Choosing the Best Smart Thermostat!

February 28, 2022

Consumer Advice




Most people think that all smart stats are equal, but there is a key difference that no one seems to be talking about, and a little bit of information here really does go a long way.

What is a Smart Stat (Smart Thermostat)?

The term smart thermostat typically just means it's able to connect to the internet, which can be handy in some scenarios. 

For example, most thermostats have a feature called geolocation which can see when you're in the home and when you're out of the home and adjust the temperature accordingly.  

This is quite handy, but compared to the differences I'm going to talk about today, is pretty much just a gimmick. 

Your system could be loads more efficient and more comfortable, simply by swapping two wires over. 

So the first main thing you should be looking at is whether that specific thermostat or smart stat can modulate your specific boiler or heat pump.

So What Do We Mean by Modulating the Boiler or Heat Pump?

Modulating the boiler or heat pump essentially just means that it can turn down the appliance output when you have reduced demand in the property. 

Say it's not particularly cold outside, 14 degrees for example, and you just need a little bit of heat energy in the house.  

The control will output a much lower flow temperature from the boiler to just maintain a comfortable temperature within the house, rather than switching the boiler on to full blast to drive the house temperature up to your target room temperature as quickly as possible. 

Because what happens then is the room temperature overshoots and has to turn off the boiler, then when the room temperature drops back down, turns on the boiler full blast again, and this 'on and off' cycling continues.  This is known as an 'on off' type thermostat. 

Instead, a correctly installed modulating thermostat releases just the right amount of heat from the heat pump or boiler to come up to the correct room temperature then lowers that heat output to just maintain your set room temperature and continue running gently.  Rather than blasts of 'on and off' known as cycling.

Read more on boiler cycling and modulation in this article

Best Types of Smart Stat

The best types of smart stat, as mentioned above, are modulating smart stats and there are two main types.  

Weather compensation and load compensation.  

Load compensation is simply where the thermostat measures the temperature inside the house and calculates how hot to make the radiators just come up to temperature, and then maintain temperature. 

Weather compensation, however, measures the outside temperature, which is the real main variable when heating.  

When you're heating a home, you typically only heat to one or two comfort temperatures, the main thing that varies is the outside temperature.

So a sensor is installed on an outside north-facing wall, (out of the sun) to tell the boiler if it's the depth of winter, or if it's spring or autumn, and it then calculates the desired radiator temperature based on that information. 

The way I explain this to older customers is, I ask them if they remember 'back before room thermostats and remember turning up the boiler in the winter to keep them warmer, and down a bit in the spring and autumn?' 

Well, this does exactly that for you. 

And that's precisely the case, it makes the radiators hotter when it's colder outside, and cooler when it's warmer outside. Nice and simple.

Without trying to go into too much detail, there are three basic types of weather compensation. The first type is known as advanced weather compensation.

Advanced weather compensation 

This can read the temperature outside and achieve an accurate temperature indoors without any sensors or thermostats actually installed indoors.  

You may still require a sensor or thermostat in some scenarios, but it still has the ability and technology to achieve this.  It also has the ability to adjust times and temperatures built-in.

This utilises the lowest temperatures of all the different types of controls and is, therefore, the most efficient, comfortable and reliable controller you can get for reasons we're going to go into below, but essentially it's the best type.

Basic weather compensation

The second option is basic weather compensation, which is very similar to advanced weather compensation, except for it doesn't have the time and temperature controls and it does need an internal sensor or thermostat to achieve an accurate indoor temperature.

Basic weather compensation typically comes built into the boiler.

Internet weather compensation 

The third type of weather compensation uses internet weather data.  

Now there's two main sorts of problems here, the first problem is the weather station might not be anywhere near your house or you could be in a valley, or on a hill, so it doesn't represent an accurate temperature near your home.  

The second issue we found is that it doesn't actually seem to have that much influence on the flow temperature from the boiler. 

So typically if you see a controller that says it has weather compensation that uses internet data, that would typically be more just load compensation and isn't really weather compensation at all in our opinion.

Read more here on our article weather compensation or load compensation article

So, What Are the Benefits of Modulating Smart Stats and Controls? 

There are so many benefits to modulating controls and all of them are listed in our benefits of low-temperature article.  However, here are some hand-picked ones just to illustrate our point.

