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Heat pumps
16/10/2024

What is low-temperature system design?

Lower central heating temperatures doesn't mean a cold home. Find out why.
Ian Mclaren
Ian Mclaren
,
Co-founder
5 min read

In a nutshell, a low-temperature system design means that the water that flows through your central heating is at a lower temperature than you would typically find with a system powered by a boiler.

Let us start with boilers. If you have a boiler today, you’ll probably find it set up to heat the water in your central heating to around 65-70 degrees Celsius. Heating engineers call this the flow temperature. The hot water then circulates around your central heating, transferring heat into your home - through your radiators and / or underfloor heating. Getting water up to this temperature (especially if you switch your heating on and off) uses a lot of energy. That means high gas or oil bills. 

In low-temperature system design, which applies equally to a system powered by a boiler or an air source heat pump, the aim is to have the water circulating through your system to be at a much lower temperature (typically 40-50 degrees Celsius), whilst maintaining the temperature you want your rooms to be heated to. 

Why is this important? 

Really, it comes down to saving you money. Heating up the water less, means you use less energy and lower your bills. If doing your bit for the planet is also important to you - it has the added benefit of reducing emissions. Either directly from your home as you burn less gas or oil, or at the point electricity is generated.

Making your boiler or heat pump work less hard to heat water, means less wear and tear - and a longer lifespan.

Is it as simple as just lowering the temperature? 

In most cases, it’s not. But that’s where good heating system design comes in. 

If you’ve read our guide on how heat pumps work, you may remember how we talked about radiators as heat exchangers. When water flows around a central heating system and gets to a radiator this is where most heat is ‘exchanged’ with the air in your room.

When the water flowing through your radiators is scorching hot, you don’t need much surface area to transfer that heat to your room. However, when using a low-temperature design, the surface area of your radiators needs to be larger to transfer the same amount of heat into your room. 

The analogy I like to use here is cooking roast potatoes! When preparing your Sunday roast, you try to get all your potatoes to be about the same size. This means cutting up the bigger ones. What you’re doing here is increasing the surface area of the potato so more heat from your oven can transfer into the potatoes, leaving you with beautiful crispy roasts.

With radiators - increasing the surface area either by installing a bigger radiator or fatter radiator allows more heat to be transferred into your room.

One other important point. You also need to make sure enough water can reach all corners of your system (flow rate). This is where the size of your central heating pipes is important (more on this later, read on)

Getting your radiator sizes right

So, we now know getting the size of your radiators right is important to making a low temperature system work in your home. But how do you size radiators?

The way we do this is by carrying our a room-by-room heat loss survey of your home. What this involves is measuring your room, any openings like door and windows and looking at what the walls and ceiling are constructed of and whether they are next to another room or the outside.

Why? We’re measuring how quickly heat leaks from your house to the outside. We then need to match the output of a radiator (how much heat it can transfer), with our lower temperature water circulating through it, to the heat loss of your room. Basically you’re looking to make sure you can put at least as much heat into your room, as it leaks out.

If you have underfloor heating, or considering it, the good news is these systems already work at much lower temperatures. Water typically circulates at maximum 50 degrees Celsius to avoid scorched feet!

How does heat leak from your home?

Time for another analogy (credit to Jez Climas) - imagine a leaky bucket. You fill the bucket up with water and out of the holes at the bottom water escapes. It’s the same with your home. You fill your home up with heat, and in the winter because it’s normally colder outside, the heat tries to move from the inside to the outside to equalise. (The reverse happens in the Summer - when its hot outside, warm air moves inside your home - that’s what keeps you up all night googling air conditioning units).

Now imagine a Victorian home (with no insulation upgrades) and a new build. The Victorian home has more holes in the walls and ceilings (more holes in the bucket) whilst the new build has fewer. This means there are less holes for heat to leak out of in the new build (or heat to transfer in during the Summer) - so the home loses less heat over the same time period.

This means you need to top up the heat in the new build house (refilling the bucket) much less than the Victorian house to keep the temperature inside at the level you want to stay comfortable. Less heat top ups, means lower heating bills as you have a lower heat loss.

So back to radiators, if a room has a high heat loss (lots of leaks) then you’ll need bigger radiators to make sure enough heat is being transferred into the room to counteract the leaks. Equally less leaks = smaller radiators. 

What about flow rate?

If your home is heated to different temperatures in different rooms, heat will always transfer from the hotter rooms to the cooler rooms. 

To maintain a comfortable home, you should consider a heating system that isn’t controlled by theromstatic radiator valves (TRVs - the dials on the side of your radiators). You will probably want some control in your bedroom (lower temperatures at night), but in general to maintain efficiency you want your system to be as open as possible.

This way you’ll have a home with even temperatures throughout. It will also help keep your system running efficiently.

Key points to remember

  • Not unique to heat pumps: Low temperature system design is not unique to heat pumps. Its a way of heating your home in a more efficient way - whether you have a boiler or upgrade to a heat pump
  • Design is key: Sizing your radiators to the heat losses of your rooms is essential to getting low temperature design right. All our heat pump upgrades come with a full heat loss survey. If you’re interested - give us a call on 0330 822 2795 or get your free personalised quote online
  • It’s not just about radiators: To make it all work its also important to make sure water can flow all the way around your system and it is controlled correctly. We also cover this in our system designs.

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