Cannot run Central Heating and Hot Water at the same time - Help!

[CointreauVersial]

We have a three-year-old condensing boiler (so a hot water tank) and a problem has recently manifested itself.

The Hot Water works perfectly on its own.

The Central Heating works perfectly on its own.

But if you try to run both together only the Central Heating works. So if the kids want a bath we have to turn the Central Heating off, or the hot water tank stays resolutely cold. Really irritating when there is six inches of snow outside.

We thought it was the 3-port valve, so we had that replaced last week, but it has made no difference. The plumber seems stumped, although he's coming back tomorrow to have another look.

Any bright ideas?

Anyone??

Pigletjohn, where are you when I need you?!

What do you mean, "you have a life"?

Hi

WE have a similar problem but only when its really really cold. B.Gas told us it was because the water coming into the house was too cold and the boiler couldn't heat it up enough!

Sometimes its a careful balance of turning on the hot water for just the right amount - it runs cold for a while and I can hear the hot water come through. once it's running hot you carefully turn the tap on more. It's a real pain when I want to run a bath as you have to keep a close eye or the bath ends up freezing. Doesn't sound as bad as you and we can get some hot water,

if it used to work properly and has now stopped, then it will be a fault rather than a design or build error. However if it has never worked properly it weas probably installed wrong. If the system has been altered or enlarged, e.g. for an extension, the flow design might be bad.

If you have a hot water cylinder and a 3-port valve, then it is almost certainly not a Combi boiler, like I think Mercedes, has, so diagnosis will be quite different.

Is there a cold water tank in the loft as well? (I might need you to go and see if there is a big one and a small one)

If he's changed both halves (the motor and the valve*) of the three-port valve with no improvement, then it might be a control or a wiring problem. How old is the programmer? How old is the boiler, and what make and model?

What do you know about the qualifications and experience oif the person who tried to mend it?

*The three-port valve has a brass part, with the three copper pipes going into it, and a motor, in a galvanised or plastic box fixed to the top of it. If it has been replaced, both parts should look visibly new and shiny.

The other thing I would be looking at is the balancing of the CH and the HW primary flows. More often, the flow through the cylinder is easier than the flow through the rads, so an adjusting gate-valve is used to restrict flow through the cylinder. In your case I would be looking to see if it has been throttled back too far. A gate valve has a "wheel head" (round) not a T-handle like a stop-cock.

Ask the heating engineer if the radiators appear to have been balanced (the inlet pipe on each should be "too hot to hold" and the outlet pipes should be "too hot to hold for long"

Feel the pipes on the 3-port valve and see if they are all hot, or if the one going to/coming from the cylinder is completely cold, or a bit warm. Feel along the pipe to see if it is colder or hotter further along.

If your boiler was replaced a few years ago on an old system, it ought to have been cleaned out to remove sediment. Ask the heating engineer if it appears to be clean and if it appears to have inhibitor in it.

If you have a small tank in the loft (feed and expansion) verify that it has several inches of water in it above the outlet pipe near the bottom of the side, and that when you push the float down (like in a WC cistern) top-up water squirts in. See if it has much mud in the bottom.

Wow, PigletJohn - thanks for a comprehensive answer, as always!

No, it's not a combi boiler - it is a Worcester-Bosch Greenstar Ri, only installed three years ago; the programmer was new at the same time. Yes, we have an expansion tank in the loft.

We've only noticed the problem recently, but it's only when we do something unusual, like keep the heating on overnight, or have a bath at an odd time of day that we notice it.

The engineer who changed the 3-port valve was someone we found through Checkatrade, so has had good reviews and presumably knows his stuff. He changed both halves of the valve (I have the old one, and it was visibly broken), and the ancient CH pump too (which has worked miracles on the speed at which the heating comes on).

Anyway, he came back today to balance the rads properly. He did comment that the system was full of black grot, and almost certainly needed flushing, and several ancient rads needed replacing (some have busted valves/cold spots etc). I thought this was just a cynical attempt to extract more money from us, but judging by your post our gunged-up central heating might be a contributing factor to the whole hot water issue!

He also replaced the thermostat on the hot water tank, did all sorts of fiddling around, flow-adjusting and pipe-feeling, checked the wiring, and concluded that the valve now worked "intermittently". At the moment, the pipe from the 3-port valve into the HW tank is hot, but not scorching.

