Posted by: @jamespa

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You suggest (I think) that your data shows that oversizing is positively beneficial to performance in your case.  Have I understood that correctly and if so how does your data show that?  As you say this is an area where we do need data!

Yes, this is what I believe to be the case.

The theory has two principles.

1. Ignoring pure weather compensation, instead run the HP in its most optimal setting. Bigger HPs can dump more heat into a property in a shorter period, and at a better COP. Therefore instead of trying to “match” the heat loss, put in twice as much and then turn everything off and allow the heat loss to catchup. UFH is perfect for this application as it’s slow to react.

2. By “overloading” the property with heat you can reduce the number of heating cycles needed vs continually just maintain and stop approach you see others doing. This is the graphs above. You would think this would cause big temperature swings within the property… the data does not support that, it’s within 1c usually. I also have a set back position over night.

Posted by: @stuartornum

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Posted by: @jamespa

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You suggest (I think) that your data shows that oversizing is positively beneficial to performance in your case.  Have I understood that correctly and if so how does your data show that?  As you say this is an area where we do need data!

Yes, this is what I believe to be the case.

The theory has two principles.

1. Ignoring pure weather compensation, instead run the HP in its most optimal setting. Bigger HPs can dump more heat into a property in a shorter period, and at a better COP. Therefore instead of trying to “match” the heat loss, put in twice as much and then turn everything off and allow the heat loss to catchup. UFH is perfect for this application as it’s slow to react.

2. By “overloading” the property with heat you can reduce the number of heating cycles needed vs continually just maintain and stop approach you see others doing. This is the graphs above. You would think this would cause big temperature swings within the property… the data does not support that, it’s within 1c usually. I also have a set back position over night.

OK.

I think your theory, if it applies at all, applies to a small minority of use cases.  The vast majority of our housing stock doesn't have UFH and does lose heat quite quickly; basically the vast majority is not modern newbuild.

Essentially you are arguing for oversizing ASHPs in order that you can run them part time.  Going back to fundamentals (ie thermodynamics) all ASHPs are, at their core, more efficient at a lower flow temperature than at a higher flow temperature (by about 3% per degree at higher FTs, this figure reducing at lower FTs).  If you heat only part time you are forced to operate at a higher flow temperature to achieve the same energy dumped to the house.

The fundamentals can be overtaken by 'engineering' in edge cases, particularly at very low flow temps.  So for a very well insulated house with UFH where the operating flow temperature is 30C it may well be more efficient to run part time because the consumption of the water pump and electronics makes up a significant part of the overall consumption.  Furthermore such a property will typically have a heat demand of perhaps 2-3kW and an ASHP sized to match this is likely insufficient for the DHW.   

However for a more typical retrofit of a 6-10kW 3-4 bed semi (or detached) with radiators operating at an FT of 45C+, the thermodynamic benefits of operating at the lowest flow temperature possible (which inherently also means using WC) far outweigh the 'engineering'.  This is well established and easily proven based on the specifications.  That doesn't of course mean that an oversized ASHP is bad in these majority cases, but it does mean that your proposed operating mode (which is the reason, in your argument, for the oversizing) isn't beneficial and so this is not an argument for oversizing.

ToU tarifs also do come in.  Part time heating of a a sufficiently well insulated house with UFH and a large slab may be less efficient, but it may be cheaper.  This I think potentially extends the argument for oversizing to a greater range of houses with UFH, but still not to the majority of our sadly poorly constructed (from a heating pov) housing stock. 

Hi James, firstly - thank you for contributing to this discussion, it's very much appreciated and quite enjoying the challenging of the ideas/concepts.

Posted by: @jamespa

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If you heat only part time you are forced to operate at a higher flow temperature to achieve the same energy dumped to the house.

Just to clarify, I did not mention increasing flow temp. In fact, this is the beauty of an oversized HP. Take mine for example

8kW Riello - can deliver 2718W at 30c flow at a COP of 6.41 when 10c OAT

vs.

4kW Riello - can deliver 1768W at 30c flow at a COP of 5.67 when 10c OAT

In fact, the 8kW Riello can deliver 3080W at 25c flow at a COP of 8.30 when 10c OAT

Note: these numbers are when both HP sizes are running at minimum/lowest level of modulation, which is when it's most performant.

Source:

(p30 and p36)

What this data is showing is that a larger HP can deliver heat, more heat... at lower flow temps and at better COP in a shorter period of time. This is exactly what I'm trying to leverage.

