Central heating - room stat & tank stat operation

[Haynes67]

I have a central heating system run from a Potterton Suprima 60L condensor boiler which operates radiators and hot water through a 3 way diverter valve, which i have recently had replaced along with my radiators flushed. The reason being I was getting hot water but the radiators were taking hours to heat up. Even with the new valve after the problem appears to perssist. So my questions are -
1) Have I got another sticky diverter valve, perhaps.
2) At what setting do a need to set my room stat in order to maintain a 21 C tmeperature? I know that the heat swiches off when up to temperature but at what point does it switch back on?
3) What is the tolerance i.e 23 C for max, then off; dropping to 19 C for min, then back on; perhaps? Any advice with regard to the room stat would be welcome.
4)Also at what temepertaure would anyone, with knowledge, recomend my tank temperture be set at. originally it was set at 80 C then after the new diverter valve was fitted it the gas fitter set it at 60 C, but the shower tap needed to be turned up, so I reset it at 80C. Any advice here would also be welcome.
Thanks in advance

[buildersmate]

There are some unusual aspects to the problems that you report so I need to go back a bit.
The Potterton Suprima range is a conventional heat-only boiler - it is not a combi. The diverter valve thus switches the hot water being supplied by the boiler heat exchange (the primary circuit) between the CH or the DHW feed (or both at the same time).
The boiler switch-off temperature (on the boiler itself) should be set to 75-80. This is the temperature at which the boiler turns off, once the water in the primary circuit has hit 75 degrees. If you now go and set the tank stat at 80, the temperature in the DHW will atempt to get to 80 before it turns off. This is dangerously hot, so the gas fitter was correct in turning that down. It is never necessary to have DHW that hot. Water for a comfortable shower is just around (or over) 40 degrees C, so there is something seriously wrong with your shower set-up if you can't get a decent shower when fed by DHW at close to 80!
Can you confirm that the pressure fed to the cold feed of the shower comes from the same pressure source as the hot? - because I suspect that you have got the cold feed connected to the mains pressure, whilst the hot feed comes from a cold-water tank in the loft? That would explain why the supply to the shower is so lop-sided - but it could be another reason.
(continued)

[buildersmate]

Now to the radiators.
Test the CH separately by turning down the tank stat to minimum (to force the feed to the tank off) and turn the room stat right up. The rad should all get hot within about 10 minutes and feel roughly the same temperature. Is the temperature gradiant the same across each radiator? Coolness at the top ONLY, indictates gas at the top of the rad. Coolness in the bottom centre of a rad indicates sludge still in there. The (partial) problem you will have created thus far, by having the tank stat set so high, is that the poor old boiler is fighting hard to drive the temperature in the DHW tank is extremely high values - that alone will limit the rate of temperature rise in the rads - but it doesn't explain it fully.
Do these tests and report back.
I'll deal with your question on the hysteresis of the room stat on a third answer.

[buildersmate]

Room stats vary in quality but I guess that most have a hysteresis (the difference in temperature between the switch-on and switch-off points) of about 2 degrees maximum. You can hear the 'click' presumably, when you move it manually?
Have you got Thermostatic Radiator Valves (TRVs) on each rad? - most modern systems have these nowadays. If so, you should be using the setting of these to efficiently control the comfort temperature in each room, holding the one room stat (that is generally mounted in the hallway) at a slightly higher temperature. That way, the one room stat doesn't turn off the boiler BEFORE the TRVs have themselves individually turned off as the individual room temperatures reach their set values. Use a separate thermometer to establish individual room temperatures, if need be.

[Haynes67]

Thanks to Buildersmate; those were good answers and contained sound information.
I have now turned my tank stat back to 60 C, the sytem appears to be working fine. in fact as soon as I turned it down a little more it clicked and opened the diverter valve to the CH.
I do have valves on each radiator except the one in the hall. I do set the valves between 3 and 5 (top) depending on the room (bearing in mind it's winter).
The shower is working fine now. It is a power shower.I believe supplied from the storage tank in the loft, as when the house was built 7 years ago the builders had to raise all the loft tanks for the showers to work properly.
I am considering having a Magnaclean device fitted, I have 28mm pipes and the Heating engineer who fluched my CH only had a 22 mm at the time.
Any further advice would be welcome. Thanks again for your help

[buildersmate]

Power showers work by pressurising the cold and hot in-feeds to the shower by an equal amount. They only work well if the static pressure to the in-feeds on the pump is the same between the hot and the cold. That means both sources coming from the tank in the loft. The hot will automatically do so - so should the cold - only the kitchen tap should be fed directly off the mains supply pressure. I'm puzzled now - because you say the shower works again - and yet you have merely returned to the situation that you said didn't work before - cold water from out the shower.
Maybe the thermostat in the power shower is on the blink (technical term) or the hot water impeller of the shower pump?
I don't have a MagnaClean though have heard good reports about them.

[Haynes67]

Thanks for your last answer builders mate - the shower is no longer an issue and may well have been my family using both showers and along other heavy demands for HW following the days shut down during flushing.
However I am still having trouble maintaining heat in the rads. It appears they remain cool even with the HW switched off at the controls and the room stat set to 22 C. The room temp feels very cool around 17 C.
I would have thought that the divertor valve would have directed all heat to the rads, and they would heat up quickly - but the boiler only swicthes in for a short time; pauses then switches in again and continues to do this without heating up the CH. When my son advanced the HW at the controls, I heard the divertor valve operate and within minutes the rads started to heat up. I also noticed the boiler operating continuously without much pausing.
Surely the rads should heat up quicker with the HW swicthed off. As said at the start do you think I have had a sticky diverter valve fitted?
Thanks in advance

[gucciman]

from what you've said it sounds to me like the boiler is cycling on and off excessively due to air in the system rather than the diverter valve playing up. do you hear any rumbling or boiling noises coming from the boiler just before the burner shuts off?

you can test the diverter valve out for heating by manually locking the lever over on the valve.

and honeywell room stats are very accurate as they use a heat anticipator.

[Loopy]

My radiators arnt heating up. They do heat up when I push to switch to manual so one asssumes my diverter valve is working and both lights are lighting up. I have plenty of hot water from my condenser boiler. What does switching the diverter valve to manual overide do? What does it divert?the room thermostat? Maybe this is broken? Thanks