Need advice on which time delay relay to use

[SteveP86]

Hello everyone.

I have an ageing central heating system that still works fine and outputs plenty heat however it bangs loudly from the boiler area when switching off due to possible build up of sludge.

The symptom is as follows...

I have gravity hot water and pumped central heating with an injector tee for the water circuit. When the heating is on and the thermostat is calling for heat everything is fine. When the room thermostat is satisfied the power to the pump is cut however the boiler remains lit, as it's supposed to, to continue heating the gravity hot water. This results in very loud banging if the boiler stat is set to anything other than LOW as a result of the water not escaping the heat exchanger fast enough.

What I did to try and combat this was to also wire the boiler to the room stat so when the room stat is satisfied the pump AND boiler turn off. This is working great if I keep the boiler at LOW or just above however this means the rads only heat to approx. 46 deg. This wiring setup is ok for winter when the heating will be on long enough per day to ensure the hot water tank remains hot. During summer months I will revert wiring so hot water ONLY can be selected.

The solution I have thought of is to include some sort of time delay realy for the pump power supply

1. room stat will switch off once ideal temperature is reached
2. power to boiler is instantly cut extinguishing flame
3. time delay will kick in (approx. 5-8 seconds)
4. after time delay pump will switch off

As a result of the above what will happen is I can have the boiler set to 70 deg for rads and when the boiler is off and the pump continues for the 5 or so seconds this will be sufficient in drawing enough heat from the boiler therefor no loud banging from pipework.

I can do this manually by turning boiler stat to 0, wait 5 seconds then turn down room stat to turn off pump = no banging so the time delay relay will automate this all for me.

What I think I need is a time delay relay that starts the power off count down when a voltage is removed ie the power supply to the pump that the room stat cuts.

If anyone can recommend a particular switch and possibly supply a link that would be appreciated.

The alternative to all this is to convert various rad valves and get a power flush at many hundred pounds so am willing to try this electrical solution first.

PS the boiler is an 18 year old cast iron baxi Bermuda inset 2 50/4 (power requirement 4 watt!)

Many thanks and sorry for the long ramble!

Steve

 

[SteveP86]

//www.diynot.com/forums/viewtopic.php?t=62214

just found this thread which explains my problem in about a tenth of the time, doh!

So yes, basically what he said but the Pipe thermostat is a no go for me because of the pump location. I feel this pump overrun option is the best for me

Best

Steve

 

[AlanE]

Fit something like this in boiler-
http://www.ebay.co.uk/itm/vokera-li...arden_Hearing_Cooling_Air&hash=item2edae9a68d

 

[DaveHerns]

I don't know but would it be possible to rewire so you have pumped hot water instead of gravity circulation?

 

[SteveP86]

I don't know but would it be possible to rewire so you have pumped hot water instead of gravity circulation?

There are no two port valves on my system so it's impossible to have pumped hot water without getting a plumber in to repipe accordingly. All the pipework is approx. 40 years old with one boiler change which was almost a like for like, old back boiler out, new back boiler in.

The pump is under the living room floor so would be a nightmare to get to (furniture, carpet, floorboards etc.)

The time delay relay would be the easiest as the pump power supply along with the rest of the heating electrics culminate nicely in the cupboard under the stairs

Steve

 

[AlanE]

As posted above fit an easily obtainable pump over-run stat , cheaper than a time delay relay.

This was the system that was standard fitment at the time your boiler was produced. Electronic pump overrun did not come onto the scene until PCB's started to be fitted in boilers. There was also 'upgrade' kits available for some boilers at the time where it was found beneficial for the pump to continue running after the boiler stopped.

You could find yourself replacing the pump in the not too distant future as it sounds as if it is not shifting the heat fast enough.

With regards powerflushing this might be a bad idea on a system as old as yours. A better solution would be a simple mains flush. Remove radiators one at a time and flush through with clean mains pressure water. Also flush pipework with mains pressure water.

Any flushing process using chemicals could well cause your system to leak to some considerable extent.

 

[SteveP86]

As posted above fit an easily obtainable pump over-run stat , cheaper than a time delay relay.

This was the system that was standard fitment at the time your boiler was produced. Electronic pump overrun did not come onto the scene until PCB's started to be fitted in boilers. There was also 'upgrade' kits available for some boilers at the time where it was found beneficial for the pump to continue running after the boiler stopped.

You could find yourself replacing the pump in the not too distant future as it sounds as if it is not shifting the heat fast enough.

With regards powerflushing this might be a bad idea on a system as old as yours. A better solution would be a simple mains flush. Remove radiators one at a time and flush through with clean mains pressure water. Also flush pipework with mains pressure water.

Any flushing process using chemicals could well cause your system to leak to some considerable extent.

I put boiler silencer in a few weeks ago (x200) and it did make a slight improvement. I also replaced 1 rad earlier this year due to being underpowered and plan on replacing another 6 along with TRV's next summer as well as draining and refilling numerous times so all rads should be modern.

Would something like this do?

http://www.plumbworld.co.uk/honeywell-l641b-tamperproof-761-12679

would this mean the pump will effectively have two switched live feeds supplying it?

steve

 

[Spark123]

In essence it will have two switches controlling it, the trick is to do it so you are not back feeding a supply to the boiler so it keeps firing. I think you can get relay boards for this.

 

[AlanE]

As spark said you would need to ensure no back feeding occurred. How you do it you would have to decide since there is noway anybody on an internet forum can ensure any modification is working as intended and not causing harm to boiler.

Proper, and best, solution would be to flush out existing radiators and pipework with the idea of getting the system to work correctly. Start by draining a small quantity of water from system, and ensure the expansion tank refills for the water loss. This will ensure the system is really full of water. Just because there is water in expansion tank proves nothing.

 

[SteveP86]

Thanks for all the advice so far.

The system will be drained and filled a good few times in the nicer weather however for the time being - due to pipework inaccessibility - I am going to go for a power off delay relay from RS Components (the kind that can be mounted on a DIN rail in a CU). Just need to figure out how to wire the thing as I've not had the need to use one before.

If anyone can point me in the direction of a suitable relay from RS Components that would be appreciated as they seem to have literally hundreds of them


Many thanks for the advice.

Steve