A lot has changed within the industry since this article was originally written in 2016. This is now a common question among installers and is now also even being asked by savvy homeowners. Great!
There is no real right or wrong answer to which is 'better', different control strategies with work better in different situations, there are even scenarios where on-off control may be the best answer. And to be honest, most controls will 'work' in most circumstances. But for educational purposes, and to pedantically find what the true best solution is for your property or job, read on.
For typical domestic heating installations, it's about a balance between comfort and efficiency. The main advantage of these controls is lower boiler temperatures which give more condensing efficiency, and much less boiler cycling which extends boiler life and again further increases efficiency.
Similarly, with heat pumps, COP is improved and cycling reduced. This is a strategy to help people come to a more solid conclusion and highlighting the benefits and pitfalls of each strategy. There are small variations in comfort and efficiency but why not aim to maximise both!
So which strategy is best for your heating? Room compensation (aka load compensation) or weather compensation, and to which level of both?
First lets explain that when we refer to weather compensation, we're referring to ‘intelligent weather compensation’ like that found on the Vaillant range, the CDI Worcester and the Viessmann 200 boiler.
This can give an accurate indoor temperature without using an indoor sensor or thermostat, clever eh! (indoor sensing and influence still available).
When referring to room compensation, we're referring to any internally located controller that modulates the boiler flow temperature to achieve the required indoor temperature. This may or may not use the open therm protocol, but will use a similar ‘bus’ type communication.
We mainly refer to the use of oil and gas boilers as these are the most widely used appliances, although the same strategy can be applied to heat pumps (hydraulics must also be taken into consideration here) with even higher notable effects on efficiency.
It is also beneficial to understand condensing theory and that larger emitter (radiator) surface area will give in lower required flow temperatures, and in turn, efficiency gained from condensing.
Both control methods use more intelligent communication between the boiler and sensors allowing for better diagnostics and more system control along with gas savings and allow more system integration.
The first thing to understand is that weather compensation is a more gentle approach that’s proactive. It alters the radiator output before the house drops in temperature therefore not needing a higher temperature to ‘catch up’ with property heat loss.
Whereas room compensation is more reactive and responsive to changes in demand. This gives weather compensation a slight advantage over room compensation in both efficiency gains from running cooler and less cycling and additionally is gentler on the appliance.
Room compensation is a more reactive approach, as soon as demand is perceived it will jump into action and immediately provide heat or reduce the output of the boiler to compensate for an internal heat gain.
This gives a potential gas saving on properties with irregular usage and a more responsive system as well as compensating for internal heat sources like solar gain or an oven.
However, if you have a larger property the controller must be strategically placed for this or the whole property could be thrown out. Similarly, because it's a reactive strategy, it will have to burn hotter to replace the heat that's left the building when it finally sees a room temperature drop.
In either case, they both ensure lower heating temperatures, this gives us the following benefits;
• Slower corrosion rates
• Less O2 released to attack metals
• Less Thermal Shock to the system and components
• Better on the expansion vessel as the membrane is kept cooler
• Reduces cavitation at pump and fittings
• Less noise/ creaking in the system
• Increased comfort through reduced heat gradient in the room
• Increased comfort through steady emitter output
• Cleaner Air within the house
• Less loss through pipes in unheated areas
• higher comfort at lower room temperatures
• More efficient combustion/ heat transfer
• Condensate can clean heat exchanger and ensure better heat transfer
• More extracted latent heat from added condensing
• Longer burn times, less cycling of the boiler and boiler wear and tear from turning on and off the ignition sequence.
Each of these above points is addressed in more detail in our article about the benefits of low-temperature systems.
To find what best suits your property you should consider the following, insulation levels (internal walls and external), the thermal mass of the property, property size and layout, your emitter types, usage pattern i.e. high /low, regular or irregular, and occupancy (high or low).
We would suggest this to be the most important factor in your decision. If you have a property that’s used infrequently or with a random pattern and little usage, there is no point in heating it to comfort levels when no one’s there, additionally you will want heat as quickly as possible when someone is, this requires a more responsive type of control so leads us toward a room compensation type.
However, if you have an irregular pattern yet are frequently at the property, then to allow the property to cool, only to have to reheat will result in high emitter temperatures and less efficient heat, as well as more wear on the boiler. Also, why not have the property comfortable before you arrive! This leads us back toward weather compensation. Here you start to build the picture
From this, it is clear that with high usage or a regular pattern, it is more often than not more efficient to keep the property heated rather than intermittently turn the heating down to attempt to save heating, but perhaps not if your property has little thermal mass…
If your property has little thermal mass, internal influences such as a window or door being left open will quickly cool the property, to return to a more comfortable room temperature as quickly as possible will require more responsive room compensation.
