Myson Marathon 700B gas boiler programme timer

[Myson700B]

The program timer on my Myson Marathon 700B gas boiler has failed.
My central heating is a gravity system.

I have options on what to do to fix the problem.

1. Obtain a replacement timer. It is an old mechanical one driven by an electric motor but a replacement is expensive and I'm reluctant to replace like with like. Also I have so far only found the combined Programme selector and Program timer as a kit at £162 from HCCF http://hccfspares.com . Ouch!

2. Buy an external electronic timer and connect the existing 6 timer wires from the programme selector to it. But which one and what are the connections?

3. Get an external 2 channel programmer and connect the existing 5 wires from the existing 6 way plug & socket to it. Again, which programmer and what are the connections?

I found a wiring diagram on the web and have posted the image here:

Advise will be very much appreciated.

 

[nickso]

erm it tells you what to do at the bottom.

 

[D_Hailsham]

My central heating is a gravity system.

Presumably the boiler is still in good nick

Q1 No pump at all? (There is room for one within the boiler case.)

A1.

Q2. How do you control the temperature of the central heating and hot water?

A2.

Q3. What options do you have for separate control of HW and CH; i.e. how do you turn the CH off in the summer?

A3.

I have options on what to do to fix the problem.

Get an external 2 channel programmer and connect the existing 5 wires from the existing 6 way plug & socket to it. Again, which programmer and what are the connections?

Probably the best solution. But which one will depend on what control you have over HW and CH.

In any case, forget about wiring to the 6-way socket; wire it to the boiler terminal block.

 

[Myson700B]

Thanks for drawing my attention to the Note at the bottom.

However, I think that this only applies to the boiler terminal block connections if I use an external 2 channel programmer (option 3.) I'm still no wiser on connecting the existing 5 wires from the existing 6 way plug & socket into an external 2 channel programmer. Perhaps those connection instructions will be supplied with the make of external 2 channel programmer I buy. Are there any suggestion of what make to buy please?

 

[Myson700B]

To avoid confusion my reply "Posted: Sun Nov 11, 2007 6:45 pm Post Subject: Thanks thus far" was in reply to nikso.

D_Hailsham

Q1 No pump at all? (There is room for one within the boiler case.)

A1. Yes the boiler is fitted with a pump for the central heating circuit.

Q2. How do you control the temperature of the central heating and hot water?

A2. The central heating temperature is controlled be wired programmable Drayton Digistat 3 wall thermostat.

Q3. What options do you have for separate control of HW and CH; i.e. how do you turn the CH off in the summer?

A3. The boiler has a programme selector to the left of the timer with 6 options: Off, HW twice, HW once, HW & CH twice, HW & CH once, Continuous.
So the Digistat 3 thermostat will not cause the CH pump to go on when the air temperature is above that programmed for the selected periods.

Probably the best solution. But which one will depend on what control you have over HW and CH.

In any case, forget about wiring to the 6-way socket; wire it to the boiler terminal block.

Yes, thanks for pointing that out.

 

[D_Hailsham]

Q1 No pump at all? (There is room for one within the boiler case.)

A1. Yes the boiler is fitted with a pump for the central heating circuit.

Good

The central heating temperature is controlled be wired programmable Drayton Digistat 3 wall thermostat.

But what about the Hot water temperature?

Do you have a stat on the HW cylinder controlling a motorized valve on the heating circuit to the cylinder?

The boiler has a programme selector to the left of the timer with 6 options: Off, HW twice, HW once, HW & CH twice, HW & CH once, Continuous.
So the Digistat 3 thermostat will not cause the CH pump to go on when the air temperature is above that programmed for the selected periods.

I am confused The Digistat 3 is a 7 Day Programmable thermostat so it incorporates the time control as well as temperature control.

If you have temperature control over the hot water, you are not limited to having the CH on only with the HW. All you will need is a simple single channel time switch to control when the hot water is on

The Digistat and the HW timer will then replace both the defunct timer and the Programme Selector.

Look at the Honeywell Range or the Drayton Lifestyle and Tempus Ranges

 

[namsag]

Starting from scratch straight forward wiring job take about half hour, but you need to clear up what sort of room stat you have

 

[Myson700B]

Reply #2 to D_Hailsham

But what about the Hot water temperature?

There is a rotary thermostat knob to the left of the program selector. It sets the maximum boiler water temperature.

Do you have a stat on the HW cylinder controlling a motorized valve on the heating circuit to the cylinder?

No.

I am confused. The Digistat 3 is a 7 Day Programmable thermostat so it incorporates the time control as well as temperature control.

Time control: yes. Temperature control: yes, of room air.

If you have temperature control over the hot water, you are not limited to having the CH on only with the HW.

That's not how my system would allow with the current controls when the timer on the boiler was working. As a temporary measure instead of the program selector being set at CH & HW on twice I have set it at Continuous. This means that the boiler will go on whenever the water temperature drops below the setting. As a result at night when the boiler goes on warm water convects to the upstairs radiators and make our main bedroom warmer than we like to sleep in unless we turn the radiator valve off.

All you will need is a simple single channel time switch to control when the hot water is on

So do I
1. Replace the failed timer arrowed in my diagram with an external timer?
or
2. Replace the Programme selector and Timer with an external timer?

 

[D_Hailsham]

So you have no proper control over the Hot Water temperature. Ideally the hot water from the tap should not be more than 60degC; the water temperature in the radiators needs to be higher than this, up to 80degC, for the radiators to give their designed output.

The way to do this is to convert the system to a C-Plan. This involves the installation of a motorized valve in the return from the Hot Water cylinder to the boiler and a cylinder thermostat.

details can be seen at:

http://content.honeywell.com/uk/homes/systems.htm#CPlan

The valve costs about £60 and the cylinder stat about £15.

If you just want to "get the boiler working again", the best thing to do is your second option.

Take a look at:

http://content.honeywell.com/uk/homes/Catalogue/Heating Controls/2.12 ST6200.pdf

If you do convert to C-plan, then the HOneywell ST6400 is suitable:


http://content.honeywell.com/uk/homes/Catalogue/Heating Controls/2.14 ST6400.pdf

 

[Myson700B]

Somehow my message posted yesterday did not get appended to this thread.

D_Hailsham, belated thanks for the info.

I may have to go away at short notice so I asked for an estimate from a local firm to supply and fit a Myson timer. They stated an astounding £280.

I then asked for an estimate to supply and fit an external programmer; an even more astounding £172 (£120 for a Drayton LP522 programmer and cable, £52 labour).) given that the Screwfix price for an LP522 is £49.

If I do find the time I'll probably get the programmer work done first then later convert my system to Honeywell C-plan.