At least there was no charge for today's efforts. He has left us to consider next steps......... In the meantime we'll continue programmer-juggling so we get hot showers in the morning.

I have no idea if the system was flushed when the new boiler went in - it was a bit of a crisis installation on the day we completed on the house purchase, minus 4 outside and removal men in and out - I wasn't really watching what he was doing.

So maybe the engineer wasn't blowing smoke, and a system flush is what we need to sort things out. Hmm....

I forgot to say - according to the engineer our system doesn't have a gate-valve to adjust the flow.

Do you know where the drain-cock is?

Er, no, 'fraid not.

ours does this, it is meant to have "hot water override". We have lived here for 11 years and I can't remember whether it worked in the beginning, but it certainly didn't work for a number of years. We have had the switch replaced and I think it worked last winter, but it isn't working now.
I just have the heating programmed to come on for 40 mins before I wake up and go off so I can then have a shower. It doesn't seem to matter so much with baths as it runs hot then cold, but with the shower it can be a killer!

CointreauVersial "Er, no, 'fraid not."

then you won't be able to do a simple chemical clean until you find it

PigletJohn - That's not to say we don't have a drain-cock, just that I don't know where it is!

I would leave a chemical flush to the experts, but it is handy to be armed with a bit of insight as to how things work/what the problems are so I don't find myself paying for something unnecessary.

Is it possible that flushing the central heating system/replacing faulty rads etc might actually improve the problem with the hot water?? I'm still trying to visualise how it would have an impact. I guess they share the same water - doesn't the hot water tank have an element, which is just like a "big radiator"?

Chasingsquirrels - yes, I have learned to juggle the programme timings, but in this cold weather we have wanted the heating on moreorless constantly, which has buggered things up somewhat.

there is a simple DIY clean you can do with Sentinel X400, which is a non-aggressive chemical that loosens old sediment. You just need access to the F&E tank and the drain cock. It usually does a lot of good and often a powerflush is then not needed. It is very often needed on older open-vented systems.

From what you say, the new boiler was fitted on a dirty system. A power flush takes about half a day, with the workman going round all the radiators to turn them on and off individually, and pumping dirty water down a drain with hoses, and using a large machine with a powerful pump and a vat of chemicals.

A chemical clean takes about as long, but with X400, you just leave it circulating through the system while it is working for up to four weeks before draining and rinsing, so it is an easy DIY job.

Sediment deposits interfere with proper flow, and cause poor heating and balancing, sometimes cause blockages which are very troublesome. They also cause working parts such as the 3-port valve and the pump to wear out early or become clogged, and cause the boiler to make crackling or clonking noises. The sediment is mostly due to iron oxide from corrosion of the insides of the radiators, and to some extent also limescale.

The primary circuits from the boiler send the same water circulating round the radiators and/or cylinder depending on the calls on the 3-port valve. Yes, inside the cylinder is a coil of copper tube, that the boiler water circulates through, and is immersed in the tapwater, which it heats (they do not mix)

This is brilliant information, thank you.

Based on what you say, I think we will budget for some new rads and a good chemical clean in time for next winter (just spent £1200 on our two cars, so money's too tight to mention at the moment).

Spring can't arrive too soon......

the sooner you do the clean the better.

tell me when you have found the drain cock

if you are swapping rads, change all the lockshields for new, and fit TRVS everywhere except in the room with the wall stat

remember to include inhibitor on final fill

As you have a history of sediment problems, also budget for a sediment filter such as a Sentinel, Fernox or (my favourite) a Spirovent. It will cost roughly £100 plus fitting. The Magnaclean is very effective, but poor materials and build quality means they often drip and leak after a year or two.

Better start saving.

If you do a chemical clean I assume you have to do it while the heating is on (i.e. in the winter) so the stuff is circulated.

A filter sounds like a good plan too.

X400 is used while the system is in normal use, and I recommend it for a DIY clean. There are other cleaners which are stronger and faster.

p.s.

A typical house needs one litre of X400, which costs about £15, and, after cleaning , draining and rinsing, a litre of X100 inhibitor to prevent future corrosion, cost the same.

Not much, is it?

I have to confess I'm a little wary of the DIY option. I would probably stick to GSI on this occasion (Get Someone In). It isn't just draining the system, it's getting it all balanced and working again afterwards (it took the plumber ages).

In addition I am blessed with a husband who doesn't know one end of a spanner from the other.