2025: The Year Heating Technology Steps Up

2025 is shaping up to be an exciting year for heating technology. There’s some truly interesting innovation on the horizon that could finally drag this industry into the 21st century. Over the next couple of weeks, I’ll explore some of these new ideas.

This Week’s Topic: Flow

One of the biggest challenges in heat pump implementation, if we set aside cost, paperwork, and general lack of interest, is flow rate.

Back in the day, someone (no one seems to know who) decided that heat pumps should send water out into the heating system at a specific temperature and expect it to return exactly 5°C cooler after passing through the radiators. This principle, known as a delta T of 5, appears to have been invented over drinks one evening and has since become the industry standard.

For the last 15 years, heat pumps have operated on this delta T of 5, requiring water to move rapidly through the system. Achieving this isn’t complicated – just fit a large water pump, and you’re good to go. However, when water moves faster than 1.2 m/s (walking speed or 2.7 mph), it gets noisy. To avoid this, the flow speed is kept below that threshold, which means the only way to move more heat is to use larger pipes. Bigger pipes = slower-moving water.

This is where the misconception that microbore pipes can’t work with heat pumps originated. For the record, microbore pipes can work perfectly well, and it’s about to become even easier.

Enter 2025 and Variable Delta T

In 2025, we’re in for a treat. Some heat pumps now offer variable or adjustable delta T. This means you can ask the heat pump to run with a 7°C or even a 10°C delta T. The water can leave the heat pump, take its time moving around the system, and return 10°C cooler than when it started. This adjustment makes heat pumps even easier to install.

Let’s take a closer look at the numbers for a 5°C delta T.

With a 5°C delta T:

• You can serve 13.5 kW of heating load with 28mm pipework.
• If the heat pump uses a variable speed pump, the water speed adjusts based on the load. For example, at full load, you’d need around 38.8 l/min to deliver 13.5 kW. At half load, the water speed would be halved.

Over Christmas, someone contacted me about a 16 kW heat pump (actual output: 13.5 kW) and was advised to use 35mm pipework. This isn’t necessary. Even if the system delivers 14.5 kW at peak output, 28mm pipework would suffice. It would only run at 100% speed for a few hours each year. I could tolerate a little noise or slightly higher delta T during those times.

But with a variable delta T (available on some units now), you could simply increase the delta to 7°C, allowing slower water speeds:

With a 7°C delta T:

• You can now push 19 kW through the same 28mm pipework.

• For reference, no domestic single-phase heat pump on the market delivers 19 kW output.

Daikin’s New Recommendation: A 10°C Delta T

Daikin has upped the ante, now recommending a 10°C delta T for radiator circuits. Here’s what that looks like:

• With a 10°C delta T, you can move 16 kW through 22mm pipework.
• And for those sceptical about microbore pipes: you can deliver 3 kW through 10mm microbore. For context, a 3 kW radiator is enormous!

The Bottom Line

System designers may argue that you need to account for resistances in bends, etc., which is true. But the key takeaway is that you don’t – and never have – needed 35mm pipework for any domestic single-phase heat pump. Even 28mm pipework might become a relic of the past.

2025 is set to make heat pump installations simpler and more accessible than ever.