pumps, and as the pumps is big then it needs a minimum water volume. Quite frankly you ( and the others on your site) may have got bigger problems than heating an extra 50ml of water.

if you club together with other owners get a Heatgeek elite guy in to survey what needs changing. Then your builders plumber can do the changes needed.

I agree we need the buffer tank when using thermostats (not ideal), but my main question is, should it be placed in the primary circuit i.e. in series with the HP? Would it not be better placed on the other side of the controller i.e. in the UFH circuit before the manifolds? At least we're not heating it then in summer.
And if so, should it be 'in series' with the pump or in parallel as if it was a extra radiator?   

Posted by: @roel

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50L buffer tank in line with the hot pipe of the ASHP i.e. on the primary side.

So I take you mean not a buffer but a volumiser. So hot in, hot out, and installed upstream of the diverter valve. It really needs to on the CH side of the diverter only. Other wise you are heating an additional 50L of water for no reason.

With all those zones the heat pump will have no place to pump all the water around 2m³ an hour. If it is volumiser and not a buffer it's not going to help much unless you also have min flow valve. System is just chasing its own tail flow rate wise.

@johnmo 

Posted by: @johnmo

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So I take you mean not a buffer but a volumiser. So hot in, hot out, and installed upstream of the diverter valve. It really needs to on the CH side of the diverter only. Other wise you are heating an additional 50L of water for no reason.

Ah, didn't know it's called a volumiser when placed upstream of the diverter, anyway extacly my point heating 50L for no reason.

Posted by: @johnmo

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With all those zones the heat pump will have no place to pump all the water around 2m³ an hour. If it is volumiser and not a buffer it's not going to help much unless you also have min flow valve. System is just chasing its own tail flow rate wise.

I don't know this detail. I'll speak to the plumber soon and see what he says.

Thanks so far.

@roel welcome to the forums. Please post a photo of your system.

Historically, buffer tanks played a crucial role in heating systems, designed to address the limitations of on-off fossil fuel boilers that operated at high temperatures to prevent corrosion. These boilers required a low flow rate, and buffer tanks helped manage the differing demands of connected circuits by blending down high temperatures efficiently. However, this approach is outdated when it comes to heat pumps, but inexperienced installers still use them, as was the case in your system (and ours).

The notion that buffer tanks help heat pumps "smooth out load" is a misconception. In reality, a 100-litre buffer tank holds only 1.17kW of energy at a 10°C higher temperature than required, providing just 7 minutes of buffer time for a system with a 10kW load. Achieving this extra temperature can also increase energy bills by up to 25%.

Modern heating systems with modulating boilers and heat pumps are designed for efficiency and precise control, making buffer tanks unnecessary and even detrimental by blending down temperatures. Bizarelly, most manufacturers still recommend buffers due to design limitations in their heat pumps, but this is a workaround rather than a true solution, often masking underlying design flaws. 

Posted by: @roel

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- I noticed the legionaires cycle seems to be done using the immersion heater only heating the water up to 60C. The HP does not seem to be used here. I need to go through the settings, but I'm thinking about disabling it altogether anyway. (Unvented tank, Chlorine in water etc.

Hi @roel. Welcome 🙂

The Samsung Gen6 12kW ASHP can heat water up to 65degC, which would charge your DHW tank to 60degC within a reasonably short time, especially using a 12kW unit! If you were happy to have DHW at 60degC then it's more cost effective to use the ASHP to do this, and provide for Legionella sterilisation, rather than using a 3kW direct electric immersion. You'll probably find the cylinder contents heating is more uniform and effective with the ASHP than with the immersion anyway. System volume isn't an issue with DHW generation as the cylinder contents absorb heat quickly, and also provide a source of heat into the hydronic circuit for defrost should this be required mid-charge of the DHW cycle.

For the m2 size of the bungalow and UFH I'm at a loss as to why the developers thought it necessary to use 8 zones all with room thermostats. They usually do this through ignorance and a lack of understanding about hydronic heating, minimum circuit volume scenarios, and how to maximise efficiency.

You've got a scenario [if I'm correct] where one zone may call for heating, and energise a 12kW heat source. The heat source is way too big for the zone heating load, even on minimum turn down, which will be around 3 - 3.5kW. The ASHP will cycle on and off and it will be impossible to have a stable weather compensated leaving water temperature. That's inefficient. You'll probably find this problem persists even when multiple zones are calling for heat, as the combined thermal load demand may still be lower than the ASHP minimum turn-down output.

12kW sounds oversized, probably getting on for 100% oversized, if a 180m2 new build is well insulated and compliant to current Part L building regs. That fundamentally compromises the efficiency of the ASHP as it will spend a lot of time cycling on and off at minimum turn down when the dwelling heating demand is low. Buffers and volumisers add volume on the primary side, and they have their uses, but in your case they are a sticking plaster over what appears to be a fundamental problem caused by a poor hydronic system design with 8 thermostat controlled zones mis-matched to an oversized ASHP. When just one or two zones are calling for heat there will be inadequate system volume in circuit to meet the ASHP's minimum volume requirements, hence to buffer/volumiser, hence likely also the issue the developer has cited about problematic defrost.

@judith has an excellent idea. Talk to other owners in your development with identical systems, club together and ask a Heat Geek or an independent expert renewable heating consultant to review the design and fitness for purpose. The recommendation they will likely come up with is to get rid of a far-too-complicated multi-zone control, open zone the system control, to bring all system volume into play all the time, and thereby give the ASHP a chance to achieve a stable weather compensated control with constant volume flow on primary and secondary side. Manual balancing can adjust for preferences to have some areas warmer than others.

They would also be able to review or perform a new heat loss calculation to check if the 12kW ASHP is grossly oversized. NHBC guide to heat pump design and specification has some good stuff, but is very top level. It uses data borrowed from CIBSE et al for infiltration/exfiltration AC/h and zone design temperatures. AC/h values are probably over-estimated however. It doesn't cover the detail of the hydronic circuit design, and this is where things usually go wrong.

Posted by: @allyfish

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The Samsung Gen6 12kW ASHP can heat water up to 65degC, which would charge your DHW tank to 60degC within a reasonably short time, especially using a 12kW unit!

The maximum DHW temperature on the 12 kW Gen 6 Samsung is 55C. The Gen 7 may be higher. 

Posted by: @mike-h

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Posted by: @allyfish

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The Samsung Gen6 12kW ASHP can heat water up to 65degC, which would charge your DHW tank to 60degC within a reasonably short time, especially using a 12kW unit!

The maximum DHW temperature on the 12 kW Gen 6 Samsung is 55C. The Gen 7 may be higher. 

Hi @Mike, says 65degC on the Samsung Data sheet

Other data sources support this:

https://www.elementshop.co.uk/samsung-12kw-air-source-heat-pump-170l-cylinder

https://www.theheatpumpwarehouse.co.uk/shop/heat-pumps/air-source-heat-pumps/samsung-heat-pumps/samsung-12kw-r32-monobloc-air-source-heat-pump/

Posted by: @roel

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@johnmo 
Agree. It's a 180m2 bungalow with modern 150mm cavity insulation.
When questioned the developer, he said 'well, just to be sure it will handle it'... 

This site (a distributor) says the minimum volume is 50L so that is where the value comes from. I suspect it has to go in the primary because there needs to be system volume when neither dhw nor heating is active. https://midsummerwholesale.co.uk/buy/samsung-heat-pumps/samsung-stealth-12-heat-pump. I don’t know if this is gen 6 or 7 however.