CH okay, but no HW

15 Dec 2007
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United Kingdom
I was hoping someone could help me find the source of the problem with our hot water.

We have a gas boiler (not combi) and a programmer that allows us to set HW, or CH+HW, or both off. The CH is working fine, but we get no hot water, or mildly luke warm. We have an immersion heater in the cylinder which we have turned off for now to find the source of the problem.

I've tried reseting the programmer, but the results are the same. I can hear a noise like a pump in either HW or HW+CH modes. When it is set to HW only, the boiler does not fire up (i.e. only pilot), however it appears to fire up for about 1 min every 20 mins or so. The pipes going to/from the cylinder do not appear to be hot. If the boiler fires up for 1 min, the top pipe on the side of the boiler is briefly warm.

I've found a pump under the floor boards with an intake and outtake and a speed setting. After the pump there is a branch with thinner pipes going one way (HW?) and a thicker pipe going another way (CH?). On the CH branch there is a eletrically controlled valve. When both CH+HW are on the CH branch is warm, but the HW branch is not.

I put "HW only" on for 12 hours and I was able to have a very hot shower, but now the hot water is gone. I don't know if this was a glitch or if it just took forever for the HW to warm up.

Here are some details:

Boiler: Glow-worm Space-Saver 45-60B
Programmer: Landis & Gyr RWB252
Immersion heater: Redring P27 3kw
Pump: Grundfos Type UPS 15-50x18 MkII
Valve: Honeywell V4043H

Any help would be most appreciated!

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could be air at coil con on cylinder, or if there is a stat on side of cylinder mv somewhere not opening
I can't see any other valves or thermostats around the cylinder, boiler or pump.

Might the pump be on it's last legs and unable to push water around the thinner HW pipes, but can pump around the CH okay?

Might there be a blockage or something restricting the flow in the HW pipes, such as a dodgy power shower pump?
yes it could be a blockage in flow or return to cyl.
no ti wont be the shower pump
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The pump is on it's highest setting. It is very quiet, but appears to be running in both HW and HW+CH modes. Water pressure is fine on all taps and no spluttering or drop in pressure, so I largely discounted the possibility of an air lock.

I guess sludge is a possiblity. What would be the solution to that?
When set to HW+CH we get virtually no HW, but when set to HW only we get some hot water after many hours. So perhaps the pump is dying. However, when we set HW+CH then the radiators fill very quickly with hot water, so the pump seems to be fine. Surely the HW circuit would be much easier to push, even if it has thinner pipes?
Simplest and cheapest method to clear sludge is to add a cleaner.
I find HYPERFLUSH to be the best, but fernox and sentinel are okay too.
Leave it in for a few weeks, and drain, fill and repeat until water is clear.
Best method is a powerflush, but not cheap
Gentlemen you are making assumptions without seeking further info.

Has the system been working correctly till this fault arose.

Has the system been drained down and bacterial growth been drawn in to the system from the header tank and this has caused the DHW circuit to be partially blocked?

Has the header tank water level dropped to a point where air is being drawn into the system, but perhaps not as the fellow from Manchester states that the pump runs quietly. I mention this because I come across ball vales in F & E tanks that appear suspended in mid air and require a knock to free them so allowing water to enter the tank and then the system.

Also although the CH appears to be working, are all of the rads turned on, is the pump capable of circulation through the whole system (CH side)

The fact that the cylinder has become heated after a number of hours could be partly due to natural convection assisted by a weak pump.

OK, its easier for me as I have recently asked to look at a similar problem so have a working example to refer to, although in this instance the CH 2 port valve is fitted on the return just before the pump, the DHW return connects at this point. (Probably an old M plan layout)

I have not carried out a detailed wiring check at this stage as the relevant parts of the system respond to demands from the timer/programmer.

In this instance I have suggested changing the pump because should it not prove successful I can remove the pump and take the job forward from that point.

Another thing is that the pump is in a four story house, it may not be a six meter head so could have had years of over work. (could be five stories in Manchester)

This side of Christmas most plumbing heating blokes are trying to respond favorably to customers in trouble so it may be good sense to go for a pump change as this seems the most time efficient way of fixing the problem or at least ruling out one possible cause.

Should any one following this thread fancy doing a pump them selves then a hose on a drain point and the ball valve tied up in the F & E tank will allow a quick drain if the isolation valves let by.

Oh and checking the F & E tank for slime mold that resemble Jelly Fish is recommended.

this might be all true tim, but here is a question for you:
when was the last time you saw a back boiler fed system that was clean?
1963, about 2 hours after it was installed?

There should be a small manual air vent near the cylinder on the flow pipe. Have you bled the air out of it? Can you post a pic?
There should be a small manual air vent near the cylinder on the flow pipe. Have you bled the air out of it? Can you post a pic?

