Its becoming an ever more increasingly popular question within our industry (well amongst ecotechnicians anyway), and one that has no real right or wrong answer, and they most certainly overlap, but there are some situations where one will outperform the other, It’s all about a balance between comfort and efficiency. The main advantage of these controls is lower boiler temperatures which gives more condensing efficiency, and much less boiler cycling which extends boiler life and again further increases efficiency. This is a strategy to help people come to a more solid conclusion and highlighting the benefits and pitfalls of each method. There are small variations in comfort and efficiency but why not aim to maximise both! So what method is best for your heating? Room compensation, weather compensation, or to what level of both?

First let me explain that when I refer to weather compensation otherwise known as 'weather comp' I’m 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, however indoor sensing and influence is available. When referring to room compensation I’m 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 also assume the use of gas as this is the most widely used fuel, and can obtain the most efficiency gains from modulating controls. It is also beneficial to understand condensing theory and that larger emitter surface area results in lower required flow temperatures and efficiency gained from condensing.

Both control methods use a 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 and are a basic standard for Ecotechnician installers.

TPI is also an option and most definitely has its uses, but as we are Ecotechnicians this article is for ‘what’s best’, and optimum performance not what’s cheapest.

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 heat lost, whereas room compensation is more reactive and responsive to changes in demand. This gives weather compensation a slight advantage over room compensation in efficiency and is gentler on the boiler.

Room compensation or 'room comp' is a more reactive approach, as soon as demand is perceived it will jump in to action and immediately provide heat or reduce output of the boiler to compensate for internal influence, this gives a potential gas saving on properties with irregular usage and a more responsive system as well as compensating for internal heat sources, however the controller must be strategically placed for this or the whole property could be thrown out, and because its reactive will have to burn hotter to replace the heat thats left the building when it finally sees a room temprature drop.

In either case they both ensure lower heating temperatures this gives us the following benefits

• More efficient combustion/ Heat transfer
• More extracted latent heat from added condensing
• Slower corrosion rates
• Less O2 released to attack metals 
• Less Thermal Shock to the system
• Better on the expansion vessel
• Reduces cavitation 
• Longer burn times, Less cycling of the boiler and boiler wear and tear from turning on and off the ignition sequence
• Less noise/ creaking in system 
• Increased comfort through reduced heat gradient in room
• Increased comfort through steady emitter output
• Safer 
• Condensate can clean heat exchanger 
• Less loss through pipes in unheated areas

To find what best suits your property you should consider the following, insulation levels (internal walls and external), thermal mass of the property, Property size and layout, your emitter types, usage pattern high /low regular or irregular, and occupancy (high or low).

Usage pattern, is it high or low? Regular or irregular (random)?

I 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, to allow the property to cool only to have to reheat will result in high emitter temperatures and  a less efficient heat as well as more wear on the boiler, also why not have the property hot before you arrive! This leads us back toward weather compensation. Here you start to build the picture

From this it is clear that with a 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…

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 the 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 you room sensor in a room that has a localised gain this 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 bias 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.

Insulation

Like low thermal mass properties, properties with high insulation levels are at the mercy of small internal heat sources. Cooking, using a hair dryer 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 buildings 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 in to account). Multi room sensors would be best placed here, but will cover these later.

Property type, size and layout.

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 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 and either side, this will may throw your weather compensation out of balance.

How Air tight and how exposed?

When properties aren’t very air tight they are subject to more ‘air changes’ this can very quickly remove the heated air in place of cooler air from outside, especially for on a windy day in an exposed area.

If they have MHR (mechanical heat recovery) installed this should also be taken in to account as this will increase air changes and in turn need a higher degree of internal reference. Interestingly this can also be used in a way to spread heat gain and so internal reference can be more accurate across multiple room without zoning 

Emitters

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 under floor heating system will work but a 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 house hold. Again an internal reference should be made if there is low thermal mass, as internal influence will play a larger role.  

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!

Trvs

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 over heating 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 boiler.

In general it’s important to set TRVs above the required room temps as setting them too low will cause them to prematurly close and reduce the available emitter surface area, this will lead to requiring higher boiler temperatures to heat the property, or simply a cold property.

Room influence 

Room influence is a feature on some weather compensated controls that gives the ability to adjust the flow temperature from weather compensation curve to account for internal influence, and make the system slightly more responsive although not quite as responsive as room compensation. Again in larger houses the % of room influence will need to be reduced.

Zoning

Clearly we all understand heating an unused space is inefficient, however I am mainly looking in to the use of condensing theory here. Here we look in to internal usage pattern, size of the property, layout and internal insulation. If the upper half of the house is rarely used then zoning off a lower temp 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 room temperature as heat is lost in to unheated rooms, losing efficiency. I won’t go in to detail here as the rest is self-explanatory, the main point to take away is over zoning or too differing temperatures between zones can result in higher flow temperatures and less efficient heat being used, this is also down to education of the customer. Consideration must also be placed on how lower your potential demand could be, if you zone too small this may increase cycling and boiler wear, the cycling and low load will also cause hotter temperatures to be used.

Multi room sensor compensation control (such as Evohome with OpenTherm Bridge)

This is essentially controls both the boiler 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, there for 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 the efficiency the answer is slightly more in depth. The main factor regarding efficiency with this product is the customers use in relation to the information above, for optimum use you would only set a slight differential between rooms, based on occupancy levels and internal wall insulation. To use this type of control as simple ‘on off’ (in other word setting unused rooms to say 5oc) will result in the boiler requiring much higher temperatures to heat the on rooms, as heat is lost in to ‘off’ rooms. However this cannot be blamed on the control but is non the less 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 boiler, although again that is not as such an issue with the controller, but a boiler spec issue, however boilers are due out very soon with much greater modulation rates. If the property has a middle to high occupancy using this control with all rooms set to similar temps could be very efficient and highly comfortable with well insulated buildings, but with less insulated, not really needed. All in all this is where controls will inevitably lead to in the domestic market. Setting up correctly specified parameters based on the above information and customer education is an Ecotechnicians key role here.

From a purely green perspective consideration should also be placed on manufacturing and installation of these controls as well as battery life if used and glitches/ required maintenance etc The main question to ask when looking at the value of multi room sensor compensation control is occupancy.

High occupancy (regularly) will have much less benefit from a efficiency standpoint especially in low heat loss houses where internal influence make 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 this day this control will come in to 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!!

In summary

The benefits of weather compensation over room compensation are, room temps are based on heat loss rather than one area which may not reflect the rest of the property’s needs, less wear on the boiler, longer boiler run times, no knee jerking and the lowest possible flow temperatures from the boiler and there for the maximum efficiency from condensing. Room compensation benefits are more responsive and rapid heat up, responds to internal influence but both of these are dependent on property type and use, and may only have minimal effect on comfort and efficiency. Typically in the UK we have brick built housing and 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 most likely adding comfort. However as insulation improves on properties, more people work from home, and extensions are built with different thermal properties to the existing, these other strategies can be employed.

Updated versions at http://www.heatgeek.com/

Written by Adam Chapman - Ecotechnician

www.ecotechnicians.co.uk