Back boiler issue

I have moved to a house that has a back boiler system for hot water and central heating. I plan on having a combi installed when I have saved enough money. The issue I have is that the central heating doesn't work without the hot water being switched on. When I switch the CH on it sounds like the pump starts but the boiler doesn't fire up. The HW works as it should and the CH works fine as long as the hot water is on. As the CH does come on albeit when the HW is on I am starting to think the problem may be electrical. The boiler is a Baxi Bermuda 57/3M with a Honeywell ST499A time controller. Any help appreciated.

Show us a pic of pipework / valve etc around the hot water cylinder.

You have a gravity HW system, that is how it is supposed to work, your programmer just has not been set up properly, you cant get CH without HW that is just the way it is, you will regret getting a combi

You probably have what is known as a gravity hot water, pumped heating system.
When you turn the hot water on on the clock it just fires the boiler up and as the water in the pipes heat up.
The hot water will rise causing it to circulate by gravity heating up your hot water cylinder.
When you turn on the heating it only makes the pump run, which makes the water flow through the radiators.
So you will also need to turn the hot water on to fire the boiler up.
Some clock will automatically turn the hot water on when you turn the heating on if set to gravity mode but I don't think your's does that.

Edit beaten to it...

This is the setup. I don't believe I can't have CH without HW. Why would I regret a combi?

I am afraid you can't , Ian and John's posts above are correct.

Andrew Hunter said:

I have moved to a house that has a back boiler system for hot water and central heating. I plan on having a combi installed when I have saved enough money. The issue I have is that the central heating doesn't work without the hot water being switched on. When I switch the CH on it sounds like the pump starts but the boiler doesn't fire up. The HW works as it should and the CH works fine as long as the hot water is on. As the CH does come on albeit when the HW is on I am starting to think the problem may be electrical. The boiler is a Baxi Bermuda 57/3M with a Honeywell ST499A time controller. Any help appreciated.

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You can get independent heating and hot water by fitting a 2 port valve ( if 28mm) to the gravity circuit and fitting cylinder stat. This will heat water to set temperature and also allow you to get independent channel selection either being temperature control with appropriate thermostats.

I have wired controls without additional zone valve to give tempered water during summer by fitting cylinder thermostat but this does not work if heating is on in which case hot water will attain temperature set on back boiler.

Back boiler set up is unique and belt and braces. With no boiler function, you can get hot water as well as heat in one room. No other system allows that

if you are looking for improving the system, convert the system to fully pumped, if not at least fit a motorised valve to the cylinder and fit Evohome controls. Now you have heat where you need it and facilities that are leading edge technology. Best thing is, if you move, you can take the controls with you

While present system is not gravity system ( was pumped primaries to cylinder) I have fitted additional motorised valve for a client and tomorrow will install 8 smart Evohome heads to a old boiler that is bullet proof ( like your back boiler). She already has temperature controlled water heating and independent control of her system to finally be able to programme almost every radiator to run on its own timetable for different temperatures. This should cut her fuel bill drastically

Andrew Hunter said:

This is the setup. I don't believe I can't have CH without HW. Why would I regret a combi?

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Get an immersion heater fitted and pipes are ideal for conversion to addition of a valve. I wonder if that cylinder coil is suitable for gravity circulation as many cylinders off the shelf are only suitable for pumped primaries due to length of the indirect coil that results in poor gravity circulation.

One time replaced a leaking cylinder and upon filling up, would not heat up. Had to convert to pumped, result was rapid heating of water.

Thanks for the help on this. I will get a heating engineer out and put your suggestions to him. Sounds like with the correct additions I could continue using my back boiler for a few years before upgrading is required. Much appreciated.

Have to agree with the others above, that Back Boiler, if well looked after is virtually bombproof, but its is vital it is serviced annually, and get a CO Detector in the room if you haven't already got one. A decent Heating Engineer will be able to convert the system to Fully Pumped without too much ado, and that will give you much better control over the heating and hot water.

Combi's have their place but also their limitations, hot water delivery can be slow, especially during the winter months, they cant do Heating and hot water simultaneously, so whilst your bath is filling (slowly) the house is getting colder again, and if it breaks down, you've nothing. Get an immersion fitted in the cylinder and you've Hot Water back up.

+1 with keeping your existing setup. Friends of mine have the same system, theirs was fitted early 1970`s, they have lovingly maintained it since day one and apart from a couple of thermocouples/regular service every autumn it has cost them nothing. Still works well, no noises or problems - you only know it`s on when the gas valve clicks as it fires up

Johnmdc said:

Some clock will automatically turn the hot water on when you turn the heating on if set to gravity mode but I don't think your's does that.

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On that clock you remove it and turn a little link thing, then when you slide the CH on the HW moves too, but most of them break , they are very flimsy