@squeakysim, the document you’re after is Microgeneration Installation Standard: MIS 3005 and you can download it here and go to page 14: https://renewableheatinghub.co.uk/renewable-heating-hub-download-directory

Posted by: @squeakysim

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I assume to compensate for this, you can just get it designed to a higher flow temp and then just keep it lower in the real world?

Yes, that’s right, provided that it’s not set too low as you want to avoid the heat pump cycling.

@editor what does 'too low' look like? Not that we're ever likely to hit it, but from what i've read I'm guessing 35 is the minimum you'd want to run it at?

Posted by: @squeakysim

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I’ve been away doing a lot of additional reading, watching you tube and sending email enquiries since the last time I was here! In addition to having a play with the heat loss calculation from sources provided here.

Which leads me to a question I’m looking for an answer on! 

I sent this heat loss to an installer and they’ve told me by regulation, the system has to be designed to 21c for all rooms except the bath (22c) or a store room (18c). Is this true…?!!

Considering mum prefer temp at 18-19, wouldn’t this be pointless…?

So do I just design the system to a higher flow temp to counter this?

As far as I am aware, the temperatures quoted are the agreed 'standard' for properties in the UK. They are the ones used when carrying out the heat loss calculations, and are probably the temperatures that most people would find acceptable. The 'standard' allows both heat loss calculations, and the offered design, from different companies to be compared, since they should all be 'singing from the same song sheet'. 😎 

The final users can of course operate their system at different temperatures, within the capabilities of the system supplied, but it should always be able to meet the 'standard'.

Your Mum can operate the system at lower temperatures, which will no doubt be more efficient and cost effective, but should she wish to sell the property in the future, the new owners may wish to have higher temperatures. It must therefore be designed to meet the present minimum acceptable standard.

My wife would prefer a temperature of 23C or even 24C in the living areas, so I had to quote the 'standard'. Standards can be useful. 😜 

Posted by: @squeakysim

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@editor what does 'too low' look like? Not that we're ever likely to hit it, but from what i've read I'm guessing 35 is the minimum you'd want to run it at?

This’ll depend purely on the house. In our property, 35C flow temperature will struggle to get rooms above 17C when it’s less than 5C outside. Running weather compensation would be optimal in this scenario on a curve that would deliver the temperatures your mum prefers. The key is to avoid cycling.

Posted by: @squeakysim

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@editor what does 'too low' look like? Not that we're ever likely to hit it, but from what i've read I'm guessing 35 is the minimum you'd want to run it at? 

Heating one's home is a balancing act, and it is even more so with an ASHP.

Consider the following example.

If the heat loss calculation predicts a figure of 12kW per hour at an ambient temperature of -3.2C, then the heating demand for an indoor air temperature of 19C will be approximately 9.4kW at 0C, 6.5kW at +5C, 4.5kW at +10C and only 2kW at +15C.

Now consider the heat demand for an indoor air temperature of 21C. 10.4kW at 0C, 7.9kW at +5C, 5.5kW at +10C and 3kW at +15C.

If a 14kW Ecodan heat pump was installed, then for an indoor temperature of 21C and an amb. of 0C it would need to run continuously to meet the heat demand. The same would be true at +5C and +10C, but as the amb. air temperature rises above 10C the heat pump would be operating at its minimum conditions, but would now be producing more heat energy than is required to meet the demand. If running on WC mode the heat pump would start to stop the compressor for a period of time, with the running period reducing as the amb. air temperature increases, until at +15C the compressor may only be running for 45% of the time. The compressor would therefore be cycling.

With an indoor air temperature of 19C, the cycling would commence at approximately +8C.

I do believe that most if not all modern ASHP are designed to cope with cycling, but it would be wise to check with any prospective supplier as to the maximum frequency acceptable.

One possible way to reduce the frequency of cycling during milder weather, may be to use an on - off type thermostat during such operating conditions. A thermostat may then run the heat pump for a slightly extended period and raise the indoor air temperature slightly higher, which would therefore take longer to cool down before once more running the heat pump.

Posted by: @derek-m

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My wife would prefer a temperature of 23C or even 24C in the living areas, so I had to quote the 'standard'. Standards can be useful.  

Hahaha!

@derek-m thanks for that, you have basically prempted my next question around cycling and what it actually is! However I will still clarify (in even simpler terms!) - a cycle is the point from when the heat pump switches on, to the point it reaches the desired temp at which it switches off again?

You mention a room thermostat being the switch that turns it off, but how would it do it otherwise? Is there a sensor that detects return temp or something?

Posted by: @derek-m

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With an indoor air temperature of 19C, the cycling would commence at approximately +8C.

Would then reducing the flow (or employing weather comp), reduce the amount or possibility of cycling...?

Over here I've given some background information about HP cycling and the adverse effect it has on the mains grid.

Since permission is required from the regional DNO to install a Heat Pump, they can monitor these issues and calculate where the transformers require upgrade. This gets mapped onto a plan called the Distribution Future Energy Scenario.

You can see the NGED DFES displayed as an interactive map, covering their four regional licence areas up to 2050. This allows Councils and Community Energy Groups to illustrate the effect of Nett Zero investments in their local area.

