Unable to adjust temperature of unvented hot water cylinder

[tom23432]

I have a Heatrae Sadia premier plus PP250B unvented hot water cylinder and a Vaillant ecoTec plus 618 system boiler.

I want to adjust the temperature of the hot water cylinder to save a bit of money, but the controls appear to have no effect.

I want to set the temperature to around 50/55ish °C. I am aware of the risks of legionnaires.

Page 19 of the manual shows how to adjust the cylinder temperature: https://mediacdn.heatraesadia.com/-...s_7032982_issue_08.pdf?v=1&d=20181205T160145Z

However changes I make here appear to have no affect.

To test this I do the following:
1. Adjust the cylinder spindle around 50ish °C
2. Run the hot taps and shower for 5mins to bring the temperate of the cylinder down
3. Set the hot water thermostat to constant
4. Monitor the return temperature on the boiler

The boiler fires up and after around 10 minutes of the return temperature steadily increasing, it reaches 65°C and then stops because the cylinder is up to temperature.

No matter what adjustment I make to the cylinder spindle, the boiler return always goes up to 65°C, when it should be maxing out at around 54 °C I guess as I am aware it will go over the set temp by a few degrees. I have tried a few different spindle positions to see if I am somehow confused and putting it in the wrong position, but the instructions are pretty clear and I am confident I am setting it correctly.

The boiler flow temp is set to 75°C.

There are two temperature controls on the cylinder, one for boiler control and another for element control. I assume the element control is for the immersion heater, but I am not certain on this. To rule out some sort of wiring issue I have adjusted both the boiler and element control to around 50°C.

I don't have a lot of confidence the plumbers installed this all correctly as there have been several plumbing issues since we moved in to this new build.

Before I do back to the developers and get them to bring the plumbers back, I wanted to rule out anything obvious that I might be doing wrong here.

 

[fixitflav]

I don't understand your problem. You say "I want to adjust the temperature of the hot water cylinder to save a bit of money, but the controls appear to have no effect."
And then "The boiler fires up and after around 10 minutes of the return temperature steadily increasing, it reaches 65°C and then stops because the cylinder is up to temperature."
If the boiler stops because the cylinder is up to temperature, isn't that what you want? It's irrelevant that the boiler return temperature reaches 65°C

 

[Madrab]

By setting the cylinder temp to 50/55deg, that won't make any difference to the boiler's return temp, all that means is that the cylinder stat will stop calling for heat once the HW reaches 50/55deg. The boiler will modulated down once the flow temp has reached its set 75Deg until the cylinder has reached the set temp, then the cylinder stat will stop calling and the boiler will shut down.

If you want to lower the boiler temp (return) then that has to be adjusted at the boiler stat

 

[tom23432]

By setting the cylinder temp to 50/55deg, that won't make any difference to the boiler's return temp, all that means is that the cylinder stat will stop calling for heat once the HW reaches 50/55deg. The boiler will modulated down once the flow temp has reached its set 75Deg until the cylinder has reached the set temp, then the cylinder stat will stop calling and the boiler will shut down.

If you want to lower the boiler temp (return) then that has to be adjusted at the boiler stat

Thanks. I assumed that the return temp would be lower if the cylinder was set lower.

I can't quite get my head around how the return temp on the boiler gets up to 65deg if the cylinder stops calling for heat at 50deg.

Am I wrong in my understanding of how this system would work.
1. Cylinder falls a few degrees below the set temp. Thermostat is set to constant so when it falls below the set temp it calls the boiler for hot water.
2. Cylinder is sending the cold water to the boiler, boiler heats it up and sends it back to cylinder. This loop continues until the thermostat is turned off or the cylinder gets back above the set temp.

I thought the return temp on the boiler for this system was basically the temp of the water coming from the cylinder. So I thought if it stops calling for heat at 50deg, how is it ever returning to the boiler 15degrees warmer than this. I was expecting the max it to reach would be 54ish.

 

[tom23432]

I don't understand your problem. You say "I want to adjust the temperature of the hot water cylinder to save a bit of money, but the controls appear to have no effect."
And then "The boiler fires up and after around 10 minutes of the return temperature steadily increasing, it reaches 65°C and then stops because the cylinder is up to temperature."
If the boiler stops because the cylinder is up to temperature, isn't that what you want? It's irrelevant that the boiler return temperature reaches 65°C

I guess I have a misunderstanding of how the boiler the cylinder system works.

I assumed the return temp on the boiler in this scenario would be basically the same temp as the water coming from the cylinder. Does the cylinder not send its cold water to the boiler to heat up, then the boiler sends it back to thee cylinder? So in my mind the return temp would never get much above the max temp set for the cylinder since it would stop calling for heat at that point.

 

[ian1182]

The hot water from the boiler circulates around a coil in the tank rather than mixing with the water in the cylinder. It's a gradual process for the cylinder to heat up and it's unlikely the water returning to the boiler will match the cylinder thermostat value for a period of time. There is no cold water as such returning to the boiler just slightly cooler than it entered.

 

[tom23432]

The hot water from the boiler circulates around a coil in the tank rather than mixing with the water in the cylinder. It's a gradual process for the cylinder to heat up and it's unlikely the water returning to the boiler will have dropped to the cylinder thermostat value for a period of time. There is no cold water as such returning to the boiler just a little less warm than it entered.

Ah ok that's really useful thanks. I think I understand how it works now.

 

[Madrab]

Yup, it's like a kettle, the hot water from the boiler runs through a coil within the cylinder, (like the element in your kettle), so the boiler keeps on sending hot water though that until the hot water in the cylinder absorbs enough heat to get up to the desired temperature. Once that hot enough it tells the boiler to stop.
It's called an indirect HW cylinder, as the hot water from the system that heats it up is separate from the hot water that comes out of your taps.

 

[fixitflav]

I guess I have a misunderstanding of how the boiler the cylinder system works.

I assumed the return temp on the boiler in this scenario would be basically the same temp as the water coming from the cylinder. Does the cylinder not send its cold water to the boiler to heat up, then the boiler sends it back to thee cylinder? So in my mind the return temp would never get much above the max temp set for the cylinder since it would stop calling for heat at that point.

Good that we've cleared it up. I can't add anything to what other posters have said.

 

[dilalio]

It's quite surprising how many people don't understand that the hot water that comes out of their taps, from a cylinder, is not directly heated via a boiler/flame.

This is why cylinders that are heated by a boiler are referred to as "indirect".

Inside the cylinder is a coil which is "in effect" just another radiator on the CH circuit. Instead of heating the "air" around it, this coil/radiator heats the "water" around it. Thus transferring energy, via heat exchange, which makes the water (either mains or tank supplied) warm enough to shower/bathe in.

 

[Rodders53]

You need a thermometer to stick in a plastic jug or cup of the hot water collected exiting the cylinder at a nearby tap to test the water temperature. I have used a stainless steel digital meat thermometer for this exact purpose myself.

You need to adjust the cylinder thermostat and test after a day or two's use, then repeat until you find the setting you need for the hot water temperature you desire.

 

[dilalio]

You need a thermometer to stick in a plastic jug or cup of the hot water collected exiting the cylinder at a nearby tap to test the water temperature. I have used a stainless steel digital meat thermometer for this exact purpose myself.

You need to adjust the cylinder thermostat and test after a day or two's use, then repeat until you find the setting you need for the hot water temperature you desire.

Or a multimeter with a temp probe that you can attach to the cylinder hot outlet.