Posted by: @jamespa

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I think your theory, if it applies at all, applies to a small minority of use cases.  The vast majority of our housing stock doesn't have UFH and does lose heat quite quickly; basically the vast majority is not modern newbuild

The 8 recent projects we've done/currently doing, four are large extensions/retrofits with UFH in most of the downstairs and just rads upstairs, two are completely UFH (one being a passivhaus) and two being 1930's with just rads. So, for us at least, UFH is very common. However, I do appreciate this is not the majority nationwide. What we are seeing is people are not moving house, instead they are extending and renovating.

Can I check I’ve understood the arguments here?

Large heat pumps are more efficient for some manufacturer’s ranges, I accept that. And if the control range can go low enough then it should be more efficient overall. Also if it cycles cleanly (not short cycling) with long periods say 30mins off on on then there is no cycling efficiency penalty to speak of. 
So an over size ratio of 50% would not be a significant problem with efficiency? That I can also accept. 

However what some people “have been given” are heat pumps which are 2-3x oversized. See openenergymonitor for details. Surely you are not claiming that oversizing is OK in those circumstances since it clearly (by measuring) isn’t!

Posted by: @stuartornum

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Just to clarify, I did not mention increasing flow temp. In fact, this is the beauty of an oversized HP. Take mine for example...

Sorry but fundamentally you have to.  

The rate at which energy is absorbed by the emitters is determined by the flow temperature and the pipe area not by the heat pump.  Over the day the total energy absorbed (and emitted) by the emitters must equal the energy lost from the house.  If you reduce the time for which you heat, then the only way to get the emitters to absorb the same energy is to raise the flow temperature.  

With UFH the increase in FT required might be quite small (perhaps a couple of degrees or less, depending on your starting point) but it will need to increase.  However because the increase is quite small and anyway low the loss in thermodynamic efficiency will be low, and so engineering considerations and ToU tariffs may well dominate.

With radiators absorption (by the radiator) and emission (by the radiator) happen simultaneously and the amount you will need to raise the FT to compensate for part time heating will be much larger.  For example if the house temperature is maintained by heating full time at an FT of 40, it will require a FT of 65 to maintain the same house temperature if you only heat 50% of the time.  In the case of radiators this means that the thermodynamic considerations will almost always dominate the engineering considerations.

Posted by: @stuartornum

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The 8 recent projects we've done/currently doing, four are large extensions/retrofits with UFH in most of the downstairs and just rads upstairs, two are completely UFH (one being a passivhaus) and two being 1930's with just rads. So, for us at least, UFH is very common. However, I do appreciate this is not the majority nationwide. What we are seeing is people are not moving house, instead they are extending and renovating.

Doubtless thats the case, but its clearly nothing like the majority.  We replace about 1.4M gas boilers each year and build less than 200,000 houses.  I don't know the number of extensions but its nothing like 1.4M per year.  The majority, by a big margin, of what we need to do is retrofits of existing houses

Posted by: @judith

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However what some people “have been given” are heat pumps which are 2-3x oversized. See openenergymonitor for details. Surely you are not claiming that oversizing is OK in those circumstances since it clearly (by measuring) isn’t!

Great point to clarify, thank you for commenting!

I'm absolutely not wanting to be associated with being 3x oversized, that is just insane. We're currently doing a (uncertified) Passivhaus, the "heating engineer" before us recommended a twin fan, 12kW unit!! I burst out laughing when I heard this. At most this property has a heat loss of 3kW.

For my own house the actual heat loss is 4.5kW (the heat loss survey was 6.7kW'ish I think), I have an 8kW heat pump... (77% oversized actual heat loss) not quite double and it works amazingly well (with highly customised controls). I'm also not saying if you have a 16kW heat loss property, you should put in a ~28kW HP. I don't know what the UK average heat loss is, my guess would be 5kW or 6kW...? Also, emitters are a big factor to consider when sizing a HP. 

It's also worth noting that I did not get the bigger HP with the intention of proving bigger/oversized HP's are better, this is purely my observations after the fact. I initially went bigger as I wanted to heat a hot tub whilst doing the central heating/DHW at the same time.

Posted by: @jamespa

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Doubtless thats the case, but its clearly nothing like the majority.  We replace about 1.4M gas boilers each year and build less than 200,000 houses.  I don't know the number of extensions but its nothing like 1.4M per year.  The majority, by a big margin, of what we need to do is retrofits of existing houses

First of all, let me set the scene for our "average" customer who wants a HP (South Devon):

HP's still costs significantly more than a direct replacement combi boiler and the available capital the customer needs for a HP is significant, £6k/£7k average, this is after the grant has been paid.

You can get a gas boiler with "interest free" credit and pay ~£100/mo over 3 years... with no deposit and no need to find space in your home for a cylinder. This is what HP's are competing with... this appeals to the vast majority of the population. Therefore, your 1.4m boilers number is irrelevant and not the correct starting point. Instead, cut that 1.4m number to the number of people that can afford to drop £7k on a heating system...

