Oh! And that was with the same Daikin model and size ASHP as is actually installed now. I don’t recall any discussion with the engineers who came to survey our property as to what temperature they would set things up for. They suggested the same radiators be uprated and apart from the DHW system, (we have an entirely separate Sunamp Thermino eP210 unit which was outside OE’s remit) we were thinking about a very similar set up. Regards, Toodles.

I see BG say servicing is around £200 a year and includes cleaning the evaporator, checking the fan for wear and 'ensuring the settings are correct'. So I wonder if they would want to reset my (if I have ) changed settings to the 'correct 'ones! It sounds like a very basic service to be honest. I will report back on Friday after the survey.

@editor

In the first instance the system with a buffer tank would require two water pumps rather than just one, so is very likely to use more electrical energy.

By adding a buffer tank the system now has a primary water circuit and a secondary water circuit, which of course adds complexity to the system control.

For maximum overall efficiency the temperature of the water entering the heat emitters should be the same temperature as the water leaving the heat pump. This is much easier to achieve when there is only one water pump and water circuit.

Considering the scenario that you suggest, in the system without a buffer tank the water is leaving the heat pump at a temperature of 45C, and is entering the heat emitters at 45C. Assuming a DT of 5C, with a LWT of 45C the RWT would be 40C, giving an average heat emitter water temperature of 42.5C. The quantity of thermal energy that is absorbed and emitted by the heat emitters is very much dependent upon this average water temperature.

In the system with the buffer tank, the water temperature entering the heat emitters will be dependent upon the primary and secondary flow rates. If the water is leaving the heat pump at 45C at a flow rate of 10 lpm, and the water in the secondary circuit is also flowing at a rate of 10 lpm, then it is likely that the water entering the heat emitters will also be at 45C. The quantity of thermal energy being moved from heat pump to heat emitters should therefore be approximately the same.

If the water flow rate around the secondary circuit is now increased from 10 lpm to 15 lpm, what will be the likely effect? If there is 10 lpm flowing into the upper part of the buffer tank but, 15 lpm flowing out of the upper part of the buffer tank, then the extra 5 lpm must come from somewhere. The only source is the return water from the heat emitters, so the net effect is water flowing from the bottom of the buffer tank to the upper part at a rate of 5 lpm.

This is where the complexity that Brendon mentioned starts to rear its ugly head. If there is 10 lpm flowing into the buffer tank at 45C, and 5 lpm flowing through the buffer tank from the heat emitter return at 40C, then when these two flows mix, the resulting flow from the buffer tank to the heat emitters will be at a temperature of approximately 43.3C.

The temperature of the water entering the heat emitters will therefore be 43.3C rather than 45C, giving an average water temperature of 41.67C rather than 42.5C, assuming that the RWT remains at 40C. While this may not seem to be much of a change, the quantity of thermal energy being absorbed and dissipated by the heat emitters will reduce from 3480W to 3305W, a reduction of approximately 5%. The net effect would be that as the system balances, the IAT would reduce from 21C to approximately 20.2C.

To maintain the IAT at 21C it would therefore be necessary to increase the LWT coming from the heat pump from 45C to 47.5C, which manufacturer's data would normally indicate that the quantity of electrical energy required would need to increase to supply the warmer water.

While it may be possible to balance flows through a system containing a buffer tank to obtain maximum efficiency, the likelihood that this would not be the case is ever present. For this reason I fully agree with Brendon that installing a buffer tank or similar additional complexity within a heat pump system should be avoided if at all possible.

Posted by: @toodles

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@lucia For us, it a shame that OE could not accommodate us in the early days of their heat pump installation service.I think that they are genuinely trying to offer a fair service to GB Domestic.

I am not sure that the 50 degree C. rating is such a good idea but then, our own installer was working on everything running well with 50 degrees rather than our actual ~35 degrees which seems to work very well for us. Regards, Toodles.

Yeah, I'm waiting for the written report before I return to the 50c design temp issue with them. So much of what they offer is good but I need this stuff addressing. 

However, I have my British Gas survey on Thursday so it will be an interesting comparison.... 

Posted by: @toodles

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I am not sure that the 50 degree C. rating is such a good idea but then, our own installer was working on everything running well with 50 degrees rather than our actual ~35 degrees which seems to work very well for us.

Also, I wouldn't mind the 50c design temp quite so much if they upgraded more radiators. That would give me more room for manoeuvre with lowering flow temps.  But I need to wait for the written spec so can't say anything yet. 🙊

Posted by: @lucia

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Posted by: @toodles

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@lucia For us, it a shame that OE could not accommodate us in the early days of their heat pump installation service.I think that they are genuinely trying to offer a fair service to GB Domestic.

I am not sure that the 50 degree C. rating is such a good idea but then, our own installer was working on everything running well with 50 degrees rather than our actual ~35 degrees which seems to work very well for us. Regards, Toodles.

Yeah, I'm waiting for the written report before I return to the 50c design temp issue with them. So much of what they offer is good but I need this stuff addressing. 

However, I have my British Gas survey on Thursday so it will be an interesting comparison.... 

