boiler

[COG]

If you have a flow rate of 9ltrs per min you would have to have an inlet temp of 20 deg C as the temp dial on max is 60degC,in other words it wont modulate down at full flow if the dial is on full so would assume you have a gas pressure problem at the meter or the gas valve (check the meter valve is fully on) either way you need someone registerd to sort it out. ps try running the hot tap on full then drop down the controll panel down and find the gas valve test switch it shoul be set at normal, turn it to max then time the meter again this switch will hold the gas on full no mater what, if your timeing is 36 seconds ish then you have a faulty dhw sthermister if its still 96 then as said gas pressure prob PSS dont forget to put test switch back to normal position

 

[bippih]

Just to check last test,when i put the test switch on max the boiler fired up the central heating light came on,[ is this normal] then i opened the hot water tap fully,the time was still 92 secs.Am i correct in thinking that the switch overrides any modulateing output from the board thus eliminating possible faulty board? ps ive also timed it with two taps open,same result.

 

[COG]

Yes but later boards flash the ch and dhw lights together so your boiler must be at least 4 year old. You realy need to get someone to put a manometer on the gas valve to check the gas pressures .

 

[bippih]

Thanks for all the advice, ill get someone in and let you know the result.

 

[Agile]

It almost sounds as if the HE is scaled up!

I would measure the DHW NTC temperature and resistance and the CH one while I was about it. I have known them fail.

Tony

 

[bippih]

The fact that the timeings for the gas rate are not correct in override,how would the heat exchanger being scaled up influence this. ps ive already checked the resistance,and changed the thermisters.

 

[Agile]

Might be the primary flow is reduced perhaps because of a pump go-slow.

Could also be a blockage in the primary circuit.

You really need to get an idea of the temperatures on either side of the plate heat exchanger. Should be about 70*C on the inlet and about 50*C on the outlet.

The outlet should be too hot to keep you hand on it and not too great a temperature difference between the flow and return if the flow is correct.

Tony

 

[bippih]

When the dhw is flowing approx 4 litres/min the temp on the primary flow and return to the water to water heat exchanger is too hot to touch for any length of time.When the flow is at 9 litres/min the inlet and outlet are luke warm.why would the temp on the primary flow and return on a closed loop system drop off when the the duty hot water flow increases?

 

[ChrisR]

... because the primary water is going so slow through the h/e that the secondary water is taking much of its heat away. See?

 

[Agile]

Actually if you think about it the primary pump speed is correct other wise the temp differential would be increased.

The problem is lack of enough heat input to properly heat 9 li/min.

This would appear to be a control or gas related problem.

The next check a CORGI registered engineer would do is to measure the gas inlet pressure at the boiler. It sounds as if this is low.

One assumes you have checked the meter control valve is fully on and that any secondary valves are also fully open. Just possible the meter regulator has failed or a blockage has occured in the gas supply. Alternatively it might be the control circuit is not fully activating the modulating coil.

Tony Glazier

 

[ChrisR]

You could do the electrical checks - the sensors(ch and HW are the same ) should be between 1 and 30kOhms. You could swap them tho I expect they're "wet".
Next, monitor the voltage on the HW sensor as the water temp rises, and the voltage on the mod coil of the gas valve, blue wires.
High flame should be about 34Volts, low about 9 Volts.

 

[bippih]

Tested the voltage at the mod coil,removed wires from coil and opened tap full voltage was 34 volts.what seemed strange was that the dwh was still hot at half bore and dropping off fully open with the wires disconnected.Ran for a period of time but didn't go into low flame,is this maybe because the temp is not hot enough, thus not getting the signal from the thermistor to go into low flame. Did you mean check the resistance on the thermistor in relation to the temp.

 

[ChrisR]

34 volts and not enough gas narrows it to the meter regulator, blocked gas pipe, mod coil or gas valve itself. Anyone think of anything else?

Sorry mate it's pooch time.