Central Heating OK but hot taps are only lukewarm

[DaveColchester]

I've just moved into a new home and have discovered that that although the central heating (underfloor heating in this case) and water fed heated towel rails work fine, the hot water taps only provide warmish water that soon goes cold; maybe enough for one quick lukewarm shower but no good for a bath.

There is a gas fired Vaillant boiler and a storage tank. There is a thermostat on the storage tank set to 60 degrees C while the boiler has an adjustable electronic dial set to 70 degrees C.

Untitled

• DaveColchester
• 19 Oct 2012
• 1

Untitled

• DaveColchester
• 19 Oct 2012
• 1

I understand these boilers normally have 5 pipes attached (Central heating in/out, hot water in/out, and gas in). In my case there are only 4 pipes and it seems it is the hot water out pipe that is missing. Following the central heating out pipe from the boiler it looks like it splits up and feeds into the water tank at multiple points (Bloke next door says there are actually two separate hot water tanks?)

When the central heating comes on, or somebody runs a hot tap, the boiler fires up successfully. I felt the central heating output pipe from the boiler - it is very hot (and actually the two pipes delivering water into the boiler are also hot and look like they take water from the hot water tank)

The funny thing is that there is an electric wire coming out of the storage tank to a switch on the wall labelled "WATER HEATER". This however was turned off when we moved in - I assume that's how the previous owner had it all the time.

I turned this heater on for a few minutes: immediately there was loud noise from the storage tank and pretty soon the hot water taps did feel hotter, but I didn't want to leave it on for any great length of time.

So maybe that will fix the problem and if so I'll reluctantly do it, but I'm very puzzled by the set up. I've never heard of a gas boiler feeding water into an electrically heated water tank, and I can't see how the gas boiler seems to easily make the water in the towel rails extremely hot but can't do the same for the hot water taps?

 

[dcawkwell]

Can't make it out on the photo.

The normal arrangement would be

flow from boiler -> pump ->
then either a diverter valve and separated off to
tank and central heating.

Or you might have

flow form boiler -> pump -> then 2 port valve for central heating
or 2 port valve for hot water.

I can see the pump in bottom right corner but don't see how it's connected
or any diverter valves. Looks wrong but can't make it out would need closer photo of the right hand corner

 

[David937]

I know this isn't relelvant to your post but the pressure on your boiler seems to be on the high side, what does it read?

So maybe that will fix the problem and if so I'll reluctantly do it, but I'm very puzzled by the set up. I've never heard of a gas boiler feeding water into an electrically heated water tank, and I can't see how the gas boiler seems to easily make the water in the towel rails extremely hot but can't do the same for the hot water taps?

It is quite common to have an indirect cylinder heated by the boiler but an electric immersion heater to heat the tank in case the boiler fails.

This will give you some more information: http://www.gasapplianceguide.co.uk/Direct and Indirect Cylinders Explained.htm

 

[DaveColchester]

Pressure reads about 2.7 bar

If I turn on the immersion heater on the water cylinder it goes even higher! That's mainly why I don't want to turn it on.

Is this safe? How can I get it down?

 

[DaveColchester]

Some more info:

The cyclinder is one of these I think:

http://www.nu-heat.co.uk/s.nl/it.I/id.706/.f?sc=7&category=250

But maybe a slightly earlier model as it doesn't seem to have a "shunt pump"

 

[picasso]

Do you have under floor heating ? maybe that is what the pump is on the left of the cylinder, I think the blending valve has gone and that is why you have no hot water, I have to go out but I am sure someone else will tell you how to check.

 

[DaveColchester]

Yes we do have underfloor heating

 

[David937]

Pressure reads about 2.7 bar

If I turn on the immersion heater on the water cylinder it goes even higher! That's mainly why I don't want to turn it on.

Is this safe? How can I get it down?

It should read 2 bar at the most when the system is hot and around 1.2 - 1.5 when cold. Drain some water from a drain cock on the system to bring the pressure down and keep an eye on it afterwards. I can't really comment on why it goes higher when you use the immersion heater so I shall leave that to the more experienced posters!

 

[Agile]

Fairly obviously the stat in the immersion is set too high!

It seems to be a heat only boiler, gas, flow return and condensate.

How its connected to the cylinder and UHF is unknown until we have a photo of the pipes under the boiler.

Tony

 

[Boilerman2]

Dave you are right this is a System boiler linked to a thermal store, the Hot water is heated by a coil within the tank, the Thermostatic mixing valve to the right hand side of the store controls the final hot water temperature to the taps, this may be faulty, or the store may not be hot enough the water pressure is too high,

From the evidence provided I suggest you contact a local heating company to carry out a Service to the boiler and investigate the problem with the hot water, the solution to which will never be "guessed" on here due to the complexity of the system