energy efficiency with Tado X relay on/off vs Tado V3+ modulating/opentherm

maurello

I purchased a Tado X wired thermostat along with 12 Tado X radiator valves, assuming they would be compatible with my system in the same way as the V3+ version. However, I discovered that my Ariston boiler uses BridgeNet instead of Opentherm, and the X version isn’t compatible with my boiler’s digital interface. As a result, I can only use the thermostat in a simple relay on/off mode, which doesn’t allow for boiler modulation.

Tado support recommended that I return the X version and switch to the V3+ model, which would allow boiler modulation and potentially improve energy efficiency.

Could someone clarify why the V3+ version with modulation would be more efficient compared to using the X version in on/off relay mode? Since I have individual room temperature control through the radiator valves, I’m wondering if the efficiency difference is minimal. For example, if the boiler heats a room for 10 minutes at 50°C versus 60 minutes at 30°C, wouldn’t the total heat output be the same? Where does the extra energy efficiency come from in the V3+ version?

eezytiger

For a condensing boiler, efficiency increases the lower the return water temperature back to the boiler. Ideally it would be no more than 50C, but the lower the better.

If your boiler is able to modulate to sufficiently low output power to keep the flow temps warm (say 30C) rather than hot (say 50C+) then you should have a very efficient system with low thermal stress.

My boiler has a minimum output of 9 kW, which is too high to maintain low flow temps. Left running long enough the water temperature will rise and rise and rise, even with all radiators (nine in my house) wide open and emitting as much heat as they possibly can. This problem becomes far worse (for my home) if several radiators decide to close off. I do have the choice of eBus (Vaillant version of Opentherm) or relay. I've tried eBus twice in three years and it simply doesn't work due to high minimum boiler output and runaway temps. It is simply not possible for my boiler to maintain the tepid flow temps that eBus typically requests. Relay makes far more sense for my boiler in order to avoid needless boiler cycling.

Your situation may be different. It really depends on how well you can manage flow/return temps for maximum efficiency. That depends on the boiler output, outside temp, insulation, desired inside temp and whether you micro-zone your rooms with individual temperature targets including, perhaps, turning unused rooms off.

maurello

How do you account for the optimal efficiency of radiators? Most homes have radiators that are designed to work efficiently at higher temperatures, typically between 55-70°C. If the flow temperature decreases due to boiler modulation, the boiler may become more efficient, but the radiators could lose efficiency. This would mean it takes longer to heat the rooms.

In the end, I’m unsure if there’s any real energy savings unless all the radiators are resized to match this new approach of managing the flow temperature.

eezytiger

There's a difference between efficiency and effectiveness.

Boiler efficiency is achieved (for a condensing boiler) by keeping low return temperatures. The balancing act is to have the water sufficiently hot for your radiators to keep up with colder temperatures outside. The bigger the radiators the more heat they can emit, which makes lower flow temps and return temps a reality. This is why radiators may need upsizing when fitting a heat pump, for best efficiency, not of the radiators, but of the heat pump. The radiators need to be more "effective" in order to improve boiler/heat pump "efficiency".

Efficiency of radiators isn't really a thing, IMHO. Effectiveness is more relevant - how well they can emit the energy out into the room. They need to be large enough to heat the room sufficiently for the weather conditions and the desired flow/return temps of your boiler. Don't box them in. Don't stick them behind furniture or curtains. Don't block airflow. Don't cover them with laundry and towels. Some people do. These things can reduce boiler efficiency because the radiators can't get rid of the heat quickly and return temps remain higher as a result. Also, the rooms are not heated effectively so the boiler works harder to get the warmth out to the rooms. It's not that the radiators are inefficient. They are ineffective. That makes the heat source become inefficient.

Sure, once upon a time, radiators were sized for higher flow temps, necessary for the coldest temperatures expected. But, most of the time, winter temperatures are well above the coldest temperatures expected. On such days you could run the heating system at lower temperatures.

Of course, lower temps do mean slower heat-up times, so again it is a question of setting up your system to suit individual needs. We are a retired couple and, when needed, the heating is on all day from 07:00 to 22:00. We seldom get out of bed before 08:00 so the heating has an hour to get things comfortable. We don't need a rapid sprint from the boiler for a quick blast before leaving the house for the day and another before return in the evening. Low and slow works for us, even with our 36 year old radiators, sized for higher flow temps. We get by just fine (in the UK) with a max flow limit of 58C, even then only reached for a few minutes per day, if at all. Return temps are usually kept below 40C, in autumn/spring, maybe 50C in mid winter. Most of the time the boiler runs at far lower temps than that, at least after the initial warm up in the morning.

Oh, and my boiler is permanently range rated down to its minimum output of 9 kW. The house simply doesn't need more than that, ever. It doesn't even need 9 kW on the coldest winter day, more like 5 kW max. Low and slow is absolutely my heating motto.

maurello

@eezytiger thanks for the suggestion! I changed the X system back to the V3+ and finally I got the thermostat to modulate the Ariston Clas One boiler. I am learning how to use the system. My system has one wired thermostat connected to the Ariston Clas One boiler, and 11 smart thermostatic valves controlling each radiator in the house. All radiators have one.
One issue is that I do not find anywhere a setting in Tado to manage the hot water nor to adjust the maximum temperature of the heating water. As a matter of fact I do not see at all the hot water icon with the faucet depicted in the instructions. Any idea how to activate that part?

eezytiger

@maurello I only ever got to see a hot water tile, in which I could adjust hot water temp, when I used eBus boiler control. With relay control there is no hot water tile.

