Hive with Viessmann 100-W heat only

[rkjj22]

My elderly parents are having a new heat only boiler fitted (it appears Viessmann makes the best one on the market- one seller even said he didn't have any any in stock, which was a pity because it's miles better than any other heat only boiler on the market), and I wanted to get some advice for them. They'd like a Smart Control, and the Hive has been recommended to them. I know it's an on/off only control, rather than an OpenTherm modulating control like Nest, and this is my question:

Would an on/off smart control effectively override the modulation of the boiler and just switch the thing off, before it has the chance to modulate right down?

Many thanks!

 

[CBW]

Opentherm will work better with modulation if Veissmann supports it. On off will do just that, on and off, with no modulation. It might work similar to TPI (wouldn’t recommend Salus btw, just linking purposes only).

 

[stem]

Basic thermostats control the room temperature by switching the boiler on and off. The boiler will heat the water going to the radiators to the temperature selected on the boiler thermostat. Hive will simply switch the boiler off when the room reaches its set point and back on when the room cools. However, in order to maintain the temperature set on the boiler thermostat a modulating boiler will adjust its burner as required, to maintain the set output temperature, but the output temperature won't change.

With OT. The boiler water temperature is reduced over time as the room temperature approaches set point. This means that the boiler will run for longer periods but it will be producing water at lower temperatures, which can improve the efficiency of condensing boilers. Some users also find the modulation produces a more consistent temperature than radiators blasting out lots of heat when they are 'on' and none at all when they are 'off'.

Summary

1. With simple on/off thermostats the boiler thermostat controls how hot the radiators get to the touch the room thermostat simply switches them on and off.

2. OT will reduce the temperature of the radiators gradually as the target temperature is approached.

 

[rkjj22]

Thank you very much. I read that to mean the modulating down won't occur with a simple on/off control, because it will switch off and prevent that occurring.

As Tado appear to have gimped their UK smart controls not to work with OpenTherm (apparently because UK gas installers are thicker than those in any other country? Rather insulting suggestion from them), I guess that means I'm stuck with Nest?

With Nest if I set the controls to switch off at 8pm does it mean it'll automatically throttle the boiler down as that time approaches, rather than a sudden cut off?

Regarding your point 2- would the boiler itself not modulate down as the target water temperature is approaching, regardless of smart controls? A chap from Worcester Bosch customer service told me that- he was under the impression that smart controls only impacted modulation on system and combis, not heat only, and the heat only boilers so their modulation regardless.

Thanks again!

 

[stem]

Thank you very much. I read that to mean the modulating down won't occur with a simple on/off control, because it will switch off and prevent that occurring.

Yes

With Nest if I set the controls to switch off at 8pm does it mean it'll automatically throttle the boiler down as that time approaches, rather than a sudden cut off?

Not really, it's more temperature related than time related. If you set the room temperature on the thermostat to 20C as the room approaches that, the boiler output temperature will slowly decrease so that at the end when it hits 20C the radiators may only be lukewarm anyway. Which will depend upon the heat the property requires to keep the set temperature. i.e. if the radiators are big enough to allow the water temperature to drop and still maintain the room temperature, and also the outside temperature.

I don't have Nest myself, so can't be sure on what would happen at 8pm in your scenario. However, it is called a learning thermostat and some users do report it doing strange things for a while whilst it's learning how you use the heating in your home, so it might well do. I can't imagine it would do it unless the set temperature was reached as it will assume that you want to maintain the temperature you've selected until 8pm, and if it throttles down much before, the room could cool down before it's 8pm. However, in theory, if the system has been sized correctly, it will likely have already modulated to a lower setting as it should be at the set temperature by the end of the day anyway. Maybe a bone fide user will be along and be able to tell you for certain how it works in practice.

would the boiler itself not modulate down as the target water temperature is approaching, regardless of smart controls?

Their burners do modulate to control the water temperature, but AFAIK usually the water temperature doesn't change. You would have to check the specification of the one you are interested in to be sure.

Condensing boilers work more efficiently with cooler return water temperatures, so some can modulate the burner to prevent the return water getting above a certain temperature, others modulate to maintain a fixed flow temperature.