The first group of advantages are that it extends the life of your boiler and system without the need for repair.

timer concept image

We go into great detail about this on the above this article, but to give you a quick overview;

  • It reduces corrosion and dirt buildup within your system, which is the number one cause of system breakdown. (In fact if you look up collision theory, for every 10 degrees you reduce your radiators in temperature, you will reduce your corrosion by half!)
  • There's less thermal shock to the components within the system.
  • It's more gentle on the system's expansion vessel 
  • It reduces pump cavitation
  • It reduces wear on individual components due to reduced cycling  
  • It reduces scaling within the boiler 

The next group of advantages are all about how much more comfort lower temperatures bring, again we go into more details about this in our benefits of low-temperature article. But essentially it does this by:

  • Improving air quality within the house 
  • Reducing convective heat in the house and increasing radiated heat
  • Reduces noise within your system
  • Reduces temperature swings within the house
  • Reduces the heat gradient within the room 
  • Safer

 The next group of advantages are all about how fuel consumption is reduced, this is done by:

  • Reducing heat loss in uninsulated areas 
  • Increased comfort at lower room temperatures 
  • More efficient heat transfer and combustion within the boiler 
  • Lower flue gas losses 
  • Longer run times 
  • Less cycling 
  • Fewer standby losses
  • Higher part load efficiency
  • Reduced electrical consumption
  • Last but by no means least increased efficiency by added condensing at your boiler, but also increases heat pump COP

What is the Added Boiler Condensing that Modulating Smart Stats Promote?

Added condensing essentially means that we've extracted so much heat out of the products of combustion (which are the fumes that get blown outside) that those fumes condense into liquid water. 

The more of this condensed water we can collect, the more latent heat we've dragged out of the gas, and the lower the flow temperatures, the more condensed water collected.

If you'd like more information on that, there's more on the condensing theory article​​. 

It's also worth noting this is mainly for gas boilers.  Oil boilers are much less likely to modulate and much less likely to accept modulating controls. 

Heat pumps benefit EVEN MORE from this…

condensation on window in black and white

How Much Energy Will Modulating Smart Stats Save on Boilers and Heat Pumps? 

People always ask for a percentage of the efficiency modulating smart stat or controllers add. 

We're going to do a separate article on this subject, but our quick answer for the sake of this article is that:

For gas boilers, it can reduce efficiency by 5% or improve efficiency by up to 45 for gas boilers.  It could save literally anywhere in there, there are just too many variables. 

For oil boilers, it could decrease efficiency from 10% and increase to 20%. However, it's also highly unlikely you'll find an oil boiler that accepts them.

For heat pumps, it will increase efficiency between 10% and 70%! 

It really depends on how efficient your current system is, as well as how you use the system and many other variables.  

measuring efficiency

Also remember there are two ways of measuring efficiency.  

The first is how much it reduces fuel consumption by, the second, however, is how reliable your system is without failing and how long it lasts over its lifetime.

Modulating controls address both of these though whilst improving comfort, so when we give a percentage that could be for the length of the boiler or the fuel consumption. 

In reality, all these little individual advantages are small but added together make a much bigger difference.

How Do I Know Which Modulating Smart Stat to Get or IF We Can Have One?

So now you know why modulating smart stats and controls are so much better than typical 'on off' thermostats, whether they're connected to the internet or not. 

Now here's why you can't have them. 

Your boiler or heat pump will need to be a specific modulating appliance that accepts bus control.  

Bus controls are an intelligent communication between a thermostat and a boiler, meaning they can pass data from the thermostat to the boiler, and back to the thermostat along the same two wires. 

Without this, you won't be able to modulate down your flow temperature from the boiler.  

The next issue is that different boilers have different bus languages, so for example the Viessmann 200 range and their heat pump range speak KM bus or plus bus; Worcester boiler and heat pumps speak EMS bus; Vaillant boilers and heat pumps speak E bus..

An e-bus controller (which will be made by Vaillant because it's their language) won't be able to speak to a KM bus heat pump and vice versa. 

There is, however, an open language that many boilers speak instead called OpenTherm.

Because this language is open, it means we can install some third-party controls that can speak to the boiler in an intelligent way. 

To clarify, by third-party controls we mean boiler controls that aren't made by boiler manufacturers, for example, Google Nest or Honeywell Evohome

Just look for the OpenTherm badge and those controllers will be able to modulate down your OpenTherm boiler accordingly.

open therm logo

Another advantage you get from making sure you use the correct bus control is that, because the boiler is speaking to the controller intelligently if you have any issues or faults, they will come up on your controller screen or your app.  

Are All Modulating Smart Stats the Same? 

Well as mentioned above, many stats such as the Hive and now Tado, simply switch the heat source (be it boiler or heat pump) on or off. 

Of the smart stats that speak a bit more intelligently to the heat source, as well as having different bus languages available, you also have some stats that are a bit more responsive or aggressive with the flow temperature than others.

