Ovehead - boiler flow v cylinder desired temperature

[Harry Bloomfield]

To heat up an open vented HW cylinder to the desired set temperature, the boilers flow temperature obviously has to be higher, but how many degrees higher?

Too low a flow and the cylinder called for temperature, would never be satisfied.

Previously, with mechanical stats, I have just ensured the cylinder stat does open and set the flow to match a satisfactory cylinder heat level.

 

[ScottishGasMan]

Rule of thumb is normally 10*c higher boiler flow than set cylinder temperature. The higher the boiler flow the quicker the cylinder will reheat also.

 

[Harry Bloomfield]

Thanks for that - I am just setting up with my VFC470F controls and its the first time I have had access to such precise temperature values and settings.

 

[Dan Robinson]

Set your hot water reheat flow temp to 85 or 80 the Lakh around with the heating flow temperature.

If you are not using weather comp, start at 60 and see if you can get it lower.

 

[ScottishGasMan]

The 470F is a weather Comp controller, so will be priority Hot Water, Designed for a high recovery unvented cylinder, if your using it on an open vented cylinder as Dan says set the boiler to run a flow temp of 85 to get it heated as fast as possible so the heating isnt held off for too long,

it will return to the flow temperature dictated by the heat curve setting for heating anyway, if you find the boiler cycling too much on HW reheat you can also change the power output when its heating HW only so you essentialy can range rate the boiler with a different power output for heat and hw

 

[Dan Robinson]

Mind you if the cylinder is old, the coil might have a very low rating and take eons to reheat..

 

[Harry Bloomfield]

Mind you if the cylinder is old, the coil might have a very low rating and take eons to reheat..

The cylinder is old = 1984 Hercol 117L 0.44 Sq M cylinder. With the new ECOfit 418 boiler set (as it was on the old controls) at max 75C flow, it would reheat from supplying a full bath in around 20 minutes.

When I was running round headless, trying to find out why the newly set up VCR470F + VR65 would not heat the cylinder properly and before I reset the boiler to factory - the boiler had an unlisted in the manual d.?? setting, d.73 (I think) which suggested it allowed HW priority or mid-position of the valve. That setting disappeared and my HW troubles went with a factory reset of the boiler. Now the boiler settings do suggest it can use separate flow temperatures for HW /CH and I have set both to max.. The boiler only seems to take notice of the CH max and only reaches that when HW is called for.

On CH most of the time it seems to stay most of the time sub 50C, unless there is a sudden large demand for heating. Which was the whole point of pulling out the old on/off control system to replace it with the VCR470 + VR65. Before, the boiler was simply working flat out or off.

 

[ScottishGasMan]

on the boiler - D.78 Maximum Charging Temp To Cylinder - Change to 80 degrees (or 85 if it allows but think 80 is limit) for faster HW recovery.

On the control, In the installer menu, there will be an option in system config I belive, where the cylinder settings are for "Load Pump" which can be set from 0-10min. Set it to 1min. This is the pump overrun for cylinder heating, again less time spent on this setting is quicker change over to heating

 

[ScottishGasMan]

Also your heating temp is altered using the "heat curve" number on the 470. It alters this based on the outside sensor you have (on a north facing wall) Higher heat curve number is higher heat to radiators. The max temp for radiators is just the limit they will ever go to, not the target their aiming for.

 

[Harry Bloomfield]

Thanks, Load Pump now set to 1 minute (default was 5 minutes). Both flow temperatures I had already set to max., and I knew about the heat curve - I still busy fine tuning that.

I have the place well insulated, so time without heat whilst it does HW for 20 minutes is not a problem.

 

[ScottishGasMan]

Should help as the load pump just means the boiler would have been on HW mode for 5 minutes after the cylinder was heated without firing. so yove reduced HW heat up/switch over to heating by 4 minutes just doing that