Indirect Unvented cylinder, Combi Boiler and Nest Thermostat

[ShaunCorbs]

30kw WB gas boiler. It is my partners house, so something I am aware of. Pipework all mine, except the gas, so no bother there.
I need to investigate it more.

Yea should be able to heat that no problem

 

[Welder]

What flow temp is that at tho?
I've just installed twin 150L OSOs and they state recovery rates at 80 degs flow!! Still, makes it look good

 

[Chris1012]

What flow temp is that at tho?
I've just installed twin 150L OSOs and they state recovery rates at 80 degs flow!! Still, makes it look good

How can the flow rate be confirmed/checked?
The cylinders temp is around 65c

 

[Welder]

Its the boiler flow temp that OSO quote at 80 degs. Most boilers closer to 70 so recovery rate way longer than stated depending on coil design (ability to transfer heat).

 

[Chris1012]

Its the boiler flow temp that OSO quote at 80 degs. Most boilers closer to 70 so recovery rate way longer than stated depending on coil design (ability to transfer heat).

apologies for my ignorance, I am not technical at all.
My understanding was once the cylinder is heated up, the outlets would draw the hot water from the cylinder and not the boiler anymore...?

 

[Welder]

We are at cross purposes.

The outlets do indeed draw their water from the cylinder. As the stored HW is used then temperatures at the outlets reduce.

What I was talking about was the time taken for the temps to recover (go back up). Most manufacturers give an indication of the time taken to heat a cylinder of cold water to approx 60 degs C. To understand whether this is realistic (indicative of the world we inhabit not the one manufacturers inhabit) we need to know the temp of the water going in (the lower the temp (say winter) the longer the recovery time, and the temp of the heating water through the coil. Most heating coils operate at approx 70 deg C buit the one I was referring to said it assumed a heating coil temp of 80 degs C. Frankly its unrealistic.

Bottom line. Take ALL numbers issued by manufacturers with a BUCKET of salt. YOUR circumstances dictate what will happen as every installation is different.

HTH

 

[AWheating]

The cylinder will only heat up at the time in the instructions in ideal conditions. Unlikely in real world situations. Time the hot water to be on without the heating so it's coil gets the boilers full output and it won't take too long. Have the heating on and it will get a share of the boilers output. Best to set hot water to come on one hour before heating times.

 

[AWheating]

a combi with an unvented cylinder is fine, all manufacturers will happily agree to this. Most manufacturers will have a drawing showing there boilers with cylinders. They won't show the full technical spec of an unvented cylinder of course as that's down to the cylinders manufacturer to spec.

 

[Chris1012]

The cylinder will only heat up at the time in the instructions in ideal conditions. Unlikely in real world situations. Time the hot water to be on without the heating so it's coil gets the boilers full output and it won't take too long. Have the heating on and it will get a share of the boilers output. Best to set hot water to come on one hour before heating times.

During last weeks heat wave, I was testing the hot water at each outlet. As the weather has been pretty mild since last week, the heating has not been on at all.
When I am testing the hot water at each tap, no other service is in use, including rads or UFH.
However it seems to still be the same heat up time from last week until today - average of 40seconds to get hot water!

 

[Welder]

Ahh. Is what you are talking about is the time for the hot water to reach the taps? If it is there is nothing you can do without massive investment. This would be the same issue regardless of whether an Unvented cylinder or combi is installed in the same place. It is a product of the distance the water has to travel and an UVC cannot, for obvious reasons, be located as closely as say a combi can in a kitchen.