Boiler never reaches temperature

[DavidJW]

I have a Myson Apollo 15kW boiler with just a two position HI-Low temperature switch. It does not matter which position the switch is in, the flow temperature at the boiler never gets to more than 55degC (measured with a clip on thermometer). I turned the CH thermostat way up to 30degC, to make sure that it was not switching the boiler off, but it made no difference. The house got hotter (very) but the pipe temperature only crept up a few degrees and the boiler just ran and ran without cutting out.

I wondered if it could be that the pump is set to high (Grundfos Selectric 15-50 set to speed 3). The boiler installation instructions say that the pump must be able to deliver 1.145 cu metres/hour at a head of 3.35 metres, which the pump can do (just) on speed 3. So I am reluctant to turn the pump down in case it damages the boiler.

Does any one have any thoughts or suggestions as to how I can get the boiler running at a higher temperature?

 

[gas4you]

An average house will only need the pump on setting 2. Try this but I dont think it will make any difference.

Could be the boiler needs the gas valve resetting to the correct burner pressure, lack of working pressure from the meter govenor or even a faulty boiler stat.

 

[DavidJW]

Could be the boiler needs the gas valve resetting to the correct burner pressure, lack of working pressure from the meter govenor or even a faulty boiler stat.

It can't be the boiler stat. If it had failed, open, then the boiler would never light. If it had failed, closed, then the boiler would heat up until the overheat stat switched it off.

Gas is a Corgi thing - not for me!! The boiler needs a service in any case.

Any Corgi Registered readers near Hailsham, East Sussex or is it a Yellow Pages job? [/b]

 

[ChrisR]

Could be a clogged heat exchanger which just isn't getting much heat into the water.

What's the return temperature?

How many rads/rooms?

 

[Dan Robinson]

Any Corgi Registered readers near Hailsham, East Sussex or is it a Yellow Pages job

If you can wait until the bank hoiliday weekend I am going to see my dad for his birthday - he lives in Boreham Street.

Usual rates though .

 

[DavidJW]

Could be a clogged heat exchanger which just isn't getting much heat into the water.

What's the return temperature?

How many rads/rooms?

The return temperature is about 10degC lower than the flow (Boiler insts specify a 11degC drop across the rads and a 9degC drop at the boiler.)

There are 13 rads. I don't have their specs but have used the outputs from the current Stelrad Elite range, which are similar, to estimate their output. This comes to a total of 13kW. The rads are nearly 20 years old, so they are probably less efficient than the current version, say 12kW. Adding 2kW for hot water gives 14kW required from the boiler. This is set at max output (14.7kW), so it should be up to the task.

The house reaches the set temperature (21degC) ok, but the gas consumption seems a bit high. It works out at about 29000kWH per year.

 

[DavidJW]

Any Corgi Registered readers near Hailsham, East Sussex or is it a Yellow Pages job

If you can wait until the bank holiday weekend I am going to see my dad for his birthday - he lives in Boreham Street.

Usual rates though .

Thanks for the offer; I may take you up on this. Though I wouldn't like you to get into your dad's bad book!

Will email you for info about rates etc.

 

[gas4you]

It can't be the boiler stat. If it had failed, open, then the boiler would never light. If it had failed, closed, then the boiler would heat up until the overheat stat switched it off.

I've had several stats that have only failed at a certain setting. Last one was a Ravenheat combi where it failed when on full but worked fine up until about 3/4 setting.

 

[Dan Robinson]

You should never need to have the boiler on a high setting. My brother maintains an apartment complex with several hunderd of these 'orrible things and everyone works fine on low, but gets twitchy when on high - not your problem here though.

Can you do a gas rate for us? fire up the boiler and time how long it takes to spin the red dial on your gas meter one complete revolution.

If you have a metric meter then see how much the last three numbers go up by in 2 minutes.

 

[DavidJW]

Can you do a gas rate for us? fire up the boiler and time how long it takes to spin the red dial on your gas meter one complete revolution.

It's a bit hot now (38 in the conservatory, 28 in the living room) so I don't feel like getting the house any hotter at the moment

Will post the answer later, possibly tomorrow.

 

[DavidJW]

Just done a gas rate test. It took 65 seconds for one revolution of the dial. This works out at about 55 cuft/hour. The manual says 62, so the rate is a bit below spec; but is it enough to make such a difference?