For example, the Nest is actually very good because it's a gentle modulating controller, provided it's a Nest version three or later.  It doesn't jump up the boiler demand too high but instead calls for a much lower gentler heat to the radiators keeping them nice and cool.

heatgeek thermostat smart control

On the other hand, we have Honeywell EvoHome, now this is a great controller that has multi-zoning, meaning you can have different rooms at different temperatures (note zoning like this can be terrible for efficiency, watch this video to see why

honeywell thermostat controller

However, because of this, it's very aggressive with the boiler flow temperature. 

It will jump up high in flow temperature before backing down quite a lot later, which as you can see above, is less efficient. 

Then you have controllers that don't use bus control at all or modulate the boiler and just turn the boiler on and off such as Hive.

hive thermostat controller

Then there's a Tado control

Now this controller used to be able simulate any boiler bus language and what's more, it's very gentle with the boiler, it used to keep the flow temperatures nice and low, and it even had a user feature on it that means you can adjust just how aggressive it is with the boiler.  However this was removed in the UK due to lack of use!

tado thermostat controller

We will be doing articles comparing the main controls in the UK soon, so you'll know which best suits you, but we can't complete this article without highlighting the potential downsides of modulating controls. 

4 Downsides of Modulating Smart Stats and Controls.

  1. One we hear reports of is that customers might say ‘the radiators aren't hot enough’.  The installer should obviously explain to the customer what sort of controller they're having installed.  But the important question here is, “what temperature is your room?” not what temperature is your radiator?

Remember your body is 37°C so you may touch a 30°C radiator and it feels cold, but it's not, it's just putting in a gentle amount of heat over time, rather than blasts of high-temperature heating. 

  1. Another potential downside is that your installer has to understand ‘how to install the smart stat control’ and hot water priority on your specific system.  The bus terminals from the controller must simply be connected to the bus terminals from the heat source, then the smart stat will modulate intelligently.  If not connected this way it will simply be an ‘on/off’ control.

Please be sure to have this discussion with your installer before going ahead. If they sent you this video, they'll most certainly already understand. Or, of course, if they're a Heat Geek. (check out the map).

  1. One other concern that people might have is that, because the boiler has to heat up hot water (because it's hot water priority) before it moves over to heating, the heating might be off heating for too long and you get a dip in room temperature. 

This isn't the case, we've installed thousands of these systems and it's almost never happened. 

The only times it has happened, we've had a scaled hot water cylinder, which meant it took too long to heat up and needed replacing anyway.  Modulating controls can also highlight other pre-existing faults like this that have been undetected for years. 

4) The last potential negative, although I've not seen it myself personally, is that you might get more bacterial growth within the system. 

If you've got underfloor heating you're going to get that potentially anyway, so although there's reduced corrosion from the collision theory I mentioned earlier, there's also a potential higher corrosion rate from this bacteria growing due to lower temperatures. 

However, this can be fixed by biocide and water treatment.  It's something that needs to be addressed regardless.  We fully cover that in this water treatment video/article here

Overwhelmingly, so the feedback from modulating controls is nearly always positive for us. 

In fact, we're often asking customers, how their heating's been, which will most likely be advanced weather compensation for us personally, and they'll kind of sit back and think “oh, how has my heating been?” and won't realise because they haven't been fighting with an on/off thermostat throughout the year to get comfortable.  Not to mention the money saved and all the other benefits listed above and in this in-depth article.

What is the Best Smart Stat for Vaillant Boilers?

Vaillant boilers speak E BUS - use a vaillant controller like the VRT 350 found here, or the VRC 700 found here. 

Buy Now: VRT 350

Buy Now: VRC 700

What is the Best Smart Stat for Worcester Boilers?

Worcester speak EMS Bus, use Worcester's own controller found here

What is the Best Stat for Viessmann Boilers?

So Viessmann 100 boiler speak both OpenTherm OR Plus Bus so any of the OpenTherm mentioned or the Vicare option. The Viessmann 200 option only speaks Plus Bus, so just the Vicare option: but only IF you need a room controller! Remember pure weather compensation is best and this comes built into the boiler with the 100 and 200 range.

What is the Best Smart Stat for Heat Pumps?

This is a simple answer, heat pumps use proprietary controls. That means they use their own manufacturers' controls, and for good reason.  

Heat pumps are even more susceptible to higher flow temperatures than boilers, the minute they allow third party controls on to their platform, they allow potentially poor controls. 

So they ONLY allow on/off controls (to link in to a larger control system) OR their own controls which speak intelligently. 

That is to say, if you have a Vaillant Heat pump, use a Vaillant control, if you have a Worcester heat pump, use a Worcester control, and if you have a Viessmann use a Viessmann control etc. 

Those putting 3rd party ‘smart stats’ on heat pumps are falling for gimmicks and not realising the true heat pump COP.

But remember, higher quality heat pumps like those listed above don’t necessarily need a controller as they have advanced weather compensation built-in.

