Low-temperature systems are where all domestic heating systems are headed. Its what’s required if we are to install more heat pumps but also has huge benefits to gas boiler installations, and paves the way for other cleaner sources.
What do we mean by mean by a ‘low-temperature heating system’?
Well we don’t mean the property is cold, we simply mean that a comfortable property temperature is reached while only having a relatively cool heating system. So rather than having 70°c radiators, your radiators could be 25°c to 50°c yet still give comfort and even improve it. Low-temperature heating systems are typically ones that don’t exceed 35-55°c for space heating. This is absolutely achievable for most UK heating systems now, however for this article we are referring to the benefits of running any system at a temperature lower than its currently running.
Importantly though, these should ideally also have low temperatures within your heat source. These two don’t necessarily go hand in hand.
There are 3 ways of achieving a low temperature heating system. Increased emitter (radiator) sizes, increased insulation, and low-temperature controls. These will all work individually but the best way to get your temperature as low as possible and benefit from all the below attributes is to implement them together.
However, the single most important and easy part is to set up and use modulating controls such as weather compensation or load compensation properly. This would give you a maximum return on the following advantages.
Slower corrosion rates
Once your system is up and running your main battle is against corrosion. This mainly comes from the radiators which deposit dirt into the system water. This creates issues such as damage to the pump and valves, scaling in the main heat exchanger in the case of gas boilers, balancing issues and reduces emitter efficiency.
Corrosion is a chemical reaction. As with any chemical reaction, the hotter the chemical, the faster the reaction. This is because as any molecule heats up it gets excited and vibrates at a higher frequency. This increases its ‘collision rate’ with other chemicals that it may react with, and in turn increases the chemical reaction rate.
In fact, there’s a rule of thumb that the corrosion rate of a metal doubles for every 10°C increase in temperature. So if for example, if the corrosion rate is 10 mpa (mils per anum) at 50°C, expect it to be 20 mpa at 60°C.
Oxygen level is the main variable here. Oxygen is the active ingredient of corrosion, hence the term ‘oxidation ‘. This is apparent when we compare open vent systems vs sealed systems.
In an open vented system, the water rapidly reduces its ‘dissolved oxygen’ content as the system heats up above 80°c. The fact it’s open to the atmosphere means this oxygen can leave the system and so after this temperature the corrosion rate begins to drop rapidly.
In a sealed system however, the fact its slightly pressurised increases this oxygen saturation temperature and so the oxygen stays within the water. The fact the system is sealed also gives nowhere for the oxygen to go, and so the corrosion rate steadily increases.
Most systems in this day and age will run at a maximum flow temperature of 80°c and return of 60°c, which will not even begin to see a drop in corrosion. In fact the corrosion rate in an open vent system won’t drop until the flow temperature gets up to nearly 90oc, and never for a sealed system.
Corrosion can never be stopped, only slowed, and minimising flow temperatures does exactly that.
Less Thermal Shock to the system
All materials are subject to thermal stress, however some more than others. In boilers and heating equipment, components are chosen that withstand high temperatures but the issue is the cooling back down, and heating up again.
This repeated heating and cooling can result in fractures in the materials. Especially where two materials of different thermal characteristics are in contact due to their different expansion rates. It’s one of the reasons some engineers like to see all brass and copper within boilers as well as the denial of the ability to create a composite that can withstand temperature to the same degree.
This heating and cooling can also cause drying out in greased mechanical components, and the deterioration of rubber seals within the joints and vales. You may notice similar effects in other materials that are left to the elements outside, especially rubber.
Of course materials are chosen that are less susceptible to this. But if you run at more steady lower temperatures, this will give even more longevity before repairs are needed, which is an increase in efficiency in real terms.
Better on the expansion vessel
An expansion vessel is what takes up the thermal expansion of the water when it’s heated. They have an internal rubber membrane and are filled with air, this air depletes over time.
By running the system cooler, there will be less system water expansion into the vessel and so the vessel will flex less. This will mean the membrane will last longer before rupturing, and also discharge more slowly. Keeping the rubber at a more stable, and cooler temperature is also healthy due to the thermal stress mentioned above.
Reduces cavitation and protects system pump
Cavitation is the process in which your heating pump effectively boils the water on its inlet side due to low pressure. This leads to wasted energy running the pump and breaking down of the pump leading to pump failure as well as noise.
By reducing the temperature of the heating water you reduce this boiling process. This cavitation also happens at fittings within the system. Additionally, as you modulate the boiler down, provided you have an internal boiler pump, you pump slows, reducing cavitation further. An article on cavitation to follow soon…
Cleaner and safer Air within the house
Despite being called ‘radiators’, most of the heat from a radiator is actually convected heat not radiated. In fact, it’s stated that 80% of the heat from a radiator is convected. In reality, the proportion of radiant heat vs convected heat depends on the temperature of the radiator. This convection draws air up through the radiator and circulates heat around the room by creating a convection current.
