Our system is old with gas fired indirect system, pumped rads & gravity to tank, recently we noticed that the hot water isn't getting hot enough altho the rads do get very hot.
Putting it to heat the water with pump off only makes a little difference & the boiler thermostat appears to turn it off before it gets hot enough, very hot water also pushes into part of the rad system instead of the tank it seems altho has it always done this
My guess is poor circulation thru the coil in the tank? our water is not really bad but we do get a bit of scale in the kettle etc.
The two flow pipes from the boiler both connect to the same fitting on the side of the tank ( & share the expansion pipe) with the central heating one continuing on round the house & I guess its robbing most of the heated water that should go to the tank
Try using a commercial descaling liquid. Obtainable from B&Q, Screwfix, BES etc. Before adding it, I would turn off all the rads except one so that it is concentrated on the gravity hot water side. Follow the instructions carefully. Removing scale may expose leaks in the system, so be prepared.
Sounds like an airlock to me, try putting the thermostat up, then see if theres any venting points at the flow/return at the cylinder, if not, try cracking open the connections at the cylinder and let some water pour out(see if you get any air). Of course, it could be a blockage but an airlock is far mor likely. Not the easiest of airlocks to clear in my experiences.
Also, To be honest, your system is a bit dated. Think you should really be looking to upgrading boiler/system or going fully pumped at least. Im assuming its an old back boiler though.
I wouldn't look too much to your system layout as the problem. Whilst old and not ideal in light of current installations, it has not changed recently, whereas your problems are described as recent.
I would focus your thoughts on two possibilities, the first being poor circulation through the cylinder coil and the second being poor heat transfer.
Poor circulation can be caused by an air lock as suggested (check the vent pipe isn't blocked) or by sediment build up (particularly gooey sludge) in the pipe.
If you're confident the flow through the cylinder coil is satisfactory, check for scale build up on the outside of the coil.
None of the above is particularly easy to verify in practice and requires access to pipework likely to be carrying very hot water. It is not in my opinion a DIY job and could be expensive to rectify.
Personally, I would recommend saving money on expensive diagnostic work and instead investing in a new cylinder with the addition of suitable controls such as twin channel programmer, cylinder thermostat and zone valves to both heating and hot water circuits.
An open flued,open vented,gravity hw system with an ancient marathon boiler!!. Just take the hit and get a 'decent' combi put in like a 'vokera linea HE'. 2k to 3k if you know any good installers.
Otherwise try clearing the airlock or blockage.....for now.
And i disagree with one previous post implying you should keep your old system because its more reliable. Well depends what new system is put in but in my opinion open flued boilers should be long gone!!!! Other reasons inlude efficiency/performance of hw+ch/SAFETY.
I didn't say they were more reliable or they break down less than any other boiler. I said there's LESS to go wrong with them.
Thermocouple Gas valve
Gas valve pump
air pressure switch
flame sensing leads and electrodes
So when i said there's less to go wrong - I was right ?
At the moment I repair on average 12 boilers per day, At least half are open flued appliances. ?
why should they be gone ?
I agree they are dated, and aren't as efficient as combi boilers but... if serviced regularly they're not a problem. You get the problems with homeowners that decide having their gas service carried out annually is too much hassle or not worth it. That's when things go wrong and open flued appliance become dangerous.
All I was saying is that, in a post before mine someone said:
'Also, To be honest, your system is a bit dated. Think you should really be looking to upgrading boiler/system or going fully pumped at least.
That's great if you have a couple of grand sitting in the bank - people that can't afford to upgrade will be very happy with their open flued gas boiler.
You don't know capnbirdseye (or anyone else's) financial position - he didn't ask about a new boiler, his query was about the marathon and what the cause of lack of hot water could be - and there's no reason why this fault cannot be rectified.
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