 

The fact that the system may be designed to operate with a maximum Leaving Water Temperature (LWT) of 50C does not mean that it has to be operated at this temperature, in fact except for exceptional weather conditions the system will probably operate nowhere near 50C.

The important thing is to get a simple, well designed, system, which can then be optimised to operate in the most efficient manner. System efficiency can possibly be improved later, by selectively replacing some of the present radiators if and when finances allow.

@jamespa If flow temperature was the only variable, you may have a point, however, there are tens if not many hundreds of variables that need to be taken into account, I am only getting to grasps with a few of them, I have a long way and probably will never understand all.

If you want some evidence, just look at the report I published here, a rise from 36 to 48 Deg C incurred a running cost rise of 76%, as this gets worse as the flow temperature increases, I estimate the increase from 45 to 50 Deg C Wil incur a rise of the 40-70% mentioned.

Posted by: @derek-m

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Posted by: @lucia

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Posted by: @toodles

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@lucia For us, it a shame that OE could not accommodate us in the early days of their heat pump installation service.I think that they are genuinely trying to offer a fair service to GB Domestic.

I am not sure that the 50 degree C. rating is such a good idea but then, our own installer was working on everything running well with 50 degrees rather than our actual ~35 degrees which seems to work very well for us. Regards, Toodles.

Yeah, I'm waiting for the written report before I return to the 50c design temp issue with them. So much of what they offer is good but I need this stuff addressing. 

However, I have my British Gas survey on Thursday so it will be an interesting comparison.... 

 

The fact that the system may be designed to operate with a maximum Leaving Water Temperature (LWT) of 50C does not mean that it has to be operated at this temperature, in fact except for exceptional weather conditions the system will probably operate nowhere near 50C.

The important thing is to get a simple, well designed, system, which can then be optimised to operate in the most efficient manner. System efficiency can possibly be improved later, by selectively replacing some of the present radiators if and when finances allow.

 

Perhaps you can persuade them to upgrade those radiators that they do change to be a bit bigger than strictly needed.  Then as @derek-m says you can later upgrade a few more, and turn down the ft.  It anyway doesn't matter that much, heat loss for retrofit is an inexact science and rooms will share heat, so if one radiator is a bit small a slightly larger one in the connected room will compensate.

My personal experience is that it's almost impossible to get someone to quote a sensible price (or at all) for what you want (and can prove is viable).  So if someone is quoting a good price for something that is both close and readily upgradable in the future, I'd go for it.

Posted by: @derek-m

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The fact that the system may be designed to operate with a maximum Leaving Water Temperature (LWT) of 50C does not mean that it has to be operated at this temperature, in fact except for exceptional weather conditions the system will probably operate nowhere near 50C.

Yes, I do understand that but I need some of those rads in asap I think. I'm shoe string productions here trying to recuperate a freelance career that was decimated by caring for my late mum. So new rads are not going to be on a shopping list any time soon. 

I think it's partly down to the fact they've undersized the heat pump. But as I said, the written report may vary. 🤞🏻

Posted by: @jamespa

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My personal experience is that it's almost impossible to get someone to quote a sensible price (or at all) for what you want (and can prove is viable).  So if someone is quoting a good price for something that is both close and readily upgradable in the future, I'd go for it.

 

Absolutely, this is probably what it's going to come down to. And I'll be here canvassing opinions when I have both surveys in. 😁

I think the Daikin needs upping to a 6kw though. The surveyor ignored the infrared photos I'd taken with Octopus' own Flir camera apart from anything else. 

Even the install tech guy was a bit amazed the infra red was ignored. I mean that's hard data and it showed that my walls despite cavity insulation, are leaky as ... in places. 

Posted by: @lucia

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Posted by: @derek-m

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The fact that the system may be designed to operate with a maximum Leaving Water Temperature (LWT) of 50C does not mean that it has to be operated at this temperature, in fact except for exceptional weather conditions the system will probably operate nowhere near 50C.

Yes, I do understand that but I need some of those rads in asap I think. I'm shoe string productions here trying to recuperate a freelance career that was decimated by caring for my late mum. So new rads are not going to be on a shopping list any time soon. 

I think it's partly down to the fact they've undersized the heat pump. But as I said, the written report may vary. 🤞🏻

Consider going the other way then, ie asking them to operate at a higher flow temperature and upgrade fewer or no rads.  Then, with the money saved, you might get the local plumber to do the rad upgrade for you (or do them yourself, I easily did 2 per day when I did mine; they were short days and I'm not a plumber). 

... Unless of course the price includes the rad upgrades and doesn't reduce if you take them out.

Fundamentally I agree with @derek-m that if you have a good financial offer for a simple, well designed system then its a good offer.  That said, before the system is declared 'well designed', the capacity issue MUST be resolved.  Best wait until you have the written report.

@lucia if you think the heat pump size isn't right it's definitely good you are getting a second opinion with BG. I did my own room by room heat loss with heat punk which was hugely useful to compare against. 

I'm not sure bigger rads will solve a really undersized pump - if it's too small to meet the heat loss on colder days it just won't be able to heat the house? I guess it might give you a bit of headroom if the pump has a slightly higher capacity at 40 or 45C Vs 50C in colder weather.