I suppose it is the age old problem of installing a heat pump to a heating system that was not designed for a heat pump.

Daikin supply a similar system, if you follow the link below on the forum you will be able to find some details.

 Daikin ASHP experiences as part of our Italian renovation project

Posted by: @derek-m

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I suppose it is the age old problem of installing a heat pump to a heating system that was not designed for a heat pump.

It sounds a bit like creating a heat pump not designed for existing heating systems 🙁 . Design of an item involves compromises, and pushing in one direction (e.g. SCOP at a high flow rate) will result in sacrifices elsewhere. The UK housing stock is rather old and leaky, with narrow pipes; I would hope the addressable market would be large enough to merit consideration.

@chickenbig my upstairs is microbore and I have no issues. The person who told you that is full of crap.

The Daikin system I'm getting installed has a fixed DT of 8C with radiators. That should provide about 1.7kW per radiator assuming the thickness of the wall of the pipes is the same as 10mm. That could feasibly work, but you'll need a heat loss survey done, and you could have flow problems as well depending on the number of radiators. It's a Daikin EDLA 8kW.

Posted by: @chickenbig

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Posted by: @derek-m

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I suppose it is the age old problem of installing a heat pump to a heating system that was not designed for a heat pump.

It sounds a bit like creating a heat pump not designed for existing heating systems 🙁 . Design of an item involves compromises, and pushing in one direction (e.g. SCOP at a high flow rate) will result in sacrifices elsewhere. The UK housing stock is rather old and leaky, with narrow pipes; I would hope the addressable market would be large enough to merit consideration.

From what I can see looking at the numerous specifications for heat pumps, they would appear to be a reasonably good compromise in many occasions. In a system with a gas boiler with a LWT of 60C and a RWT of 40C, hence a DeltaT of 20C, the average temperature of a radiator would be 50C. In a heat pump based system with a LWT of 52.5C and a RWT of 47.5C, the average radiator temperature would also be 50C. Of course to transfer the same quantity of heat energy, the heat pump system would require a higher flow rate.

In practice I think that you would find that the gas boiler would not be running continuously, but would be operating for a period of time, and then switching off for a period of time, and hence the overall average flow rate may be the same as the heat pump running continuously.

The way that my gas boiler is operating at the moment, is that it is running for approximately 4 minutes and heating the radiators up to approximately 35C, and then is switched off for approximately 35 minutes. In the process it is controlling the indoor temperature at 20.75C +/- 0.1C.

Admittedly this is with 15mm copper pipework, but I suspect it would still perform with smaller diameter pipework. I may have to increase the water pump speed slightly, from its present setting of minimum, as the weather gets colder.

Because the cost of electricity is still over 3 times that of gas, heat pump manufacturers have to aim for maximum SCOP, to make them cost effective to operate. Otherwise very few people would buy them, even with the government incentives. 

Posted by: @batalto

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The person who told you that is full of crap.

I would tend to believe they are following conservative design rules. They've trained up and are following the system they know best/have experience of. I think in part they are reducing the risk of installing into an existing (somewhat outdated) system. An estimate of between 6k and 8k (after BUS rebate) for a 7kW heat pump and 8 new radiators suggests they believe there is a fair amount of risk.

Posted by: @oswiu

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The Daikin system I'm getting installed has a fixed DT of 8C with radiators.

Will this have some sort of buffer or low loss header to provide hydraulic (and DT) separation?

Posted by: @oswiu

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It's a Daikin EDLA 8kW.

I see the Daikin Altherma 3 M (heating only) has a minimum flow rate of 20 l/min above 5 degrees and 22 l/min below -5 degrees. Looking at an 8 radiator 8mm micro-bore system I think the ideal maximum flow rate is around 16.4 l/min. :-/ I guess operating at lower power than 8kW might bring this into the realms of an acceptable DT, but then I fear falling foul of insufficient power when defrosting.

(Assuming 8mm micro-bore has a wall thickness of 0.07cm, 8 * 60s * 100cm/s * 3.14 * (.8 cm / 2 - .07 cm)^ 2 = 16,400 cubic centimetres/minute.)

Posted by: @derek-m

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hence the overall average flow rate may be the same as the heat pump running continuously

At the risk of pedantry, peak and mean are rather different things; averaged across a working day commuter line trains are not full, but during rush hour one may struggle to get on.

In the case of 8mm micro-bore the greatest power one can get is about 144 W/K * DT (assuming a continuous (so peak == mean) flow rate of 1 m/s; power = DT * 4.2 * 100 * 3.14 * (.8 / 2 - .07)^2 ). And this assumes no restrictions, and micro-bore will have been used for convenience (i.e. ease of bending).

Posted by: @chickenbig

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Will this have some sort of buffer or low loss header to provide hydraulic (and DT) separation?

I see the Daikin Altherma 3 M (heating only) has a minimum flow rate of 20 l/min above 5 degrees and 22 l/min below -5 degrees. Looking at an 8 radiator 8mm micro-bore system I think the ideal maximum flow rate is around 16.4 l/min. :-/ I guess operating at lower power than 8kW might bring this into the realms of an acceptable DT, but then I fear falling foul of insufficient power when defrosting.

The details they've sent me include a volumiser, so not hydraulic separation, but some extra volume, but that's assuming they don't actually mean a buffer tank because apart from in the list of materials, the quote talks about the two interchangeably. 

The installers manual for the 4-8kW models specifies a minimum flow rate of 6L/min for heating, 12L/min for defrost and 25L/min for hot water, so basically you're looking at a minimum of 12L/min. You'll have to do the maths to see if 16.4L/min is good enough, but it does at least seem possible, albeit with barely any capability to modulate. 

Of course even if it's theoretically feasible, that doesn't necessarily mean that an installer will actually put it in for you... Perhaps a hybrid route would be better suited if you don't need the BUS grant, or an air to air system?