Modulate Boiler - Opentherm and integated hot water tank

Se99paj

We currently use a Netatmo thermostat to control the central heating in our house, unfortunately the version we have only controls the central heating, so we had to have a separate controller that managed the hot water.
We need to replace the existing boiler and I've discovered that Netatmo have a new thermostat that can be used to control the central heating and hot water - The problem I'm having is trying to understand if this is feasible for us.

Netatmo state that the boiler "is compatible with OpenTherm and has an integrated hot water tank (a system OpenTherm Modulating Boiler)".

We currently have a combi boiler so there is no hot water tank, but is an integrated hot water tank slightly different? I've heard of storage combi boilers and wasn't sure if they would be compatible

ThisIsWeird

In what way did/do you want to control the hot water? Surely a combi has its own DHW output control, which you set and forget?
Make and model of proposed new boiler?

FreeBear

ThisIsWeird said:

In what way did/do you want to control the hot water? Surely a combi has its own DHW output control, which you set and forget?

With OpenTherm, the thermostat tells the boiler what the flow and DHW temperatures are wanted. OT works as a Master (the thermostat) and Slave (the boiler) - Struggling to see how the OP would have a separate control for the DHW....

Tado should do both heating and DHW, but I believe you have to get creative to enable eco mode with some boilers. Would have thought (expected) other smart thermostats to offer the same functionality. Downside to using an "off the shelf" control is if it doesn't do what you want, you have to wait for the manufacturer to update the firmware (usually at a cost).
One attraction of using something like https://diyless.com/product/opentherm-thermostat2 - You get the source code, and can make changes as you see fit.

ThisIsWeird

I still don't get what any of that will do beyond what the DHW 'output' control does on the boiler. Set that to whatever you want - say, 60o - and jobbie surely jobbed?

tacpot12

The Neatatmo website does indeed say the control of domestic hot water is only available if the boiler "is compatible with OpenTherm and has an integrated hot water tank (a system OpenTherm Modulating Boiler)".  The installation manual doesn't give any further information about domestic hot water heating, and the UK website doesn't help.

Googling produces the following article, which also doesn't acutally explain anything other than how to change the setpoint on the cylinder:  How does the Domestic Hot Water control work? – Netatmo Further googling results this Reddit thread Netatmo Modulating Thermostat and hot water control : r/homeautomation (reddit.com) which concludes that it doesn't work with Viessmann boilers with an integrated hot water tank. (AKA a "storage boiler"). 

It's one of the worst pieces of marketing I've seen, but the information available for heating controls is dreadful across the industry.

You are going to need to contact Netatmo to get a proper explanation of what will work - I would ask them for a list of boilers that they have tested, or where customers have confirmed that it works, and ask to speak to the customer. 

You need to understand how OpenTherm works to consider what might be necessary to use OpenTherm with a domestic water cylinder. OpenTherm tells the boiler the temperature of the water needed in the heating system to heat the house. Your room thermostat calculates this temperature based on the current temperature of the room and set point. So if you have the set point at 21C and the room is at 18C, the thermostat will call for water at about 30C, if the room is at 14C, the thermostat will call for water at about 40C, but then lower this as the temperature in the room rises.

If you have a hot water cylinder, you are going to want the water in it to be around 60C as any lower introduces a risk of Legionella. To heat the water in the cyliner to 60C, the water from the boiler needs to be at least 60C and probably 70C to heat the cylinder in a reasonable time. 

So where there is a demand for hot water, this needs to override the temperature of the hot water generated by the boiler. This might be done by the thermostat, or might be done at the boiler, but it is still not common to find boiler manfacturers being clear about how their boilers do this, and the thermostat manufacturers are equally vague.  

But you also need to consider whether this hot water at 60-70C is allowed to flow to the radiators or not. Intergas have a Hot Water Priority design that uses a three port valve so that when the DWH cylinder is calling for heat, the boiler produces water at a temperature set on the boiler control and this only goes to the DHW cylinder, not to the radiators. If the cylinder coil has been sized correctly (i.e. it is a quick recovery coil), the DWH cylinder will be up to temperature before the room temperatures have dropped by more than a degree or two. With this design, the OpenTherm thermostat only controls the temperature of the heating system water when there is no DHW demand. 

Intergas also have a boiler with dual OpenTherm inputs, but as usual the information as to how this actually works and how you might use it is still sadly lacking. Does it average out the demanded temperatures, or does the highest temperature "win"? The UK Heating industry is just really bad at providing information to customers and tradespeople that might want it.  

FreeBear

OP - Another thought.. If you are having to replace your boiler, do have a look at a heat pump as an alternative. With a £7500 boiler upgrade grant available plus the relaxation of rules on EPC recommended insulation upgrades, installing one is starting to look very attractive. If this had been in place last year when I went through the exercise of choosing a boiler, I may well have gone for a heat pump. Octopus are currently quoting me £2700, and that includes new/larger radiators (scope to get that price down as I already have suitable rads). That £2700 is not much more than I paid for a Viessmann combi !

Rodders53

FreeBear said:

 Octopus are currently quoting me £2700

Or more likely £10,200 less the Govt Grant of £7,500.   

Plus leccy is still 24.5p vs 6.08p per kWh from tomorrow. 
Not sure any HPs will give a COP of 4 or more, reliably, even now?
Octopus say "Heat pumps use up to 4x less energy than even the most efficient boilers to generate the same amount of heat." (sic)  (my bold)

FreeBear

Rodders53 said:

FreeBear said:

 Octopus are currently quoting me £2700

Or more likely £10,200 less the Govt Grant of £7,500.   

Plus leccy is still 24.5p vs 6.08p per kWh from tomorrow. 
Not sure any HPs will give a COP of 4 or more, reliably, even now?
Octopus say "Heat pumps use up to 4x less energy than even the most efficient boilers to generate the same amount of heat." (sic)  (my bold)

Take a look at https://heatpumpmonitor.org/ - A limited number of contributors at 93, but still enough to see how a well tuned system can perform. Quite a few are returning a COP over 4.0. Yes, there are some returning a COP of 3.0 or less - I suspect either old installs or poorly configured.

Have a look at some of the manufacturer's specs, and you'll find that 5.0+ is attainable during the summer months for DHW.