My hot water gets hot then cold then hot then cold etc with the boiler firing up and down etc.
Central heating is fine.
I have a Baxi Genesis 80 combi.
I have thoroughly cleaned the DHW heat exchanger using Fernox ds-3 and using a jet washer.
AS far as I can tell the heat exchanger is clean.
What do I look for next and how do I test it for failiure before replacing any parts?
I currently have to run the bath hot water tap very slowly to get a nice hot bath.
Steve................
The boiler cycling on DHW production is normally a sign of only drawing a very low flow rate of hot water on a working boiler. However a plate HE that is not working correctly could also cause this.
Which side of the PHE did you clean? They can block with limescale on the secondary (i.e. fresh water) side, and with other "crud" on the input side.
Are you getting corrosion elsewhere in the system? i.e. lots of rads filling with gas and needing bleeding etc?
I had exactly the same fault a couple of years ago. I changed the DHW heat exchanger (after previously cleaning the old one). Changed the diverter valve (this can send the hot water to the radiators rather than keeping it for the hot water - you have checked this? ) Easy way to check, all rads cool, run the hot water for 5 minutes or so, see if the first radiator is getting warm - if so, then the divertor valve could be faulty.)
If the above have been checked then there are really only 2 probably causes - pump failing, or blocked primary heat exchanger.
How old is the boiler? When it was fitted, was it connected up to old pipework with giving that a good clean out? Has it had inhibitor in it from new?
Mine was a classic one fitted by a cowboy - fitted to old pipework, not cleaned out, no inhibitor. The HE was cast iron, so easily corroded. I attempted to get the HE out one day, but every fitting was seized into place, so I gave up, and bought a new boiler.
Alan.
I cleaned both sides of the heat exchanger.
I immersed it for around 2.5 hours in Fernox ds-3, keeping it hot and flushing it with a jet washer a few times.
Radiator water seems quite clear. There was some black flakes in the HE but not a substantial amount.
I am wondering if perhaps the diverter valve needs replacing because it's not fully switching to the hot water setting but can switch fully to the central heating setting.
I will swap the flow switches tomorrow to see if it makes any difference.
Steve..............
Excellent point
I'd call the DHW one the secondary heat exchanger, the primary being the one above the gas burner
The symptoms exactly match it still being scaled up, and I am doubtful that what you did will clean it.
I did a friend's Baxi 105 (IIRC). Put the exchanger in a large plastic washing up bowl. Mixed up a litre of DS-3 in a jug, and poured it through slowly. Beware of the acidic spray/vapour. Then transfer the collected solution back to the jug, and pour it through again. Each time the DS-2 turns from yellow to green, it's used up, and you need to mix up a fresh load. Keep doing this until you can't make it go green anymore. I think it took
3-4 litres. A heat exchanger might not seem particularly large, but it has a very large surface area which needs descaling.
Thats what I meant - if the secondary/DHW HE has been cleaned, and it still doesnt work correctly, it is likely to be the primary HE that is scaled up with rust and limescale.
Alan.
But then wouldn't it also affect the central heating side?
Steve..........
I suppose I could clean the primary one as well, but I don't understand how that could affect the hot water side of things and not the heating side.
Steve...........
That would not cause the symptom you are seeing. Basically when the primary water is diverted through the PHE its not shedding heat fast enough, and hence the primary gets hot enough for the boiler to cycle off on its internal stat. If you were bleeding water into the CH, that would consume more heat from the primary water rather than less.
I would try circulating descaler through the DHW side of the PHE rather than just soaking it.
I will do that this morning.
Steve.....
It does affect the heating side, but you cannot tell, unless you have measured the temperature of the rads every few months at the same settings. The heating side is rarely put to full output, whearas the water output is nearly always at full output when on.
Also, the radiators are usually on for a long time, so have longer to get up to a reasonable temperature. If this is 10 degrees below the maximum that was once available, you wouldnt notice, as the boiler performance has been gradually declining over the years.
Again, it may be that, or may be a faulty divertor, or possibly pump.
Alan.
Cleaned the secondary HE again and it seems to be very clean. No evidence of any dirt etc coming out of it.
I also cleaned the primary HE and that was filthy. Loads of black flakes and what appeared to be rust came out of that. I have a generous amount of inhibitor in the system so how the hell could it get that dirty?
Gave it a good clean with Fernox ds-s and then used a jet washer on it.
All back together and running but so far it doesn't seem to have made any difference to my hot water then cold water and so on problem.
I am tempted to try a new 3 way valve diverter.
Steve..........
Definitely no change to my hot water situation but I have noticed that the central heating side has become even hotter after cleaning the primary heat exchanger.
I ran the hot tap with all radiators cold and there was no heat getting to the central heating flow pipe so I guess that rules out the
3 way diverter valve as being at fault?So what next apart from taking a sledge hammer to it and replacing it with a new boiler?
Steve...
I am not familiar with that boiler, but does it have a DHW flow temperature sensor? Some modern combis have a probe that senses the outgoing water temperature and use it to control modulation etc. If yours has something similar then it could be its giving a false reading and telling the boiler PCB that the DHW is hotter than it really is.
Also is there a DHW temperature adjustment? Is that set high enough? and again, is it working correctly?
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=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D/
The temperature knob is set at full. I have tried it at several positions and it always makes a difference so is working.
There is a DHW and Heating thermistor. Perhaps I should try swapping them over as they are identical.
Steve..........
Does the service manual give a range of expected resistances? A quick check with a multimeter would probably tell you want you need to know. Still if easy to swap, then that sounds like a worthwhile test.
(I had a combi once that developed a fault on the DHW sensor. This was one where it "tempered" a couple of litres of water which it held internally to give "instant" how water. Alas the heating of this on switch on took priority over other boiler functions. So the failure of the probe also caused the software to lockout the whole boiler at startup, because it could not sense its way though the initial heating of the water. I had to fool it with a fixed resistor while a new probe was on order!
I will try swapping them over tomorrow.
I unplugged the connector from the DHW thermistor and I was surprised to see it had no effect whatsoever.
I was expecting the boiler to refuse to supply hot water, but it did and it behaved as it usually does. water gets hot then cold then hot and so on.
Steve...........
If its doing the same with it not even in circuit, it might indicate that its gone open circuit. Could you measure the resistance across the unplugged end?
Managed to source a cheap thermistor and have just replaced the old one and it's looking like that was the fault.
Hot water seems much better at the moment. Will know for sure when I run a bath later.
Thanks to all for suggestions.
Steve...........
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