Thermal mass creates a kind of ‘lag’ in time from changes in temperature within the property, it uses the mass of the building to store and release heat and can be used to our advantage to smooth heating changes.
Little thermal mass requires a more reactive system to respond to demand, however, if you locate your room sensor in a room that has a localised gain or loss such as a hairdryer in use, solar gain, or a window open, the controller will alter the rest of the properties emitter output and possibly make the rest of the house too warm or cold.
Weather compensation does not suffer from this as it is not biased to one area. Of course, if you have little internal insulation the location of an internal sensor will matter less as the heat will naturally dissipate easier but in the same token localised heat gain is less of an issue, so would lead back toward weather compensations.
Like low thermal mass properties, properties with high insulation levels are at the mercy of small internal heat sources. Cooking, using a hairdryer or even physical activity will make a much larger impact than a property with little insulation.
With little insulation internal influences such as solar gain make a much smaller impact, it is for this reason that I would suggest weather compensation is a much more likely solution, room temperatures will be as even and stable as possible and nearly always precise, additionally this is where most savings are required!
Location of insulation is important too. If the insulation is on the internal side of the external walls this will mask the use of the thermal mass of the building, if however, they are external or in a cavity, they alter the building's thermal characteristics and slow thermal inertia.
High insulation on internal walls (or partition walls) effectively creates further insulated zones within the property, this could prevent internal heat loss (such as an open window) or gain from spreading.
A badly located room sensor could then give inaccurate data for the rest of the house. This would then lean back toward weather compensation (taking all other aspects into account). Multi-room sensors would be best placed here, but will cover these later.
Badly locating a sensor in a larger property could create wild temperature swings and potentially waste energy, however on the flip side if the property was the extremely open plan the temperature would generally be much more stable and a room sensor would give a much more accurate picture of the climate. Here we also have the opportunity for zoning which I will loosely cover.
The property type (terrace/ detached etc) also has influence in combination with thermal mass and insulation, if you are looking at heating a flat, for example, you could get intermittent influence from flats above, below and either side, this will may throw your weather compensation out of balance.
When properties aren’t very airtight they are subject to more ‘air changes’ this can very quickly remove the heated air in place of cooler air from outside, especially on a windy day in an exposed area. This would mean an internal reference isn't as necessary.
If the property has MVHR (mechanical heat recovery) installed this should also be taken into account as this will increase air changes and in turn need a higher degree of internal reference due to the fact it is not constant.
Interestingly this can also be used in a way to spread heat gain and so internal reference can be more accurate across multiple rooms without zoning.
Emitters have different response rates, underfloor has a much slower response to radiators as it has to heat the floor before the air. This means that you would rather suit the emitter to the property type and usage pattern before you suit the control to the emitter.
Regardless, fitting a room compensation control to an underfloor heating system will work but more proactive weather compensated system would suit more due to altering the flow temperature before the house temperature dropped, especially in a high thermal mass household. Again an internal reference should be made if there is low thermal mass, as internal influence will play a larger role.
The lower the system temperature, the higher the 'self-regulation effect'. If you have larger radiators or even under-floor heating, particularly if your property is well insulated, your flow temperatures can come right down toward the target room temperature. When you have a system with low temperatures that are close to room temperatures, when the room's internal temperature increases due to internal gains, the emitter output is reduced by a much higher % than a high-temperature system. Similarly, when the internal room temperature drops the emitter's heat output is increased much more than a high-temperature system.
Let's take an exaggerated low-temperature system, for example, let's imagine we were targeting a room temperature of 20ºc and the emitter surface temperature was 22ºc. Because the emitter output is directly related if our room temperature if the room increased to 21 the output of the emitter would half, and if the room temperature dropped to 19 the emitter output would increase by 50%, provided if maintained its 22ºc surface temp.
If however, we had an emitter sitting at 50ºc the room would have to vary in temperature a lot more to have any effect on the output of the floor.
Now we don't necessarily require 22ºc radiators to benefit from this, but bare in mind weather compensation naturally runs at much lower temperatures throughout autumn and spring. When weather compensation runs at higher temperatures internal gains count much less in the load. As a general rule of thumb, we would say that you start to see tangible benefits from this once your system is down to a dt50 system or below, but you will again have to take the factors above into account also.
The higher the potential self-regulation effect, the more pure, advanced weather compensation you can use, but also the more accurate your room temperatures will remain in general.
All the above gives a basis to the considerations that could be taken (all be it with a pinch of salt) when selecting which control type to use, however, there are more proverbial weapons in our arsenal to combat the challenge of comfort and efficiency!