People that have/enquiring about heat pumps on average have more capital to deploy and use the opportunity when renovating their homes to install a HP. So yes, UFH in relation to HP is very common for us.

It's the same story with EV's/ICE vehicles. EV's were/are adopted by more affluent people because they have more capital and space (for charger/parking) to make it happen and be convenient.

(Apologies, I will reply to your other bit later today, busy day...)

Posted by: @stuartornum

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Posted by: @jamespa

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Doubtless thats the case, but its clearly nothing like the majority.  We replace about 1.4M gas boilers each year and build less than 200,000 houses.  I don't know the number of extensions but its nothing like 1.4M per year.  The majority, by a big margin, of what we need to do is retrofits of existing houses

First of all, let me set the scene for our "average" customer who wants a HP (South Devon):

HP's still costs significantly more than a direct replacement combi boiler and the available capital the customer needs for a HP is significant, £6k/£7k average, this is after the grant has been paid.

You can get a gas boiler with "interest free" credit and pay ~£100/mo over 3 years... with no deposit and no need to find space in your home for a cylinder. This is what HP's are competing with... this appeals to the vast majority of the population. Therefore, your 1.4m boilers number is irrelevant and not the correct starting point. Instead, cut that 1.4m number to the number of people that can afford to drop £7k on a heating system...

I realise that maybe the case for your customers now but the statement that bigger is better (and more importantly that you don't have to change the flow temperature if you heat part time, which is simply not true) wasn't qualified in any way and I doubt Octopus, for example, are installing in the circumstances you outline (actually i know they aren't).  Also 1.4M is ultimately relevant because that's what we have to achieve.

So whilst it may well be that for your customers having a bigger heat pump and running it part time requiring a modest increase if FT, which has a relatively small efficiency impact because they have new UFH, makes sense. 

That does not mean that it makes sense more generally.

Posted by: @stuartornum

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3. Pure weather compensation is bad

Disclaimer/Admission: I'm a software engineer... I'm not using the standard weather comp control from the manufacturer, I wrote my own. I appreciate this is not available to the general public. The point I'm making is, with better controls, you can achieve better efficiency. Manufacturers... DO BETTER!

Hi @stuartornum, would you be willing to share your WC control or at least the bare bones of it on github? I am a total noob as a heat pump owner but have done a ton of research. I have just had a Grant Aerona3 13kw installed on the ECO4 scheme so am already winning!!! The EPH CP04 controls that cam with it are from the dark ages and even with the GW04 gateway connected to the Ember app (easier access to setting timer etc) it is still just a bit rubbish. I have connected the modbus to my Home Assistant server via a wavesure modbus to ethernet adapter and the modbus integration so can see the data in real time. My next task is to work out how to send values to the HP and perhaps automate this pretty much what Homely is doing but locally. Weather plays a huge role where we are (coastal Pembrokeshire very wet and windy) so would love to be able to add the actual weather into the conversation rather than just AOT!

Regards

Si. 

@grantmethestrength

Be interest to know how you are getting on etc, as we have a Grant R32 13kw system and looking to use modbus to get greater controll of some settings along the HA.

We also have a EPH control and don't find them too bad and with Grant they do allow you to use any controller, even the new one for R290 HP. But they only control  part of the HP and you have to use the attached controller to actually change any settings, which is not always a bad thing as most people just want it to heat up and will never use the one directly attached

Posted by: @grantmethestrength

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would you be willing to share your WC control or at least the bare bones of it on github?

Here you go: GitHub

It works for my Riello NHXM, which should be identical to the Midea M-Thermal, Clivet Edge, Bosch CS2000AWF...

Please don't judge the quality of the code and lack of tests... I wasn't expecting to share it... I just added bits on when I needed to do something new. I'll refactor and add comments/docs if people begin to use it. This is also just the code to control the heat pump itself, the WC code is much more unique to my setup. For example:

• I have quite a lot of sensors around the inside and outside of the house that influence a decision
• You'll notice in the code "dt1sh" - this is a key param gaining more efficiency 
• Scheduling... my schedules are different to yours
• Influence of predicted weather
• Influence based on energy price and state of house batteries

I've not written any Docs, but here are the design ideas:

• Modbus is flaky, therefore, I wanted something to hold "state" and enforce it regularly
• Only one thing should query the modbus at any time, hence there being a single job on a 10s schedule that queries and writes to the modbus - this has worked flawlessly
• The API queries the cached data that's retrieved every 10s (super fast)
• Integrate easily with Home Assistant
• Unfortunately Homely does not play well with other clients on the modbus... therefore, you can't use this and Homely