combo hot water tank supplies meager flow

[Mike Halmarack]

The hot water supply to the bathroom wash hand basin here is little more than a trickle.

Being located in a flat, the top of the header tank of the combination hot water cylinder is at a height of about 229cm/7'6" The outlet to the wash hand basin stands at about 91cm/3'

That's a maximum head of 137cm/4'6" IF that's how it's correctly calculated.

Controlled by the ball valve, the header tank is set at about 60% full.

Access to the tank being a tight squeeze for a dodderer, I need to ask if there would be any advantage in terms of water flow if the arm of the ball valve was tweaked to get the tank set at 90% full? Or would this have a negligible effect?

[tabbypurr]

It would change your 4'6" head to perhaps 4'11", so it wouldn't solve the problem.

NT

[Chris Green]

Do you really mean a 'combi' or not? It sounds as if the flat

*doesn't* have a combi boiler but this needs to be clarified first before trying to diagnose the poor flow rate.

[Mr Pounder Esquire]

This sort of thing has happened to me, with both the hot and cold water taps on the wash hand basin. I've got British Gas insurance (yes, okay). They came out the same day and fitted new valves to the taps. All has been fine since.

[Chris Hogg]

He may mean a Fortic tank, one of these or similar -

If it was a combi in the modern sense, low flow rate might indicate low water pressure, or the main stop-c*ck not open very far.

With the Fortic tank, I'd check to see there isn't a dead pigeon or rat blocking the outlet from the tank, or some other obstruction.

[Mike Humphrey]

The original post never mentioned a combi at any point, and it was quite clear that this is a gravity tank-based system with a very low head. My first thought would be to change the tap - most modern taps are designed for at least 1 bar pressure, and this setup has 0.13bar at most. Does the tank supply any other hot or cold taps? If so what's the flow like from them?

Mike

[NY]

I can't imagine this would have much effect. If you use a cylinder which is fed from a header tank rather than a mains-pressure hot water cylinder, then pressure and flow rate will be reduced. That's why header-tank systems use

22 mm rather than 15 mm copper pipe for the hot water - to try to get hot/cold flow rate which are a *bit* more comparable.

In my first house I had a cylinder which had a header tank welded to the top, and I got good flow rate into the washbasin and bath with about 4 feet of head - but that was with 22 mm pipe. In our present house, there is a much bigger head (high up in a first-floor loft, to ground floor basins/baths) and the flow rate is dire, and the time for the water to run warm is about 5 minutes (!!). I suspect they've used 15 mm pipe for the hot, so the flow rate will be lower (about half the x-sect area).

If I had a choice, I'd always go for mains-pressure heat-on-demand (combi) boiler - unlimited amount of hot water, equal hot/cold flow rates (makes it easy to mix them at a tap), fast time to run hot. I'm not sure why anyone still fits header-tank systems and/or hot water cylinder systems.

[Brian Gaff]

Is there a change in the bore of the pipe along the run somewhere or air in a sagging bit of the feed lower than the tap? Brian

[Chris Green]

It said "...combination hot water...", combi is an abbreviation for combination so that's why I asked the question. I did also say that "It sounds as if the flat *doesn't* have a combi boiler ...". I was just trying to make sure we knew what we were talking about! :-)

[Tim+]

Firstly, has it ever been more than a trickle? If it has then there might be some obstruction to flow somewhere.

I think the first thing I would do is take the hot tap apart and make sure that the washer hasn?t stuck to the valve seat making water pass through the hole in the centre of the washer. Some valves use a small nut to retain the washer and if this falls off the washer may detach itself.

Raising the water level in the header tank might help a little bit but if the flow was okay in the past I suspect this isn?t the cause. As others have suggested, crud in the header tank could also obstruct the outflow but this would affect all hot taps.

You?re never going to get great flow with this sort of system in a flat though and if your cold water pressure is good you might want to think about upgrading to an unvented HW tank. The other option of course would be a combi boiler. Personally I don?t like the ?all your eggs in one basket? approach of combi boilers. It?s all too easy for a fault to leave you with no heating AND no hot water. At least with a tank and an immersion heater you have HW back-up.

Tim

[John Rumm]

For all their advantages, there are circumstances where combi boilers are not appropriate at all, or will offer lower performance:

Combi boiler tend to require decent mains pressure and flow rate;

They exclude the option of mixing other sources of heat such as solid fuel, solar thermal etc.;

Typically have a limited maximum power output that can't match the delivery rate of a well designed storage system.

Don't allow for such easy provision of backup water heating.

Combi boilers don't always allow for sophisticated controls like weather compensation.

[Peter Parry]

It would have negligible effect. If it has always been like this you may have had a tap meant for high pressure (direct from a Combi boiler) fitted rather than the low pressure one you need.

[Chris Green]

... not to mention "combo" in the subject line.

[Cynic]

Presume you're describing one of these?

[Cynic]

[tabbypurr]

And since combis generally come without any backup immersion heat, a faulty combi requires rapid fixing, whereas a stored system is a lot less urgent. That can raise repair costs a fair bit.

NT

[Steve Walker]

While I agree with the rest, other sources can be used to pre-heat the water going into the combi - although it is going to be a much more expensive option, with a mains pressure vessel or some atmospheric system with a direct pump feed or a pump and large heat exchanger.

Sensibly, you are correct though.

Grrr. I am just about to replace our boiler. Old and new ones are heat only, we have a gas fire and a couple of plug-in electric heaters (not that its that cold yet), water is stored and we have an immersion heater

- so should be no problem and no great rush to get it done (I need time to make good the old flue position and mortar in the new one). Except, the immersion has failed and is seized solid in the tank and with five of us, a tank of water doesn't last long.

The tank itself is only about 5 years old, but when I tried to remove the immersion a couple of years ago, it wouldn't go. A combination of the access being slightly impeded and an immersion spanner from Screwfix that turned out to be slightly undersized meant that force was applied to the seized immersion at an angle and the tank has buckled slightly, making it weaker and so even less likely to withstand removal.

SteveW

[Mike Halmarack]

It's a Fortic tank, as first pointed out and linked to by Chris Hogg and later by Cynic. The label on the tank reads: "Combination Tank". The visible plastic pipework to both the hot and cold tap in the bathroom is 15mm. I'm not going to rip out the tiled box section that the pipework runs through to check how far back the 15mm pipework goes.

I like the idea of a mains pressurised hot water tank as a replacement for the Fortic, though it's electric only in this building. I don't know if there are electrically heated versions. I'll check.

There's a good chance that the wash hand basin tap is of a type needing high pressure, so Ill change it for a low pressure type as suggested, this being a relatively simple and inexpensive option.

Thanks for all the useful explanations and advice.

[Tim+]

They certainly are. I assume your cold water flow/pressure is adequate? If not, you won?t gain all the potential advantages of a pressurised HW tank.

Do have a look at the washer in the tap before replacing the whole lot. It is possible for the problem just to lie in the tap.

Tim

[Mike Halmarack]

The cold pressure seem pretty good in both the kitchen and bathroom.

This is the tap:

Can anyone say if this is likely to be a high or low pressure tap? I don't see that kind of detail in the information.

Also, would this tap have a fairly normal type washer? it seems to show a replaceable cartridge.