Hoping the ST hive-mind can help me with this one!

We have moved into a house with a combi fitted, first time I’ve had one.

I’m a bit disappointed with the performance of it, tbh. CH is ok, but DHW seems lacking.

I’ve went round all the hot-water taps in the place and they all seem to be the same- I get an output of about 5.5 litres of hot water per minute. Bath fills, but takes a while.

I’m going to call in a professional soon, but a friend noticed something he thought odd- the boiler is on the 1st floor, and seems to fill from a cold-water tank in the attic. The tank is definitely filling up, and has fresh cold water in it.

Is this unusual?

combi hot water normally run from a mains inlet, not tank.

Very – I assume it was patched into an existing system (perhaps done like that because mains pressure is *very* low – what is your cold tap like)?

A good combi boiler will give you very hot water pretty much at mains pressure (if it is plumbed into the main rather than coming from a gravity-fed tank).

I bet they replaced a conventional boiler with a combi (very badly) and just plumbed it into what was already there.

Thanks all.

Cold water is not too bad, but not brilliant either. I will do a flow-rate check on all the taps when I get home to get some accurate numbers.

sounds like a cowboy install to me.

ive recently done the same conversion from gravity system to a mains pressure combi – its like night and day.

my tanks went to the scrappy and we replumbed the system to miss all that stuff out and regained a lot of indoor space back.

it takes a little while to displace the cold water in the pipes if the tap hasnt been run for a while but still superior to how long it took from the hot tank.

whats the boiler ass 5.5litres seems low- mines rated at 17litres/min it might just be a poor boiler. i have WB in my last two houses that couldnt cope with running baths yet my grant and my parents monster WB are happy enough .

Cheers TR- we’ve got a plumber coming in later this week to help me confirm that its all fed from the tank (just in case I’ve made a mistake- IANAP).

Boiler is a Vokera Compact- I’m not at home just now so can’t say if its a 24, 28 or whatever.

Getting rid of the tank will get me more loft space too, which would be great.

I bet they replaced a conventional boiler with a combi (very badly) and just plumbed it into what was already there.

+1

I think you may all be right on the idea that its been a straight replumb into what was there already.

My wife has just remembered the former owner telling her that the old standard-style boiler used to be in the kitchen, but burst and flooded out the ground floor. Insurance were all over it. This was approximately 5 years ago.

For some reason they decided to put the new boiler in on the 1st floor, and swapped to a combi (perhaps because of the age of old unit, I don’t know).

Not necessarily a bad thing is it? I’m sure our old house had a combi boiler with water header tank, the tank provided the head to run the central heating circuit with the advantage that it kept itself at the required level (our new house combi requires a small top up every year or so). If its a normal combi then it’s a bit lazy of the installer but you’ve got the advantage that its dead easy to add fernox to the system! I think it’s worth checking your incoming water pressure, it’s very cold at the moment but 5 l/min is low even for a low output boiler (perhaps the incoming pressure is so low they had to use a header tank?). I think the water people guarantee a minimum delivery pressure and if you’re not getting it they have to sort it out (if its their pipe work causing the issue).

Thanks dave, I’ll check that out.

are you sure about that dave. normally* your CH circuit on a combi is a sealed system – ie you fill it on the fill line and then close it off locking in the pressure , if it needs topped up its leaking.

dont fancy washing my face in your hot water if your adding ferenox to the header tank :s

* normally as in my day job is not plumbing but on the various systems ive had and worked on ive not seen a combi with an open feed line to the CH . admittedly all modern condensing combis…..

I’m not going completely mad i don’t think!?! Quite unusual perhaps but I’m sure it can exist: combi boiler but with cold water header tank on the central heating instead of the mains filling loop/lock off approach. The central heating circuit does the radiators (obviously!) and is pressurised by the head of water from the tank in the loft (and this tops itself up if required). The hot water is completely separate and is fed from mains water.

Quite why you’d choose to do it this way I don’t know, perhaps easier to install/cheaper boiler, or given the OP’s lack of hot water flow I wondered whether its because the mains water pressure is so low that it wouldn’t be able to pressurise a normal combi to 1-1.5 bar.

And as for top up, I’m talking a tiny amount to top the pressure back up. My understanding is corrosion disociates the water, the oxygen is used to form metal oxides (rust for want of a better description) and the Hydrogen collects in the highest radiator until you bleed it. You then need to top the water back up. That’s where Fernox and the like come in!

Seems to suggest it can be fitted with a header tank (page 6):

http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&ved=0CC8QFjAA&url=http%3A%2F%2Fcdn0.vokera.co.uk%2Fwp-content%2Fuploads%2F2009%2F04%2Fcompact-he-installation-and-servicing-manual.pdf&ei=hVIBUbWpNu_D0AXz0YDwDA&usg=AFQjCNEHudv14cuIBf0kLmFNwjjmKkcOfg

It does say:

“LOW PRESSURE SEALED SYSTEM
….an independent make-up vessel or tank
mounted in a position at least 1 metre above the
highest point in the system and at least 5 metres
above the boiler….”

I’d need to measure it out, but I think its probably about 2 metres from the top of the unit to the bottom of the tank. Will check tonight.

My understanding is corrosion disociates the water, the oxygen is used to form metal oxides (rust for want of a better description) and the Hydrogen collects in the highest radiator until you bleed it. You then need to top the water back up.

Corrosion doesn’t disociate water, it’s just dissolved oxygen in the water. Otherwise you’d blow your house appart bleeding radiators (hydrogen is flamable in very low concentrations and has very low ignition energy).

The reason reason heating (or engine cooling, I’ve run a car on plain water without any issues) systems don’t imediately dissolve in a puddle of rust is that hot water actualy contains very little disolved gas and there’s little/no contact with the air to disolve any more (compared to cold water, like rain on a bit of unpainted steel).

Hydrogen = squeeky pop no? You are of course quite correct that corrosion doesn’t disociate water (thats me writing quickly on my teeny phone when i should be working), dissimilar metals/electrical potential disociates the water. The O2 is mopped up by corroding the metals, hyrdogen is released when bled. Your explaination might be the more dominant one but i’m sure the above mechanism can also come into play.