When the air is drawn up the radiator surface and through the radiator convection fins (you know, that area that’s full of dust and cobwebs that’s rarely ever cleaned) it can kick a huge amount of allergens into the air.
These allergens mainly consist of dead skin, dust mite excrement, dead dust mites, molds, and animal hair/skin/mites etc if pets are present.
The worst is dust mite excrement though, about 10% of the population is significantly allergic to it, which causes some of the worst allergies and in particular, childhood asthma.
The higher temperature heating also has a tendency to dry out the air which can exacerbate issues for those with eczema or breathing issues.
Lowering your emitter temperature even slightly, significantly shifts the method of heat transfer from convection to more radiant heat which settles the air. An important one especially if the household has a vulnerable or allergen sensitive residence.
Less loss through pipes in unheated areas
Heat transfer is created by DT (temperature differential). The wider this DT, the more effectively the heat will transfer. Pipework that travels under suspended floors and lofts, especially if they are hot will output heat into areas that don’t necessarily require it.
Although not as effective as insulating pipework, running these cooler has the effect of reducing that heat output. What’s more, if the appliance is modulating down, this will often also slow the pump speed relatively. This reduces the flow rate and further reduces the heat transfer due to having a more ‘laminar flow’. Another very small gain but they all add up.
Less noise/ creaking in the system
This is not an issue actually created by high temperatures, but more a lack of pipe lagging and due care on installation. But if you have a nosy, creaky system, it is highly likely the creaking is due to the expansion and contraction of long pipe runs and radiator clips.
With on/off high-temperature systems these long pipe runs will continually expand and contract which rubs against floorboards and joists. Running the system more steadily and cooler will minimise this movement.
Similarly with radiators that click as they expand. They slide along the clips that hold them up causing a ‘ticking noise’, again running the system cooler helps here.
Comfort at lower room temperatures
There are many reasons low-temperature systems are more comfortable, and you’d be hard-pressed to find someone who lives with one and disagrees. There are 4 main reasons for this, the most important being an increase in radiant heat.
Radiant heat is a strange thing. It does not need a medium to travel through. It’s in fact the way the sun heats the earth through the vacuum of space.
Essentially it’s a light wave (infrared). It travels in straight lines and once they hit a surface they make the surface vibrate to a similar frequency as the source.
Radiant heating in a room warms the walls and furniture, this in turn radiates back out into the room.
If you walk into a room with low air temperature, you can also radiate back out in to that room making you cold. With more radiant heating however, walls and emitters will radiate back toward your body and clothes making you feel warmer than if you just have no to little radiant heat.
Of course these do equalise a certain amount but is notable and also leads to less dry air.
Increased comfort through reduced heat gradient in the room and steady emiter output.
Another notable effect of lower temperature heating within the room is the reduced heat gradient. When the room is heated by a cooler emitter a higher amount of radiant heat is used which, as mentioned, travels in straight lines.
The infrared light meets and heats surfaces within the room, that surface then radiates back into the room. The result is that the room heats more evenly.
A radiator with more convected heat will heat the air to above comfort temperature. As this cools the air will drop down the other side of the room, objects like sofas and beds can disrupt this current.
The nature of high temperature, on/off heating is that the radiator also pulses. The room will overshoot the selected room temperature, then undershoot before the heating then kicks back in.
A lower temperature system can minimise this ‘over and undershooting’ effect, and even simply match the heat input from the system, to the heat required to have a nice comfortable steady room.
All this means comfort can be found at lower temperatures and turning the stat down results directly in fuel bills saved.
High-temperature radiators and exposed pipework is clearly a safety hazard for the vulnerable and even for the less so, if any incidents occur.
Scaling in gas boilers occurs when Iron from radiators, or limescale from the cold water main come in to contact with the hot surface of the boiler. They can solidify and create a tough, insulating layer that can be very difficult to remove.
It’s said that 1mm of scale causes a 5% decrease in efficiency. The more worrying aspect is that most people are unlikely to even know of this efficiency drop untill they experience problems which may be much later on down the line.
More efficient combustion/ Heat transfer and cleaner emissions (Gas, oil and LPG boilers)
Heat transfer is a product of delta T (temperature difference). The wider the temperature difference between two substances, the more efficient the heat transfer, and less effective any insulating layer between them. This is called the ‘heat transfer coefficient’.