Now remember the next step…. How to commission and control your system!

Smart Thermostat and Modulating Control Roundup 

So rounding this up, if you're looking for a thermostat for a property, rather than choosing a smart stat that just connects to the internet, look for one that actually has some smart intelligence, that can speak to the boiler or heat pump, and the heat source can speak back to the thermostat, but importantly, modulates the heat source down when required. 

I'd start off by finding out if you have a modulating boiler (heat pumps always modulate flow temperature), then find out what bus language it speaks, and then look at your options and don't discount boiler manufacturers' own controls, as we found these to be among the best, if not the best.  

Or simply comment below with your boilermaker model and property type and we'll suggest the best control options for you. 

It will reduce your fuel consumption, increase the life of your system and improve comfort, but only if commissioned correctly, and not overly zoned!!! 

See the next two steps:

1) Why not to zone

2) How to commission for max efficiency

Enjoyed this article? Want to know more about system design and how to become a top heating engineer? 

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14 comments on “Choosing the Best Smart Thermostat!”

  1. I have used a Netatmo 'smart' thermostat and radiator valves in a previous property, and was looking at what is best for a 1yr old new build now.

    Weather compensation, and low flow temps look interesting, especially as a prelude to going ASHP, which looks appealing.

    So... we have a 15kW Potterton Assure system boiler, attached to a Megaflo hot water tank. No obvious Opentherm-like controls, but they do have an add-on in-flue sensor, that looks like it swaps the boiler to a proper weather compensation mode. I see something similar on other Baxi boilers too.

    We have 2 radiator zones for a largish detached house, with reasonably decent thermostats. No timed control over DHW though, oddly.

    Is it worth trying this out? Otherwise the options seem to be to add a should-be-called-dumb smart micro-zoning thermostat that will ruin any efficiency.

    1. It is difficult to add weather compensation to a heat only boiler as it will then run too cool for the hot water and only a few heat only boilers (and even system boilers) have dual temp flow. It’s best to find an engineer locally to check it out. Try our Heat Geek Map

  2. Interesting article, I have a Viessmann 100-W1BC 30kw combi boiler (not the latest version). We have a mid terraced house and just use on demand hot water.

    I need to update the controls but trying to decide on a thermostat, would like to go opentherm as this boiler has the connector for it, just never been given best advice by previous engineers.

    A Nest or Honeywell Lyric T6R was what I was considering as both support opentherm.

  3. Hi - can you help please. I have a Worcester Bosch 30 CDi classic boiler on LPG attached to a megaflo 210i unvented clyinder. The installer has completely buggered up the install. He fitted the boiler with WB's intregral diverter valve so the system could use the CT200 easy control. Trouble is the cylinder is not compatible with the IDV so it must be removed and the system converted to s-plan which means goodbye IDV and goodbye Easy control.

    My new installer who will do the conversion has asked me to select a new thermostat. I would like modulated control and advanced weather compensation. Which thermostat do you think I should go for. The house is detached with 4 bedrooms and a couple of recptions. Half the house is open plan and a couple of the bedrooms will not be used very much. No underfloor heating. Any ideas? Many thanks. TW

  4. Hi we have a 300m2 house, built c1975 but cavity wall insulated, with replacement DG windows (2011), fitted with two Sanyo SHP-C90GEN heat pumps controlled by the Sanyo SHP-ACCS controller, all installed 2011. SAP rating was calculated at 55as part of install assessment. The water store consists of two ACV SLE300 tanks that supply the hot water and heating which is to radiators. The ASHP use CO2 and we get a max flow temp from the tanks of 52-54degC.
    Heating control is via Danfoss ECL Comfort 110 controller fitted with external weather compensation control and heating flow temperature modulation. Single room stat in the hall. Been told this unit is actually a commercial controller more suited to district heating systems than a single dwelling. All rads bar one fitted with TRVs.
    The Danfoss controller has failed so we have an opportunity to simply replace it or look at a more modern solution. We’ve read about zonal room temp control systems but also your suggestion that these can degrade ASHP efficiency.
    Any advice?
    Many thanks

  5. Hi, I have just installed a grant aerona R32 heat pump to my property which was fully refurbished and am looking for the right thermostat. Would a nest work? I would love some modulated temperature to save energy. We have underfloor heating in one room and radiators elsewhere

  6. Hi, we've got a Worcester Greenstar Ri gas boiler (installed 2014) in a 3-bed Victorian semi-detached house. Thinking about getting a smart thermostat and smart radiator valves to help reduce energy bills. Any advice? Have watched some of your videos on modulation and heat zoning. Many thanks

    1. Don’t bother with valves. Just get a Worcester weather compensation controller. Call your local heat geek or Worcester directly.

Heating Sensei

February 28, 2022
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