Strange zone valve and pump behaviour?

[dpravo]

I have an S plan system and over the last few days i've noticed strange behaviour which I hope someone can help diagnose..

When the hot water comes on (typically on a timer), the h/w zone valve opens, the pump starts and then the boiler fires up.. after some time, with the hot water still 'on', the zone valve closes and the pump turns off. Then after some more time the zone valve reopens, the pump starts and the boiler fires up.. then the cycle just repeats.

What used to happen was that once the hot water came on, the zone valve would stay open and the pump would run the entire time the h/w was on.. with the boiler firing up intermittently to keep the hot water cylinder hot.

Can anyone advise what might be causing this change if behaviour?

To add to this ( and I hope its not related ) i did have a replacement pump fitted a few weeks back.

 

[cross thread]

Could be the syncron motor in the zone valve is getting “ tired” ,if it’s a Honeywell it’s easily repaired

 

[stem]

The hot water motorised valve opening depends upon two conditions being met at the same time.

1. The programmer or time switch being 'on' and
2. The hot water cylinder thermostat calling for heat.

This is usually the sequence of what happens:

1. The programmer turns 'on', and if the water in the cylinder is below the temperature set on the cylinder thermostat, both conditions are met, so the valve opens, the boiler fires up and heats the hot water.

2. The hot water cylinder reaches the temperature set on the cylinder thermostat and it switches 'off' thus closing the motorised valve.

3. The water in the hot water cylinder cools, the cylinder thermostat turns back on and the valve opens and reheats the hot water (provided the programmer is still on)

So, assuming that the hot water you are getting from the taps is at it's usual temperature and is not much hotter than it should be, that suggests that the controls are doing their job correctly and maintaining the set temperature. In which case, the boiler running is because heat has been lost and it's being replaced. That can be because someone has used some hot water, or the heat has been lost from the cylinder or adjacent pipework.

So, has anything changed? is someone washing pots earlier than usual, having a shower before going to work instead of in the evening, is there a hot tap dripping somewhere, has some insulation been moved?

As you have an S-Plan it will have a two port motorised valve. If the synchronous motor was getting tired it wouldn't have the strength to open the valve, so the water wouldn't heat at all, which is the opposite of what you are seeing.

 

[dpravo]

Thank you for the replies so far. I am getting hot water from the taps. So if I'm interpreting your comments correctly @stem then the zone valve opening and closing, and the pump turning on and off, all whilst the programmer is 'on' is normal behaviour? If so, that is great. As for something changing.. i would say approximately 2 weeks ago I had the boiler serviced and I had a new pump installed as the old pump didn't have enough grunt to heat the furthest rad. However, this new behaviour I have noticed did not start happening until the last few days. I.e. straight after the service and pump change it was still behaving in the previous way of zone valve staying open and pump continually running the whole time the programmer was 'on' (for hot water).

 

[stem]

So if I'm interpreting your comments correctly @stem then the zone valve opening and closing, and the pump turning on and off, all whilst the programmer is 'on' is normal behaviour? If so, that is great.

You are indeed interpreting them correctly.

Sometimes if the boilers thermostat is set lower than the cylinder thermostat temperature, it will run continually the whole time the programmer is 'on' because it won't ever get the water in the cylinder over to the set temperature and switch the cylinder thermostat 'off'

So unless you have increased the boiler temperature in the last few days, that won't be the cause. However, the sequence you describe sounds like the normal operating sequence of this type of system.

 

[dpravo]

Ok, that's really interesting. I have not touched the boiler thermostat. Thanks for the detailed explanations @stem - I really appreciate it. Am I right in saying that in theory I should be seeing this same behaviour when the heating is on? And the fact that I don't (i.e. the heating zone valve stays open and the pump runs the entire time the heating is on) means that the ambient temperature around the heating thermostat never gets up to the desired temp?

 

[Madrab]

Yes, ultimately the over riding control for both the CH or HW is the thermostat. If the temp hasn't reached the stats setting then they keep passing power to the zone valve to stay open and the switch in the zone valve passes it's own power to the boiler/pump to turn on.

Once the set temp is reached the stat turns the power off to the valve and in turn that stops sending power to the boiler/pump. If the temp drops then the stat calls for heat again and so on and so forth, until the programmer switches it all off.

 

[dpravo]

thank you @Madrab - one other query.. so when the stat is calling for heat, and the zone valve is open, and the pump is pumping and the boiler is firing I notice that the boiler will at some point stop actively firing/heating and then after some period of time it will fire back up - this fire up / stop firing, fire up cycle can happen multiple times without the zone valve closing or the pump stopping - is it the boiler thermostat that determines when it should fire up and then stop?

 

[stem]

The boiler thermostat simply controls the temperature of the water leaving the boiler so will cycle on / off as required. This will happen more frequently when only the hot water is on because the boiler will only be running at a fraction of its total capacity.

If the boiler kept running continually it would get hotter and hotter until the water inside was boiling. The boiler thermostat stops this happening by switching it on and off.

Newer boilers 'modulate' to achieve the same thing, i.e. the flame size rises and falls. but older boilers are simply 'on' or 'off'.

 

[dpravo]

Got it - thanks very much for all the excellent information! Much appreciated.