Posted by: @bontwoody

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you sure can at -5, but as the OAT increases your radiators cannot put out 9kW thus you are starting the cycling prematurely.

 

There sure are a lot of variables to consider here! 🙂

 

for sure there are, everything varies as much as OAT varies

Posted by: @bontwoody

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@Fazel @Derek_M

Well here is some interesting behaviour last night. I lowered the flow temperature at the top end of my WC curve by 2 degrees bringing it down from 37C at 15C OAT (top graph) to 35C at 15C OAT (bottom graph)

Although last nights was a shorter run length, cycling appeared much more quickly, COP was virtually identical. We were both warm enough in the house but the thermostat in the hall never reached the 23C set point

-- Attachment is not available --

 

Can you set the flow rate to stay high/fixed to see what difference it makes? Even if you will see a lower dT.

@fazel @bontwoody

Anyone who said central heating system design is easy lied, particularly in a retrofit situation where one of many often competing objectives is to avoid unnecessarily replacing existing elements.  It's fascinating though, if you like that sort of thing.

For years we didn't bother with design.  Just slap in a 28kW boiler, fit trvs, very roughly oversize rads, leave the flow temp at the factory default 75C, and let it sort itself out (not bothering to consider that the condensing boiler you just fitted to get higher efficiency won't actually condense, because the return temp is too high).  Simples!  We didn't even bother with weather compensation even though its been mandatory in Germany (and thus easily available) for years.

Posted by: @jamespa

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@fazel @bontwoody

Anyone who said central heating system design is easy lied, particularly in a retrofit situation where one of many often competing objectives is to avoid unnecessarily replacing existing elements.  It's fascinating though, if you like that sort of thing.

For years we didn't bother with design.  Just slap in a 28kW boiler, fit trvs, very roughly oversize rads, leave the flow temp at the factory default 75C, and let it sort itself out (not bothering to consider that the condensing boiler you just fitted to get higher efficiency won't actually condense, because the return temp is too high).  Simples!  We didn't even bother with weather compensation even though its been mandatory in Germany (and thus easily available) for years.

The above standard philosophy for gas boilers is probably one of the reasons for the existing problems with some of the present heat pump installers. Many installers may still think 'fit a higher capacity unit than is required, and that will reduce any callback problems'.

Whilst MCS seem to be setting reasonable standards in most cases, there does not appear to be any body, such as building inspector, who is actually ensuring that these standards are always being met.

Posted by: @derek-m

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Whilst MCS seem to be setting reasonable standards in most cases, there does not appear to be any body, such as building inspector, who is actually ensuring that these standards are always being met.

One of the many reasons I think that MCS should be taken entirely out of the regulatory loop.  That which needs to be regulated should be regulated through building regulations (where it affects safety or potentially violates some minimum standard of efficiency), or planning (but only to the extent that it affects other properties).  That's how everything else in the building industry works, why should heat pumps be different?

Of course government may still wish to make MCS a condition of receiving a grant, but that's a wholly separate matter to the business of regulation.

Posted by: @derek-m

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Whilst MCS seem to be setting reasonable standards in most cases, there does not appear to be any body, such as building inspector, who is actually ensuring that these standards are always being met.

Nor does any Local Planning Authority appear to have filed a submission to the House of Commons Committee which scrutinizes the Dept of Energy (DESNZ) to complain that they are required to issue Building Regulations letters of compliance to householders for work which isn't subject to inspection. Whyever not?!

I have now obtained permission to submit such evidence, although

• I don't represent a local authority
• I don't yet have a heat pump
• the closing date for such evidence has passed

🤔

Would anyone here be willing to allow me to use photographs of their heat-pump installation which has been professionally installed by an MCS-accredited company?

I don't need to identify your name or address, although the first three digits of your post-code would be useful.
Nor do I need to identify the installer.

I really need four or five photos which show incorrect/poor pipe insulation

1. where pipes enter the external unit
2. where pipes pass through an external wall
3. around valves within the house (ie the insulation only covers the pipes themselves)
4. where insulation has been compressed by cable-ties or wall-brackets
5. where rain-water can enter between the pipe and the insulation

Thanks

Posted by: @transparent

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they are required to issue Building Regulations letters of compliance to householders for work which isn't subject to inspection. Whyever not?!

Because it's been done in a completely muddle headed way.  The requirement for accessing a grant has been conflated with both planning and building regs and handed to a body that is controlled by the industry and set up as a standards body not a regulator. 

It's just all wrong and needs to be sorted out so that it fits into the way everything else works.  But MCS are apparently the darlings of government so it's unlikely to happen any time soon.

Posted by: @jamespa

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But MCS are apparently the darlings of government so it's unlikely to happen any time soon.

That's why the accreditation/certification issue needs submitting to the Commons Committee.

It's they who hold the government to account.

The Chair is Angus MacNeil MP for Na h-Eileanan an Iar (Outer Hebrides). He's now sitting as an Independent, having declined to accept the whip of the Scottish Nationalists following a dispute.

His constituents have great experience of energy matters.
For them, it's important that it works properly, rather than merely gets thrown into place by someone with accreditation.
We could hardly ask for anyone better to challenge the status-quo.

@fazel

"Can you set the flow rate to stay high/fixed to see what difference it makes? Even if you will see a lower dT."

