Emitter design considerations for natural refrigerant air source heat pumps

Your heat pump has to work as a team with your heating system to deliver heat efficiently.

So, it’s super important that your heating system is designed specifically for the heat pump selected. And that includes the emitters!

Here, we’ll cover all you need to know about natural refrigerant heat pumps and emitters – from what types of emitters work best, to the emitter design considerations to bear in mind.

What emitters work best with air source heat pumps?

There’s a common misconception that heat pumps don’t pair well with radiators.

But this isn’t true at all. Just like boilers, heat pumps are very effective when paired with correctly sized radiators.

Just like boilers, the heat emitters have to be matched to the heat pump and its refrigerant.

But unlike boilers, heat pumps are more sensitive and have lower tolerance of poorly sized emitters – see our blog on optimising control.

Can you keep your existing emitters when you retrofit a heat pump?

Most of the time, you’ll need to replace your emitters with different ones if you’re switching from a boiler to a heat pump.

This is because, as we’ve touched upon already, heat pumps operate at different flow temperatures and different temperature differentials (DTs) – meaning they require a larger surface area to achieve the same heat output.

That said, it isn’t always the case. Whether you need to replace your radiators when retrofitting a heat pump will vary based on factors like:

• Your existing boiler
• Your existing radiators
• Your heat pump and refrigerant

In particular, many buildings in the past have been fitted with oversized radiators – making them suitable for low flow temperatures already.

Likewise, if you can’t easily make changes to your building’s existing infrastructure, your system designer might recommend a high temperature heat pump that’s designed to allow you to keep your building’s existing emitters.

Our Elm heat pump has been designed especially for this purpose, and is the UK’s first direct boiler replacement heat pump.

The importance of temperature differential (DT)

Temperature differential – also known as Delta T or DT – is the difference between the flow and return temperatures.

The flow temperature is the temperature that your heat pump heats your water to before it gets sent to your radiators.

Meanwhile, the return temperature is the temperature at which it returns to your heat pump after flowing through your radiators.

Maintaining the correct DT in a heat system is essential for the operation of any heat pump.

Every refrigerant will have a different optimal DT.  However, to give you an example, CO2 (R744) is most efficient with a 70C flow and 30C return.

Yes, some CO2 heat pumps can tolerate higher return temperatures, but the efficiency will be severely affected.

With that in mind, the whole system design for a CO2 heat pump should be predicated on achieving these optimal temperatures.

After all, designing and installing your heat pump system to be as efficient as possible is key to maximising carbon savings and minimising energy costs!

The role of thermostatic radiator valves (TRVs) in maintaining DT

Because refrigerants are so sensitive to flow and return temperatures, standard thermostatic radiator valves (TRVs) aren’t recommended with heat pumps.

Instead, natural refrigerant heat pumps tend to work best with differential pressure (DP) TRVs.

Why?

Well, a system with standard TRVs will create the design DT when operating at full demand (with all TRVs open) – assuming properly sized radiators and design conditions.

But, at partial demand, standard TRVs won’t control flow properly and the return temperature will rise.

On the other hand, DP TRVs won’t allow higher than set flow rates through emitters.

This means they can control flow properly at both full and partial demand to keep the return temperature as designed.

Standard TRVs

To show you what we mean, let’s look at a CO2 heat pump with a flow of 70C.

In this example, lock shield valves provide the balancing and TRV heads provide demand control of rooms – while a constant pressure pump reacts to system pressures.

Here’s what happens when the system operates at full demand with all TRVs open:

As you can see, with all TRVs open, the return temperature matches the desired 30C.

Now let’s see what happens when the system operates at partial demand, with two TRVs open and two shut:

Here, the pressure increase leads to an elevated 50C return temperature – massively compromising the heat pump’s efficiency.

Differential pressure (DP) TRVs

Let’s take a look at that same CO2 heat pump, but switch the standard TRVs for DP TRVs.

Here, the system is again operating at partial demand, with two TRVs open and two shut. But because DP TRVs have been used, the flow is still controlled properly and the return temperature remains as designed:

As you can see, despite the pressure increase due to the partial demand, the flow through each of the open TRVs remains at 0.03kg/s – as it would if all four TRVs were open.

In this way, the return temperature is able to remain at 30C – its most efficient – allowing the end user to maximise carbon savings and reduce energy costs.

End-of-line bypass valves: the dos and don’ts

End-of-line bypass valves are typically installed to maintain minimum flow in a system, to ensure that pumps don’t operate under no-flow conditions.

In this way, they can also prevent pressure build-up when control valves in the system close.

However, when it comes to natural refrigerant heat pumps, it’s best to avoid or remove end-of-line bypass valves wherever possible.

This is because of the effect of unintended flow through the bypass.

Essentially, excessive flow can bypass heating zones, mixing warm flow water directly into the return line – and this can cause high return temperature.

So, what should be used instead?

Well, we usually advise using a low-flow bypass around pumps.

Low-flow pump bypasses should be installed with a pressure-balancing valve appropriately sized to accommodate the pump’s minimum flow rate and ensure proper operation at the valve’s opening pressure.

Let our expert system designers worry about emitter design for you

As you can see, there’s a lot to think about when it comes to emitter design.

And this stuff is mostly highly technical.

With that in mind, it’s worth noting that any technical advice provided here is for informational purposes only.

While we strive to ensure the accuracy and reliability of the information provided, it isn’t intended to be a substitute for professional advice or services tailored to specific circumstances.

Instead, our system designers can offer bespoke advice and guidance based on your unique circumstances – including your existing heating system, your building and your heat requirements.

They’ll take the time to understand your needs and to put together a complete commercial heat pump system design, tailored especially to you and your building – including the emitters!

So, what are you waiting for? Simply get in touch to start the ball rolling.