Since I had problems with the heating system behaviour (impossibly low flow temps requested) under eBus, I am using relay. I just set water temp at the boiler. It is fine like that and does not need adjustment day to day or week to week. It is the same setting all year round - 43C.

maurello

@eezytiger clear thanks for your reply. I ended up leaving it as eBus and modulating. It feels far more effective as you actually suggested to me.
Now my challenge is that I see an odd behavior from the thermostat, i.e. it is heating the whole time. My setup has 1 wired thermostat and 11 thermostatic valves, all from Tado. Basically the thermostat job would only be to manage the boiler on/off modulating. Shall I set it as zone controller for each room although each room as an independent thermostatic valve by Tado?

eezytiger

@maurello With modulating control instead of relay control it might be the case that you boiler is turned on for extended periods, but at a low flow temperature, thus improving efficiency and comfort. Mind you, low flow temp is relative to the outside temp. The colder it is the higher your flow temp is likely to be, and/or the longer the boiler will need to remain on. It's a balancing act. With eBus control it will be Tado making the decisions about flow temp vs run time.

As for independent SRTs vs zone controller connected SRTs, I've tried both ways over the past three years. Right now all my SRTs are connected to the zone controller, but that hasn't always been the case. I also have all my SRTs sitting loose on the radiators. This is a personal choice with my creative thinking about how to best for my system operation to my needs and the limits of my boiler.

I can't advise you on how to best set up your system. There are too many variables. I'm afraid you'll have to experiment to see what works best for you. Probably the best place to start is with a "conventional" setup, where all SRTs are properly mounted to the radiators and also connected to the radiators. If something does not work to your liking then you need to work out what to change. Sorry that's not residually helpful, but I just can't second guess your unique characteristics of home, size, thermal properties, micro-zoning or not, occupancy and schedules, boiler features etc.. I've spent three years experimenting in order to arrive at my current setup. Even so, I might change it again this heating season or next, depending on weather changes, comfort levels, energy consumption etc..

maurello

@eezytiger thanks! I will be experimenting. I only do not understand what the difference is between leaving the SRT as independent or setting the one thermostat I have as zone controller. What's the difference? I do not understand.
Also what I do not understand what temperature I should set on the thermostat since all temperature reading and demand for heating should come from the SRT. I have tried many settings. My learnings:
- If the thermostat has a lower temperature set than the SRT, the boiler does not start despite having many SRT asking for heating
- If the thermostat has a higher temperature set than the SRT, the boiler does start and things seem OK
- If I choose the thermostat as zone controller in every room which has one SRT, I do not see any change as compared to leaving the SRT as independent

Very confusing. Would wish for Tado to publish better documentation

eezytiger

The thermostat temperature should be set to the desired temperature for that room. Mine is in the hall. There should not be any other SRTs or dumb TRVs in that room. The thermostat will control temps in that room and fire the boiler as necessary. Indeed, any radiators in the same room as the thermostat should be left open (although balanced with all the other rooms so that they all receive a fair share of the CH water). If you have the thermostat and one or more SRT/TRVs in the same room then they might conflict in their behaviour, with one trying to heat while the other tries to stop heating. You don't need two of them fighting each other.


INDEPENDENT SRTs will open and close according to temperature measurements vs desired temperature, but they cannot force the boiler to come on. They will only be able to heat when something else has caused the boiler to turn on. They operate more like dumb TRVs, albeit with more precise temperature monitoring and actuation. For example, turn up the temperature in the (independent) spare room and the boiler will not respond immediately. The room will only start to receive heat when another room or the main thermostat turns the boiler on.

SRTs which ARE linked to the zone controller (typically (always?) the primary thermostat) can request the boiler to fire. For example, turn up the temperature in the (zone controller linked) spare room and the boiler will fire in order to heat that room immediately.

hugbilly

With regard to the above, if you have a wireless temperature sensor and one or more SRTs in the same room and they are set to a zone controller then the norm is for the sensor to control the SRVs, it is more accurate at temperature measurement. Indeed if you place the sensor in the same virtual room as the SRVs it will automatically take over control of them. The problem with not having SRTs on any radiator is that when any device in the system calls for heat then those radiators will heat up even if you don’t want them to . . .

maurello

Thanks for the help! Now I finally managed to configure everything. I moved the SRT in the same virtual room of the thermostat and set it up as measuring device. All other rooms have own SRT and they are all set to use the thermostat as zone controller. Magically things started to work and the house heats up very nicely :)

Only remaining thing to do is to set the flow water maximum temperature. My boiler can go up to 82C, which is too much, not efficient for condensation boilers. I asked Tado support if I can set a maximum value for that water (not the hot water as utility) in Tado app, but they have not responded to me yet. I suspect this is not possible in Tado and will have to set it up in the boiler itself. Value to be set, 60C. That should be it for this year and we see how it goes.