Discuss couple of questions regarding y plan and s plan heating system question in the Central Heating Systems area at ElectriciansForums.net

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egginyourface

If the cylinder isn't full of hot water and you turn a hot tap on, will boiler give you hot water or does cylinder need to fill up 1st?? daft question i know...

And connected potterton boiler and read in the manual that there was no need for a permanent live if system is completely trv'd. So this means there will be no pump over run??

Advice would be appreciated
 
You will get water at whatever temp it is in the cylinder, if there is a cylinder in the system. The boiler will be triggered by the cylinder stat (if the timer for HW is enabled) in order to heat the water in the cylinder.
 
Not sure I understand the connection between a required perm live and "Fully TRV'd" in respect of pump overrun. The TRV's are normally a wax disc type flow valve that limits the passage of hot water into the radiator(s) for the room in question. The boiler (and pump) have no knowledge of them at all and will still fire up and run the pump if the call for heat (switched live, live supply whatever you want to call it) asks for CH to kick in.

Maybe ask on the plumbers forum for that, maybe it'll make sense to a plumber as it's beyond my ken ;)
 
Just had a quick nose on the plumbers forum, if the system is fully TRV's then the boiler will keep recycling the little water that's left in the pipework (not cycling what's been locked out in the rads). Two observations modern boilers appear to have a built in pump overrun (which maybe why the perm live isn't needed?). Having a fully TRV'd system is bad design anyway, I've been given advice previously to leave the bathroom rad (or airing cupboard rad if there is one) and the rad where the thermostat is non TRV'd. Hope that helps.
 
Sorry been out for dinner :) The wiring instructions are clear, no room stat (as there'd be no point if all the rads are TRV'd) and run the pump from boiler P/F connection (not from boiler live). I'm guessing that there is a timer that will cater for pump overrun, but only when the CH timer period is active (a minor design oversight).
 
Thinking about it a bit, that's really a poor design for the system. The only time the boiler will ever shut down (from a CH perspective) is at the end of the timer period (when there will then be no pump overrun available).
 
i think i agree there... ive been looking all over the net and found that some boilers dont need a pump over run to dissapate heat and prevent kettling.

as with this potterton
 
I suspect the control board will "know" when to turn off P/F based on the return temp of the water for CH and hence prevent kettling. IE if the return temp is "close" to the feed temp of the water for CH then turn off the pump and boiler (assuming no HW call) for say 2 mins. That's why I recon they say use the P/F output and not the boiler feed.
 
"Hello",

I am definitely NOT tryimg to be pedantic here - This comment is just for future reference regarding the Pump Over-Run on `Modern Boilers`:

The Pump Over-Run is obviously to Dissipate Heat from the Boiler Heat Exchanger - This is required because Modern Boilers often do not have Cast Iron Heat Exchangers - The Over-Run is to prevent VERY Hot Water from remaining in the Heat Exchanger when the Burners stop which could Damage the Heat Exchanger by `Distortion` / Expansion and Contraction - When the Boiler and Pump start up again the still VERY Hot Water in the Heat Exchanger is pumped out of the Boiler to be replaced with what will be much less Hot Water - hence possible rapid Contraction of the Heat Exchanger Metal.

Older Boiler that have Cast Iron Heat Exchangers and Larger `Waterways`do not usually require a Pump Over-Run - where a Boiler has A Cast Iron Heat Exchanger BUT Small `Waterways` - in these cases because of the Heat Retention of the Cast Iron and the possibility of an Overheat situation IF the Water is NOT Pumped Away after the Burners go out - A Pump Over-Run is required - A Bypass configuration should usually be Installed for these Pump Over-Runs which includes an Automatic Bypass Valve - and in fact there is a Regulation about this regarding Energy Efficiency, Gate Valves / Lockshield Gate Valves must NOT be installed on Boiler Bypasses.

`Kettling` is not the reason for having a Pump Over-Run on a Heating System - Although Corrosion Inhibitor such as Fernox is not really a `Dedicated` Lime Scale Preventor it changes the `Properties of the Water within the Heating System - Primarily `Negating` the Oxygen and `Suspending` minerals etc. in the Water - but this has the added effect of preventing the Formation of Limescale within the Boiler Heat Exchanger.

Because I do not Wire Up the Heating Systems that I Install I do not know whether there should be a Permanant Live to the Boilers that require a Pump Over-Run - BUT it seems obvious that IF a Pump Over-Run is required that only having a Permanant Live Supply to the Boiler would enable the Pump Over-Run to be able to Work ?

The Boiler could easily be Firing on a Maximum Boiler Thermostat Setting when the Programmer Setting switches OFF the Heating System / Boiler - Boilers that require a Pump Over-Run would HAVE TO be `Powered` to allow that to Over-Run to Dissipate the Heat from the Boiler Heat Exchanger in the event that the Boiler Thermostat was near to Satisfied - there ARE Heat Sensors on these Boilers which cause the Pump to Over-Run until the necessary Heat is Dissipated [Possibly controlled by Thermistors ?]

While one might suppose that If the Programmer was `Turning Off` the Heating System - it would presumably have been `On` for a period of time - then the Room Thermostat would probably be the `Governing Control` - The Boiler could obviously be `Firing` at the Off Time on the Programmer or Programmable Room Thermostat - So requiring the Pump Over-Run - Depending on the Boiler Thermostat Setting to some extent - I.E. If the Boiler Thermostat was set on a Mid / Low Temperature Setting [As some people do] then possibly an Over-Run would not take place.

The comment from topquark about a Heating System Radiators NOT being Fully TRV Controlled is absolutely Correct.

There should ALWAYS be a Room Thermostat and in the Area where that Room Thermostat is located the Radiator should NOT have a TRV - In fact it should have 2 X Lockshield Radiator Valves.

As the Radiator should be the CORRECT Size / Output for the Space that it is to Heat and NEVER be Turned Off - Turning it Off would cause the Room Thermostat to NOT ever reach the correct `Off & On` Temperatures - Unless perhaps installed in a Small Hallway and reaching Temperature by `Heat Gain` of the Hallway from the other Rooms.

There are of course situations where there is NO `Ideal` / Correct location for the Room Thermostat - For example in a small Hallway where the connecting Room Doors will always be Closed in Cold Weather - That would cause the Room Thermostat to Turn Off the Boiler at intervals when the rest of the Home / Rooms had not reached the correct Temperatures - as the small Hallway would probably very quickly reach the `Off` Temperature as Set on the Room Thermostat.

I hope that this has not been `Too Boring`.


Chris [Heating Engineer]
 
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Reply to couple of questions regarding y plan and s plan heating system question in the Central Heating Systems area at ElectriciansForums.net

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