Boilers burn between 900°c and 1200°c and the lower we can get the system water temperature below this, the higher the heat transfer coefficient of our heat exchanger.
There’s plenty of information out there on heat transfer coefficients but it goes a bit beyond what we’re covering here.
Furthermore, when using modulating controls to target low temperature the boiler spends more time in a modulated state. This effectively oversizes the burner chamber and heat exchanger.
This gives more room for the combustion to effectively mix and create a cleaner burn. This is evident in lower unwanted carbon monoxide readings when heating engineers measure combustion readings at lower fire. (Can also be due to increased excess air)
It also gives a cooler combustion chamber, and in turn less harmful NOx emissions, an unwanted by-product created by excessively hot combustion. This modulated state also has the benefits of fewer stops/starts for the boiler as well as a whole host of other benefits. You can read more about the benefits of modulating a boiler here.
It also gives a larger heat exchanger surface area, relative to the heat input, to absorb the heat. This ensures that as much of the heat from the combustion is drawn out as possible before the gas is blown outside. This lowers flue gas losses.
Lower flue gas losses/stack losses (all boilers)
For every degree the flue gas is above our desired room temperature, we have wasted potential energy that could have been used in our property. To get these temperatures to actually equalise would take totally impractical radiator and heat exchanger sizes.
However, as mentioned above, we can effectively gain just that, by modulating down the boiler. Giving more time for the flue gas to be in contact with the heat exchanger and give up its heat.
Modern building regulations for insulation mean a 40 30 radiator design (very low-temperature system) isn’t completely out of the realms of possibility. If this can be achieved, in the interim seasons we could theoretically get as low as a 1% flue gas loss.
Bear in mind this doesn’t take in to account boiler cycling inefficiencies or combustion inefficiencies which are covered in other articles. This graph is also in net values (European) as opposed to gross which we typically use in the UK.
Boiler efficiency is typically calculated based on fuel composition, firing conditions and ‘stack losses’. Stack losses are represented by any heat that leaves the boiler via the flue. There are two types of stack losses, ‘Dry flue gas losses’ and ‘Flue gas loss due to moisture’.
When measuring dry flue gases you are measuring ‘sensible heat’ loss. That is all the heat energy leaving the flue above the ambient system temperature, or return water temperature. It does not include any energy lost as a result of moisture in the flue gas.
‘Flue gas loss due to moisture’ refers to the ‘latent’ heat in the flue gas that is lost by creating steam/water vapour (vapourisation) as part of the combustion process.
This flue gas loss due to moisture can be recaptured by recondensing the water vapor back to liquid. For this, we use condensing boilers.
More extracted latent heat from added condensing (all boilers)
Provided you have a condensing boiler, lower heat exchanger temperatures mean the boiler will condense more. Water is perhaps an unexpected byproduct of combustion. But if we look at the chemical equation of combustion it makes sense.
CH4 + 2O2 –> 2H2O + CO2 + Heat
In older, non-condensing boilers this water left your boiler via the flue in the form of water vapour. The creation of the water vapour (aka vaporisation) takes valuable energy, infact, up to 11 %. By allowing this vapour to recondense in older boilers would cause the boiler internals to rust and decay.
Since 2005 modern ‘condensing boilers’ have been made mandatory in the UK.
These boilers can cool the combustion gases to below 57°c, and when this is achieved the water vapour recondenses back into liquid water.
This state change from water vapour to liquid water re-releases heat. The lower we can get these flue gas temperatures the more latent heat we re-absorb.
Every liter of Condense water collected has reclaimed and additional 0.65kw of energy that would otherwise ended up in the atmosphere. A deeper explanation is available in our article on condensing theory.
This is illustrated quite well in the graph below showing lower return temperatures relating to higher efficiencies.
As well as adding efficiency this condensate also cleans the heat exchanger ensuring clean flue gas pathways and maximum heat transfer.
It’s worth mentioning this graph is only for natural gas. the maximum condensing efficiency for oil for example is 6%. Other sources also have lower condensing temperatures which make it harder to reclaim that lost energy.
Improved COP for Heat pumps
Although there’s fewer variables and complications here, there is vastly more efficiency to be gained from running heat pumps at an as lower temperature as possible.
A heat pump running radiators at 55oc could use 40% more electricity than a system at 40°c. And we all know how much electricity costs.
This is to do with the temperature/pressure relationship heat pumps rely on. That is the higher the refrigerant gas pressure, the higher the temperature of that refrigerant gas.
Smaller emitters and radiators, or a call for a higher temperature for hot water demand, will mean the compressor has to drive harder to increase the fridge gas temperature. Even a small increase in pressure results in a disproportionally higher use of power due to the square rule we’ve mentioned in other articles.
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