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What Is Temperature Differential In Heat Pumps

A lot of the discussion around heat pumps is focused on temperature these days. We’ve discussed it briefly in our blog comparing heat pumps to gas boilers and again in this blog about high temperature heat pumps.

We’ve alluded to flow/return temperatures, but not discussed it in detail. In this blog we’re going to cover what it is, and what affects it.

What is Delta Temperature?

Temperature differential, otherwise known as Delta T, DT, flow/return, is an extremely important facet of heat pump performance. It’s the difference between the temperature the heat pump puts out and the temperature which returns to it from the heating system.

While the temperature that a heat pump puts out is important, it’s arguable that the delta temperature is even more important. A high delta temperature means a more efficient heat delivery system.

Why is temperature differential important?

Imagine a transport company sending out a lorry full of produce. If it returned having only delivered half its load, you wouldn’t consider that a very efficient trip.

It’s the same with temperature differential. An efficient system should return water that is significantly colder than when it left the heat pump. The benefits of this are:

-You can have smaller pipes for the same kWh of heat delivered
-You’ll have lower pumping cost, because the volume of water is lower
-You’ll have lower heat loss from the return pipe
-more efficient heat delivery in the building

However, the benefits don’t increase linearly with the size of the temperature differential. An extremely high delta temperature will lead to similar practical challenges that old steam systems used to have. It’s the role of the system designer to achieve the right balance to make the system work as efficiently as possible to deliver the best carbon reductions and cost performance for the level of heating required.

What affects temperature differential

There is a misconception that heat pumps must have small temperature differentials. This is not true.

You may see low delta T’s in the data sheets and promotional materials released by some heat pump manufacturers – we’ve seen designs based on 5 or 10 degC delta T’s. These values, however, are only correct if the manufacturer is using synthetic refrigerants. With natural refrigerants it’s possible to design a system with much larger delta T’s. Heat pumps which use R744 (CO2) or R290 (propane) can perform very well with a much higher delta T’s.

We’ve written before about why we use CO2 in our heat pumps at Clade. CO2 is actually most efficient at a temperature differential of 30 or 40 decC. That kind of delta is ideal for many heating applications because it delivers the required temperature while maximising whole system efficiency.

Below is an illustration of relative COP (coefficient of performance – we’ve written a blog explaining it here) between CO2 and a synthetic refrigerant. You can see that at useful flow temperatures a wide delta T gives the highest COP.

The thermodynamic properties of CO2 enable high temperature heat delivery and a low return temperature. As well as the efficiency benefits of CO2, it’s also worth remembering that CO2 is not subject to the HFC phase out and therefore will not become prohibitively expensive over the coming years. Nor will it catch fire, explode or leave forever chemicals in our precious environment. Again, we’ve written about this in detail in our article about why we use CO2 as a refrigerant.

Conclusion

The general public is often subjected to misconceptions around heat pump technology and the idea that they, by necessity, have a low temperature differential is one of them.

We hope we’ve made the case here that that is not true – low DT’s are simply a result of the synthetic refrigerants chosen by some heat pump manufacturers. At Clade we use CO2, which allows us to design and install extremely efficiency systems that maintain a much higher delta T, deliver the heat required and achieve significant total cost of ownership efficiencies over other systems. If you’d like to talk to us about how we can help you with a new system, get in touch.