Cast your mind back ten years. Gas boilers and combined heat and power (CHP) were the stars of any plant room: low CAPEX, low OPEX, easy to install, simple to run, and comfortable operating at high delta Ts between flow and return. The market had moved from a temperature set of 82/71 down to 70/40 – flow temperatures falling, but delta T widening, which suited both DHW delivery and heating circuits without compromise.
First generation heat pumps changed that. Essentially chillers reversed to deliver heat, they were complicated to design and install, somewhat unpredictable in operation, and constrained to very narrow delta Ts – typically around 5K. The market was immature and implementation was patchy. Heating systems ended up being designed around the equipment rather than the load, and delta Ts collapsed as a result.
Second generation heat pumps – designed from the outset for heating rather than adapted from cooling – restore design freedom. Flow temperature and delta T become design inputs again, not constraints imposed by the equipment. The system can be designed for what the building needs, rather than rebuilt around what the heat pump can tolerate.
The Impact of High Delta Ts
The higher the delta T, the lower the flow rate required to deliver the same heat output (Q = m · Cp · dT). Lower flow rates mean smaller pipework, smaller pumps, and critically, lower electricity consumption on both the primary side of the heat pump and the secondary distribution network. In a net zero context, that matters. Every watt consumed by a pump is a watt that has to be generated, metered, and paid for. Pump energy consumption increases significantly with flow rate. High delta T is not a performance footnote; it is a direct route to lower whole-system energy consumption.
A low delta T heat pump does not force a low delta T on the network, but it does impose a penalty. With a small temperature uplift (typically 5K), the compressor must work harder to reach a usable flow temperature, consuming a disproportionate amount of electricity in the process. The efficiency loss is not in the pipework; it is in the compressor. COP suffers, and the whole-system energy case weakens.
There is a second consequence. A thermal store charged across a wide temperature band holds more usable energy in the same volume. The narrow delta T that first generation equipment imposed meant these stores were often used as buffer vessels to prevent short cycling, rather than as meaningful thermal storage. High delta T restores the store to its proper function: a thermal store large enough to decouple heat pump operation from peak coincident demand. The wider the band between flow and return, the more energy the store holds, and the more useful it becomes as a grid balancing asset.
Flow Temperature
High delta T becomes more achievable when the heat pump is capable of high flow temperatures without significant COP degradation. Most refrigerants suffer meaningful performance losses as flow temperature rises, but R290 (propane) is relatively tolerant of higher flow temperatures, making it well suited to heating-first applications. R744 (CO2) is widely used but has a practical limitation: return temperatures above approximately 37°C cause it to stop functioning effectively, which rules it out for combined heating and DHW in most UK building contexts where the delta T can often be compressed. R290 does not share that constraint.
Heating circuits have largely settled at modest flow temperatures, typically around 45/35 for radiator systems, and lower for underfloor heating. Domestic hot water (DHW) is the more demanding case, primarily because of legionella prevention requirements. Stored DHW systems tend to return water at relatively high temperatures, which narrows the delta T across the heat pump and reduces COP. Instantaneous DHW generation via a plate heat exchanger removes that problem: return temperatures are low, delta T is wide, and the absence of stored volume substantially reduces legionella risk. It is a better outcome on both counts.
The equipment to deliver high delta T at useful flow temperatures exists. The next article will cover what defines a second generation heat pump, and why it changes what is possible in system design.
