Posted by: @andymc

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@derek-m the LWT is 32 whilst the return is 29 ( 1 Deg difference from original delta T5). As the additional 1oC is on the return  (originally 5) I'm assuming we have gained an extra 1oC..?

As mentioned we have underfloor heating throughout the house. So I'm assuming the whole floor area (less plasterboard  internal walls ) is one huge emitter?

Would the lower delta T be an issue for the heat pump? Is their any benefit to having a lower delta T. I'm assuming not as the floor is acting as one large emitter?

Fully understanding what is happening within a central heating system is not that simple, I often have to go back to basics and think it through myself.

A simple example is a 3 bar electric fire, the energy source, the electricity supply, transfers 3 kW of energy into the electrical elements, which emit 3 kW of heat energy into the surroundings. If the present heat loss of the room is 2 kW, then the room temperature will start to increase, but if the heat loss of the room is 4 kW, the room temperature will start to fall. Running the electric fire for 1 hour will consume 3 kWh of electrical energy and emit 3 kWh of heat energy.

It may be best to consider your heating system as two distinct, but connected parts, the energy source, in your case the heat pump, and the energy emitters, which is the UFH. Connecting the two together is the pipework etc., with water being used as a transfer medium to convey the heat energy from the source to the emitters.

It takes approximately 1.16 Watts of energy to heat 1 litre of water by 1C, so if water at 35C is flowing into your UFH, at a rate of 20 litres/min, there is potentially 1.16 x 35 x 20 = 812 Watts of heat energy available. Of course, not all this heat energy is absorbed by the heat emitters and dissipated, so if the RWT is 32C, the amount of energy emitted will be 1.16 x (35 - 32) x 20 = 69.6 Watts. Over a 1 hour period this will equate to 4.176 kWh of heat energy.

The quantity of heat energy being absorbed and emitted by the UFH, is primarily dictated by the LWT coming from the heat pump. The higher the water temperature, the greater the heat transfer rate. If the flow rate is the same at 20 lpm, but the LWT is increased to 38C, and the quantity of heat energy being emitted is now 5 kWh, the RWT will be given by 38 - (5000 / 60 / 20 / 1.16) = 34.4C. The DeltaT between LWT and RWT has now increased from 3C to 3.6C.

If the water flow rate is now reduced to 10 lpm, the quantity of heat energy being emitted will be about the same since the LWT is still 38C. The calculated RWT will therefore be 38 - (5000 / 60 / 10 / 1.16) = 30.8C. The DeltaT between LWT and RWT has now increased from 3.6C to 7.2C.

So I think it is possible to conclude that LWT affects the quantity of heat energy being transferred and dissipated, and the flow rate dictates the expected DeltaT.

Looking at the other end of the equation, for the heat pump to produce more heat energy, it will need to increase the LWT, which it does by making the compressor work harder. If the water flow rate remains the same as before at 20 lpm, the cooler RWT will require more heat energy to increase the water temperature from 34.4C up to 38C as in the above example.

As can be seen above, lowering the flow rate from 20 lpm to 10 lpm, causes the DeltaT to become even greater, so the heat pump now needs to increase the water temperature from 30.8C back up to 38C, but because the volume of water has been halved, the quantity of heat energy remains the same.

I hope that the above makes sense, but please feel free to ask for clarification.

Posted by: @andymc

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@derek-m the LWT is 32 whilst the return is 29 ( 1 Deg difference from original delta T5). As the additional 1oC is on the return  (originally 5) I'm assuming we have gained an extra 1oC..?

As mentioned we have underfloor heating throughout the house. So I'm assuming the whole floor area (less plasterboard  internal walls ) is one huge emitter?

Would the lower delta T be an issue for the heat pump? Is their any benefit to having a lower delta T. I'm assuming not as the floor is acting as one large emitter?

Sorry, I forgot to answer your final question.

Having a lower DeltaT just means that your water flow rate is higher than is necessary to achieve the required energy transfer. I think that most heat pump manufacturers specify a minimum water flow rather than a maximum one.

Derek thanks for taking the time to reply. Basically Detla T has no real effect. The biggest influence is LWT water temperature.

I've reset everything back