Unfortunately for me, a previous owner of the property I was asked to look at appears to have watched the "hide those pipes" episode of Barry Bucknall, s DIY program. (Back in the days of black and white TV)

Re the "try cleaning it" suggestion, were it not so near to Christmas, and with my brain set in anorak mode, I would be minded to fit a Magnaclean on the return just prior to the pump, add a non acid cleanser and have the customer turn off the rads at the rad valves, and run the system in DHW setting for 24 hours to see if this would restore circulation in that area, all this assumes that the isolation valves hold up, as draining the system could disturb debris which would invalidate any assumed result.

As a gesture to the public at this the season of good will I would like to point out that if a system has been in place for lets say twenty years, has given good service, but is now starting to play up, then give serious thought to complete renewal.

Why waste money on the old when it could pay part of the cost of the new.

The sooner a new boiler is installed, the sooner it can begin earning its keep by reducing your gas consumption.

Choose a condensing appliance that comes with weather temperature compensation.

The cost of a thorough power flush could instead provide a lot of nice new rads, which the girls might see as better value than cleaning out the old ones, which may then leak!

Why give some of your hard earned money to the gas suppliers through your old boilers excessive gas consumption, when you can make better use of it by getting a far more economical bit of kit installed.

This could add value to the biggest investment that most of us make in our lifetime, your house.


PS Give your gas supplier the chance to quote, it will help you appreciate the true value of the independent installer.
Why waste money on the old when it could pay part of the cost of the new.

The sooner a new boiler is installed, the sooner it can begin earning its keep by reducing your gas consumption.

Do the sums and see!

My old Thorn Olympic [similar scenario] has been bopping away merrily for the last twenty odd years.
In that time I estimate my maintenance costs at less than £25 a year OK I am corgi but read on.

Years ago I added some fernox to the system. I topped it up with some sentinel about 6 years ago. Cost £20 for both lots of additive
The boiler has had 2 gas valves V4600C to get someone in would cost say £120 X2
3 thermocouples. Still a DIY job 3 T/C's = £15
Cleaned it out whenever the pilot looked a bit iffy X4 Say £50 a pop as an average over the years. The old pump is still fit and has never had to be slapped about to get it going in the autumn.

£475 for 20 years aint bad is it especially when thats based on local prices for getting someone in for non DIY stuff.
OK I dont do much DIY cos I don't get paid for it

Now lets look at a modern system which for a proper job would cost say £2000 and have an expected lifespan of only ten years.
If I have to take time off to do my own work then I'm not getting paid on other jobs so I work on a what it would cost basis.
Now given that over ten years I would spend another £1000 on servicing and parts. I would rather be paying my old £25 than the new £300 per annum costs. Extra cost = £275 a year!

My house is insulated and only 2 TRVs are fitted, Not maintenance items and I froze the pipe to fit them so I didn't lose my additives. During this summer I turned pilot off and just used electric shower and dishwasher.

My gas bill for the last year is around the £325 mark.

My boiler is only 65% efficient. I need £211.25 worth of heat

To move up to a 95% efficient boiler it would cost £16.25 more than that leaving my bill at £227.50.

10 years gas costs new 2275
Boiler cost + maintenance 3000
Total 5275

10 yrs gas costs old 3250
10 yrs old boiler maint 475
Total 3725


=£155.00 saving a year to keep my old boiler! To date I could take a hit of £1500 on my existing system and still be better off than if I bought a new boiler.

Well thats the way it is for me. Try it for yourself and see what happens.

I do put much of the longevity of my system down to the fact that I got some inhibitor in all those years ago.

Don't fall for the eco - hype if you want to save money.
Fall for it if you want to leave some fossil fuels for the future.
Why else would anyone want to spend £3000 to save £1500?
So with your savings you can afford to splash out on some inhibiter to top the stuff you put in 1972, why not see if one of the neighbours wants to share a bottle over Christmas ;) (X100 is £12.98p at B&Q)

Do take your point, but you may be out all day, I have a non insulated, no TRV, s, no cylinder jacket, £200 per year gas bill punter, a nicer man you could not hope to meet, but his heating is off 22 hours of the day.

Therefore a new, up to date, energy efficient boiler with W.T.C. etc would not necessarily be a good route for him to take whilst he is single.

Where as a retired Guided Missile electrician, takes pleasure in telling his mates that his new toy definitely uses a lot less gas.

He substantiates this claim with the following observation/

With his old boiler he would know when it fired up at 0600 hours as his bed head was against the out side wall by the meter cupboard.

At 0600 precisely the meter used to go tick, tick, tick, tick, but now with the new WTC controlled boiler (an AVANTA) it goes tick,




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