Alpha CB24 hot water problem

[James]

I have an Alpha CB24 combi boiler which is starting to play up. A couple of months ago, the pressure of the hot water dropped slightly, and it has continued to get worse since then. It's down to little more than a trickle now so I called in the local boiler repair people to look at it.

They have suggested that:

(a) it needs to be de-sludged; (b) it might have a blocked heat exchanger (hard water area); (c) to help with the de-sludge, I need some flocculent in the water; (d) the seasonality valve might be blocked or stuck; and (e) I need a Magnaclean magnetic filter fitted to prevent the boiler from sludging up again.

They reached these conclusions without, in fact, looking at the boiler and I haven't been able to get a straight answer about what they think the actual problem might be, or even if these measures will actually fix it.

Can someone give me some pointers about what the real problem might be? If I can talk to the boiler repair people and understand what might be the issue, I might have more chance of getting a reasonable answer. I also wouldn't be blowing up to UKP700 on what seems to be just speculative work. For about UKP1,000, I can get a replacement boiler anyway, so I can't see too much point in paying for repairs which might not do the trick. Any advice much appreciated. Ta. Also interested if there are any quick fixes I can do myself then get the repair done properly later.

-- James

[John Rumm]

I take it you mean flow rate and not pressure?

The primary effect of sludge could be a partial blockage on the primary side of the HW heat exchanger, but this is unlikely to reduce your flow rate - although it will limit the maximum temperature of the hot water delivered to the taps at a high flow rate.

Also Alpha make a selling point of their cyclone filter system in the heating circuit designed to catch crap circulating. (assuming you have the CB24 and not the cut down CB24X version)

That is going to be your number one probability; blockage caused by scale on the hot water side.

It also has a strainer/filter designed to stop debris getting into the boiler - that can be cleaned.

You need corrosion inhibitor, and see (a) again!

Well they are all plausible to some extent... but it does not really sound like much effort has gone into the diagnosis.

First thing to do would be take out the DHW secondary heat exchanger and run some descaler through it (Fernox DS3 - about £15 for a tub in a shed or plumbers merchant).

[Jim Alexander]

First of all have you got the boiler installation manual? It should have been left after installation but if not its easily downloadable from Alpha. Trace through the DHW on the boiler schemaic and you will immediately see what might be implicated in the fault. Not much. The good news is that from your description of a "trickle" the diverter valve and all the other DHW temperature regulation systems are working well. The bad news is the DHW heat exchanger is in the frame. I note from the description there is a cold water inlet filter which is not mentioned on the schematic.

I suspect you are getting a free or low cost estimate from boiler people. Its the repair job they want. In the circumstances I think they have been quite helpful even if some of the suggestions are way off. Unfortunately if you don't have a maintenance contract its at your risk. Time to spend some of the money you have saved by not having a maintenance contract. What you do yourself depends on you competence but checking the cold water inlet valve shouldn't be rocket science. No idea about the seasonal valve but its in circuit. The DHW heat exchanger is still the most likely culprit. Doesn't look a particularly difficult job.

Jim A

[kd]

Been there, had that. Heat exchanger will reduce water flow (it's obvious really) and reduce the water temperature. Simple task to remove and clean but for the cleaning use powdered citric acid. It's amazing in it's speed. I found that the temperture of the water solution works best when at 80 C. I used a big pan on the hob. I do this every year as routine now.

[James]

Gents, thanks for the advice. From my own thinking, I'd have expected it to be some kind of limescale problem, and was a bit mystified about how the sludge might cause this problem. The first boiler man to visit recommended descaling the pipes and boiler, which sounded reasonable, but when I contacted his office they seemed to have loftier aspirations. I wouldn't mind paying if they could only be more precise about the whats and whys.

Anyway, I do have the boiler schematic and the appropriate tools so I'll try taking the heat exchanger out and descaling, then add corrosion inhibitor after that. I already looked at the inlet filter and it was depressingly clean and shiny, so that didn't fix it...

Thanks again.

-- James