I think its possible, but the pump is quite inaccessable so I will try that after Ive played about some more with the WC curve

Posted by: @squiff

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Great thanks everyone, lots of food for thought.

So the key takeaway is to not trust the nameplate capacity, and you have to size instead based around the nominal capacity (usually listed as -2c).  And, for some models that's basically the same (e.g. the Ecodan 11.2kw still outputs 11.2kw down to -5c), and for others it's a massive difference (the Daikin 16kw HT drops to 11.5kw at -2c).  So in this case the 11.2 Ecodan and the 16 Daikin are on a par.  Confusing!

Posted by: @derek-m

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Question the need for such items as a buffer tank or plate heat exchanger, and if absolutely necessary then make allowance for these in the sizing of the heat pump.

I was chatting to my proposed installer yesterday, he said I would need a buffer tank (in fact 2, one for the rads and one for underfloor heating).  I asked why, he said to deal with the expansion of the water in the heating system as it's heated, and to provide a buffer of hot water used when the heatpump does a defrost cycle.  I've goggled needing a buffer tank with a heat pump and there are lots of articles saying that it's necessary.

You seem to be saying that a buffer tank is only rarely necessary.  Why do you say that?

 

Quite. It is necessary to ask a prospective installer to provide a copy of the capacity data tables for their proposed heat pump and see how that compares with accurate heat loss calculations. If an installer cannot provide such data, and justify their heat pump selection, then it may be time to search for a different installer.

To answer your query relating to buffer tanks, the first thing to clarify is if your prospective installer actually knows the difference between a buffer tank and an expansion vessel. If they don't then again it may be time to seek a different installer. A buffer tank will not deal with the expansion of the water as it is heated, whilst an expansion vessel, by virtue of its name, will.

A typical system would normally have two expansion vessels, unless the hot water cylinder is a vented type. One expansion vessel for the primary water side, the water that carries the thermal energy from the heat pump to the radiators, UFH loops, DHW cylinder etc. A second expansion vessel would be required on the hot water system if the hot water cylinder is un-vented. If you Google 'expansion vessel' you will be able to see some photo's and probably an explanation of how an expansion vessel works.

Your installer's comment about requiring two buffer tanks, one for radiators and one for UFH, would cause me concern. Is your installer also proposing a system that runs at a high, fixed, LWT, along with zone valves, room thermostats and TRV's? If so, that would probably be the reason for also installing buffer tanks. Maybe time for a different installer, since your prospective one does not appear to understand heat pumps and how they should be operated and controlled.

Let me explain the possible problems that a buffer tank and the other equipment may create and then you will be better armed for discussions with prospective installers.

Installing a buffer tank would create the requirement for an additional secondary water pump, since the primary water pump 'pushes' the water into the buffer tank whilst a secondary water pump is required to 'suck' the water out of the buffer tank.

The speed of at least one, if not both, of the water pumps needs to be controlled, so that the flow rate going into the buffer tank is the same as the flow rate coming out. It may be necessary to measure the flow rates in and out of the buffer tank, or at least the temperatures of the water flowing, so that the flow rates can be balanced.

If the flow rate going into the buffer tank is greater than that coming out, then not all the thermal energy being produced by the heat pump will be transferred to the heat emitters. This may therefore cause the IAT to fail to achieve the desired level. To achieve the desired IAT would require the LWT to be increased, which I think is now fully understood will reduce the heat pump efficiency. It could also lead to premature cycling.

If the flow rate going into the buffer tank is lower than the flow rate coming out, some of the cooler water returning from the heat emitters, will mix inside the buffer tank, with the warmer water flowing out to the heat emitters. This again would mean that the heat emitters would not receive the full quantity of thermal energy, the LWT would need to be increased, and the heat pump efficiency would be reduced.

There is a good 'heat geek' video on youtube which explains this better than mere words can do.

Having zone valves opening and closing will unbalance the flow rate through the buffer tank, as will having TRV's throttling in or opening up.

So I would suggest that if at all possible, buffer tanks, plate heat exchangers, zone valves and most TRV's should be avoided. TRV's in bedrooms may be permissible and room thermostats for possible cycling reduction during milder weather conditions.

I am a firm believer in keeping heating systems as simple as possible.

Posted by: @squiff

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I was chatting to my proposed installer yesterday, he said I would need a buffer tank (in fact 2, one for the rads and one for underfloor heating).  I asked why, he said to deal with the expansion of the water in the heating system as it's heated, and to provide a buffer of hot water used when the heatpump does a defrost cycle.  I've goggled needing a buffer tank with a heat pump and there are lots of articles saying that it's necessary.

only just saw this. A buffer tank is never ever anything to do with expansion. Could be that they are the sort of person who likes to overcomplicate to make the job bigger (more ££), and blind the customer with psuedo-expert words that don't have facts behind them . If they are saying this, who knows what else they don't understand. Who knows what else they've got wrong.

The preferred number of buffer tanks is zero (the summary reason without going into detail is for best efficiency), and this works for most situations. In some situations there's a need for one. But Two ? - in a normal domestic setup ? Nope. Fail.

My simple , very strong view: Get a new installer.

@bontwoody if the samsung control panel end is easier to get at , remove one of the PWM signal cables , then your pump will go to full speed always on, then move the pump to the switched live output.  you don't need to get to the pump itself.