OpenTherm thermostat with Domestic Hot Water control

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OpenTherm thermostat with Domestic Hot Water control

Postby texnic » Wed Apr 06, 2016 11:39 pm

My boiler (Agpo 225C) has the so called "comfort mode", in which the tap hot water (aka domestic hot water, DHW) is made immediately available (as opposed to the burner being ignited only when I open the tap, leading to a delay in hot water supply). If I understand correctly, this mode continuously consumes a bit of gas. Therefore I'd like to switch it off when going on holidays or at night. The manual also mentions the possibility to control this mode via OpenTherm.

The heating in my house is controlled with a single-zone Honeywell Round Modulation thermostat. It provides no DHW control, and also no programming or remote control functionality. I would like to replace it with something with over-internet control support, but it would also be great to have the control of DHW. I would be satisfied with the simple functionality to switch the comfort mode on/off, so no necessity to be able to set or read the hot water temperature for example. I've asked Honeywell, they told me "Unfortunately none of our thermostats have a button for this. Our thermostats in the Netherlands are only for controlling the central heating and not the hot water."

I was unable to find a thermostat that would have provided such function. Does anyone have suggestions on where I should look or why it is not available?

By the way, the Agpo manual also mentions that some thermostats don't have a "button" but switch off the comfort mode when the room temperature set point is below 15 °C or so. That would also be an option for me, but still the thermostat should implement the respective OpenTherm-ID. And it is not easy to find out which of them do.
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Re: OpenTherm thermostat with Domestic Hot Water control

Postby marcelr » Thu Apr 07, 2016 7:38 pm

Did you have a look here yet:

http://otgw.tclcode.com/matrix.cgi#boilers

Gives an overview of OT possibilities of quite a bunch of boilers and thermostats.

grtz,

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Re: OpenTherm thermostat with Domestic Hot Water control

Postby raymonvdm » Thu Apr 07, 2016 8:17 pm

You thermostat has this button but it is not made available on the outside :-) I have had an issue with a thermostat in the past and after shorting two solderpoints the DHW was working again :D

So the thermostat supports it, you just need to add a button


Image

It is also possible to set the DHW using the OpenThermGateway http://otgw.tclcode.com/
Running HS3PRO on PC with Z-Wave / OpenTherm / Plugwise / RFXcom / MQTT / XAP400 / Logitech Media Server and Squeezelite on PI`s
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Re: OpenTherm thermostat with Domestic Hot Water control

Postby hvxl » Thu Apr 07, 2016 8:44 pm

That's a very weird statement from Honeywell, given that there are two versions of the Honeywell round modulation; with or without a button for DHW. Here's a picture of the verion with the button.

Image

Actually, the version without the button has the same functionality. It's only missing the actual button. With enough determination you could hook up your own button.

Then there's the Honeywell Chronotherm IV modulation. It has a menu setting to control whether or not to enable DHW during the "SLAPEN" period and another setting for "VAKANTIE/TIMER".

Other options: The Remeha iSense switches off DHW standby when the setpoint is the same or lower than the night temperature. ThermoSmart allows you to select per mode if you want DHW enabled or not. I don't know other thermostats in such detail, but probably a number of them will also have similar features. Finally, with an OTGW you can switch DHW enable on and off whenever you want.

By the way, checking the equipment matrix will not be much help as the DHW enable bit is conveyed in OT ID #0, which all opentherm equipment must support.
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Re: OpenTherm thermostat with Domestic Hot Water control

Postby texnic » Thu Apr 07, 2016 9:07 pm

marcelr wrote:Did you have a look here yet:

http://otgw.tclcode.com/matrix.cgi#boilers

Gives an overview of OT possibilities of quite a bunch of boilers and thermostats.



Yes, I have seen this matrix. But I couldn't find out which ID corresponds to switching the comfort mode on and off.
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Re: OpenTherm thermostat with Domestic Hot Water control

Postby texnic » Thu Apr 07, 2016 9:19 pm

hvxl wrote:That's a very weird statement from Honeywell, given that there are two versions of the Honeywell round modulation; with or without a button for DHW...


Thanks a lot for the extensive answer. I am afraid I need some terminology clarification. When you speak of DHW, what exactly do you mean? More explicitly, what does the "water tap" button in your image do? Does it enable hot water, or the comfort (same as "standby"?) mode? Finally, do you know the model of the thermostat in the photo?

I have read quite a bit about the OTGW, and I like the idea. However so far it looks that one should really dive into DIY to make it do what I need, and since the needs are quite simple for now, it looked like too much effort. I need the remote (internet) control of the thing, so I would need to hook up the OTGW to something like a Raspberry Pi. I've never done it before. So I decided to postpone it till I finish my current DIY projects. In the meantime, if I find an off-the-shelf solution, I'd be happy to just use it.

I'll take a look at all thermostats you listed, thanks for that too.

Cheers
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Re: OpenTherm thermostat with Domestic Hot Water control

Postby hvxl » Fri Apr 08, 2016 12:26 pm

Within Opentherm, the DHW (Domestic Hot Water) enable bit is used to indicate a request toward the boiler to keep some amount of warm water in stock. This request will only be honoured if the boiler has this functionality and it is enabled.

Since the Honeywell Round Modulation has no idea about day or night, it normally always sends the DHW request. When the water tap button is pressed, the thermostat stops sending that request, causing a boiler that responds to such a request to stop preheating the water. The thermostat will show an icon in the display (a tap with a cross) to inform the user of that situation.

The image in my previous post was taken from the Honeywell T87M1011 product page.
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