Originally Posted by
Andy the Minion
DB
I really welcome your comments and especially the effort you spent making them. They are constructive, based on your experience in the profession, and the reason we released this trial on the forum. It is also why I’m am here.
So firstly, thank you!
In return I will try to explain some of the how or why behind the functions and what we can or can’t do. Resideo is a big company and as my user name suggests I am just a small part of the development team. The Evo system is a big part of my working life, I have had all three version one in my home, sometimes two. They will have to take it from my cold dead hands if they ever want it back again :)
So….. I will certainly accept the criticism on behalf of the team. I will try to explain where I can and not come over as defensive, but some things are outside my power to change.
If the forum can accept that position, I would greatly appreciate you allowing me to participate.
I will probably repeat this screed in the release feed as not everybody is following both.
To start, the first part of what follows won’t address the main points of your comments but I would like to explain the reasons for DHW priority.
Firstly we received a considerable amount of pressure from OpenTherm users, the argument they made was that because the boiler water temperature calculation is a sum of all the demands. When a CH system is happy ticking along at 35°C and an 80°C DHW demand appears, the heating gets an unwanted jolt of hot water.
This is not normally a huge problem (an annoyance, yes) because standard tanks recover in 10-15 minutes (usually its a top up rather than heating from the cold inlet temperature) and they can sometimes recover almost as fast as they fall.
However, for the slow cylinder being described, this is a considerable problem. The CH gets overheated for as long as the tank takes to recover.
This is the exact opposite of the argument against DHW priority. It was the opentherm guys that noticed this, but it is also completely true for all TPI systems as well.
An excessively long 100% demand caused by the cylinder will cause overshoots in the central heating. The time constants in room temperature control makes it impossible for even a HR92 to stop the overshoot because the hot water is already in the radiator by the time it notices it. We now have a lot of evidence to back this up because of remote diagnostics.
The second reason is that we are heading into a world where 3 or 6kW heatpumps and 35°C flow temperatures will become the norm. The Dutch are already there and in 2025 we won’t have the option of a big ass combi either. This is just about okay for a correctly insulated underfloor system in new build, but I know from helpdesk calls and site visits that heatpumps are being installed in conversions with large radiators.
Even in a good new build the DHW becomes really critical, 55°C flow is a real struggle, CoP values plummet and they will be relying on a direct electric element as boost/backup. In this case the expensive, high CO², high temperature MUST only be put into the tank. If the CH valve were open the tank will (again) be very slow and the heating will be partially supplied by a direct element.
It’s a personal view but if mis-selling of heatpumps becomes a big thing, zoning systems will be a potential solution to some very unhappy OTC based customers with very high bills.
The final reasons is cooling, European heatpump installers are selling them with a ‘free cooling’ sales pitch, and a non heat/cooling control will just not be acceptable. The current UK regulations exclude this from any rebates but we needed to cover this immediately. Heatpumps are two pipe systems, they can either heat or cool so again a DHW demand must close the CH valve or there will be unhappy times. The system can’t allow pump overruns into the CH system either, back in the 1990’s those new-fangled Combis had enough trouble with this, and a cooling customer will simply go up the wall :)
I know this didn’t answer the ‘why no option’ point, but this was a rational decision made with the application experts in UK sales as @mtmcgavock mentioned, and it was based on technical knowledge, a balance of probabilities and experienced feedback.
1. We made DHW priority conditional on having a CH valve, so most systems will not be DHW priority. We were actually expecting this to be picked up as a problem as it needs additional components, wiring etc in the system (short pause as the OT guys rush to the keyboard)
The CH valve is needed because not all systems are fully HR92 and the constant opening and closing of all the HR92 will bring its own problems.
2. The argument about underheating of the central heating with a slow cylinder. This is a 50/50 call, it is either going to be central heating overheating or underheating depending if there is priority or non-priority DHW.
Normal cylinders just won’t notice and will work equally well with either system.
3. We also made a call based on knowledge of the response times in central heating systems.
a. Water is fast compared to air, a stratification layer in a tank means seconds between hot and cold water, plus the Evo has an adjustable differential so a cylinder doesn’t have to fall 15°C before a demand starts like old bimetal cylinder stats.
b. Temperature fall in a room is generally in the order of 0.2 – 0.7°C/hour if there is truly no heating – and a bath of 45°C water is definitely not ‘no heating’, the room will overshoot. The water in the radiator will also need to cool before room cooling starts.
The human body cannot detect temperature changes much below 1°C so the system has over an hour to respond.
c. Finally, most cylinders do actually recover well, and because of the high flow temperature in DHW only mode this will get getter still. Importantly the slower ones will also improve because the boiler doesn’t have to drag the whole system volume up to 80°C
So having said all this, I am not closing the topic down. If we can get evidence of a significant population of Evo system that have been noticeably adversely affected, we will look to see how it can be addressed.