@squeakysim

Like a gas boiler, or an oil fired boiler, a heat pump produces heat energy, which in the UK is normally transported to the heat emitters using water as the transport medium. The most basic system could consist of a gas boiler controlled by a thermostat. When the indoor temperature falls the thermostat switches on the gas boiler and water pump. Once the indoor temperature increases to the setting of the thermostat, the gas boiler and water pump are switched off. This cycle will repeat whilst heating is required, and the frequency of the 'cycling' will be dependent upon the heat demand.

The most basic way of controlling an ASHP is in a similar manner, with the LWT set at a fixed value of say 50C, and the heat pump being started and stopped by a suitable thermostat. This is the most inefficient way of controlling a heat pump, and obviously does lead to cycling.

As far as I am aware, modern heat pumps can control the speed of the compressor, the speed of the water pump and some may also be able to control the speed of the fan. Let's consider the affect of each of these in turn.

Increasing the speed of the compressor will cause it to work harder and produce more heat energy, and in the process this will increase the LWT.

Increasing the speed of the water pump will cause the heat energy to be transported from the heat pump at a faster rate, but could lower the LWT.

Increasing the speed of the fan will provide more heat energy to the heat pump in the form of ambient air, so may improve its efficiency.

A more efficient way of controlling an ASHP is to utilise weather compensation, where a temperature sensor (normally located within the outdoor unit) measures the ambient air temperature. This temperature measurement is compared against the weather compensation curve and hence derives the calculated LWT. The speed of the compressor is therefore varied to try to achieve this calculated LWT. If the ambient air temperature were to fall from 10C to 5C, the calculated LWT may be increased from 30C to 35C and hence the compressor would need to run faster to achieve this higher temperature.

To transport the heat energy from the heat pump to the heat emitters, the water needs to be pumped around the system. It is my understanding that the speed of the water pump is regulated to produce a DeltaT of 5C between the LWT and the RWT. A reduction of RWT compared to LWT would indicate that there is insufficient heat energy being transferred from the heat pump to the heat emitters, and hence would cause the water pump to speed up. The speed of the water pump should therefore stabilise at a slightly higher speed once the required DeltaT of 5C has been achieved. Obviously the operation of the system is dynamic, with both the compressor speed and the water pump speed being adjusted to achieve the required objectives.

So to try to answer your last question. Running a heat pump in WC mode should help reduce cycling, within the confines of the system operating parameters. As ambient air temperature increases, there will be a point where both the compressor and the water pump are running at their minimum operating speed and cannot go any lower. If the LWT and/or the RWT increases outside prescribed limits, then I suspect the the ASHP will shutdown for a period of time, until the values fall back within prescribed limits. This of course will cause the heat pump to cycle.

Posted by: @derek-m

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If the LWT and/or the RWT increases outside prescribed limits, then I suspect the the ASHP will shutdown for a period of time, until the values fall back within prescribed limits. This of course will cause the heat pump to cycle.

At least on my Ecodan, I think the compressor shuts down if the difference between the water temperatures gets too small. I don't think it cares about the RWT otherwise. The water pumps continue to run for a while after that, but if the heat pump doesn't start pumping more heat in 10 minutes or so, they'll shut down too until the water temperature falls or, more rarely, the configurable timeouts expire.

Cycling near the minimum limit seems to be less efficient than pumping in a bit more heat than strictly necessary and then shutting off for an hour. Avoiding cycling at minimum may be part of the reason why the Ecodan's "auto adaptation" mode is pretty aggressive, but then it overshoots on the LWT a few times an hour and instead cycles 1-2/hour because it exceeded the target.

@mjr

I cannot state for definite, but as a Control Systems Engineer I would expect both the LWT and the RWT will be used in the control of the speed of the internal water pump of the ASHP. The DeltaT between these two measurements, as far as I can see, is the only way that the controller can be aware of what is happening to the heat demand. When the heat pump is operational, it is afterall a continual balancing act, between changes in the LWT and changes to the RWT.

I think that the Ecodan controller periodically runs the water pump for several minutes, when the compressor is stopped, to equalise the temperatures around the system, to try to ensure that it is getting reasonably accurate information.

Having thought about what may be happening when a heat pump is operating at the lower end of its capabilities, I agree that using an on - off type of control under these circumstances may be more efficient than the normal controller running WC. I have that type of control for our gas boiler, but rather than a thermostat I use an industrial PID Controller, the control parameters and response of which I have adjusted to provide indoor temperature control accurate to +/- 0.2C.

I would be interested to know the operating conditions under which you tried auto adaptation. One reason for large changes in LWT could be stratification when the compressor and water pump shutdown. Changes in temperature whilst the system is operating, could be due to an inadequately mixed water flow past the temperature sensor, which alternately heats and cools the sensor as the flow rate changes. I have experienced this on several occasions in the past on industrial systems, where the temperature sensor has been located too close to the heat source.

Posted by: @editor

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@transparent, the quick answer is if you want your government grant as part of the Boiler Upgrade Scheme (BUS) the installers must be MCS accredited. There’s no other reason to get an MCS accredited installer as it doesn’t guarantee a competent installation.

The other reason for MCS is around permitted development

"Development is permitted only if the air source heat pump installation complies with the Microgeneration Certification Scheme Planning Standards (MCS 020) or equivalent standards. Read more about the scheme."

It doesn't have to be accredited but it does have to comply.  If it doesn't you need specific planning permission.