Room influence is a feature on advanced weather compensated controls. It requires a sensor to be located in the property which feeds back the actual room temperature to the main control. You can then select the amount of influence the control has on the weather compensation control, it could give a very small shift in flow temperature, or almost take over from wether compensation.
Fine-tuning this is the ultimate in efficiency and comfort in our opinion. Again in larger houses, the % of room influence will need to be reduced as the thermostat may not be representative of the rest of the property.
TRVs can be used in conjunction with weather compensation, and efficiently too if used correctly. Set just above the target room temperature, this will prevent overheating in rooms that receive additional heat gain such as solar. Of course, this may not prevent the over-heat but it will stop putting energy in the room unnecessarily and lower the output of the appliance.
In general, it’s important to set TRVs above the required room temps as setting them too low will cause them to prematurely close and reduce the available emitter surface area, this will lead to requiring higher appliance temperatures to heat the property, or simply a cold property. It is for this reason, if you have unused bedrooms you must choose a temperature that isn't too low, a sensible 'set back' temperature is 2-3oc.
Clearly, we all understand heating an unused space is inefficient, however, I am mainly looking into the use of condensing theory here.
Here we also look into internal usage patterns, size of the property, layout and internal insulation. If the upper half of the house is rarely used then zoning off at a lower temperature may be beneficial. However, if internal insulation is low then turning off or lowering the output of half the emitters in the property will cause the remaining half to run hotter to achieve the set room temperature as heat is lost into unheated rooms.
The higher required temperatures can mean a less efficient heat generated. We won’t go into too much more detail here as the rest is self-explanatory
The main point to take away is that over zoning, or too differing temperatures between zones can save energy. But also result in higher flow temperatures, and less efficient heat being used.
This is also down to the education of the end-user. Consideration must also be placed on how to lower your potential demand could be, if you zone too small this may increase cycling and appliance wear, the cycling and low load will also cause hotter temperatures to be used and unnecessary wear on the system, more on that here.
These essentially control both the appliance flow temperature and the flow through each emitter to maintain a comfortable individual room. It has the benefits of full independent control of each room, therefore not wasting heat in unused areas, as well as dealing with internal influences independently, and diverting energy only where required.
This is clearly by some way the most expensive control, however from a purely ‘comfort’ perspective, the best option. Regarding efficiency, the answer is slightly more in-depth and complex.
The main factor regarding efficiency with this product is the customer's use in relation to the information above, for optimum use you would only set a slight difference between rooms, based on occupancy levels and internal wall insulation. To use this type of control as simple ‘on-off’ (in other words setting unused rooms to say 5oc and others at 21oc) will result in the appliance requiring much higher temperatures to heat the on rooms, as heat is lost into the ‘off’ rooms.
However, this cannot be blamed on the control but is nonetheless how I would expect it is used most of the time.
Another big disadvantage of this is the energy requirement that individual rooms could be left with, this will result in a lot of cycling and in turn wear of the appliance. Although again, that is not as such an issue with the controller, an appliance spec issue. However, appliances are constantly improving modulation rates.
If the property has a middle to high occupancy using this control with all rooms set to similar temperatures could be very efficient and highly comfortable with well-insulated buildings, but with less insulated, its not really needed.
The main question to ask when looking at the value of multi-room sensor compensation control is how often will the property be occupied.
Regular occupancy will have much less benefit from an efficiency standpoint, especially in high heat loss houses where internal influence makes much less impact.
However the world is changing, more and more people are working from home, as well as insulation constantly improving. With a low occupancy of one person in a home office during the day, this control will come into its own, putting the office at comfort with the main house on a reasonable ‘set back’ is key, but again depends on thermal mass, property size and insulation. Around we go again!!
From a purely green perspective consideration should also be placed on the manufacturing and installation of these controls as well as battery life if used and glitches/ required maintenance etc. All in all, this is where controls will inevitably lead to in the domestic market as insulation levels continually improve. Setting up correctly specified parameters based on the above information and customer education is an engineer's key role here.
The benefits of weather compensation over room compensation are that room temperatures are based on heat loss rather than one area, which may not reflect the rest of the property’s needs, less wear on the appliance, longer appliance run times, no knee-jerking and the lowest possible flow temperatures from the appliance and there for the maximum efficiency from condensing or COP.
Room compensation benefits are that its more responsive with rapid heat up, respond to internal influence. Typically in the UK, we have brick-built housing and the most stock doesn’t have particularly great insulation, which is why weather compensation has a very big potential saving for the country as a whole, whilst adding comfort.
As insulation improves on properties (insulation is still king), more people work from home, and extensions are built with different thermal properties to the existing, these other strategies can, will, and should be employed. As we focus on low carbon technologies however we will be focusing on the lowest temperature possible which should be more weather compensation based due to the self-regulation effect.
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