Only warm water at taps?

[Cr8]

Hello -

I've read through threads and haven't found the solution to my problem. Hope someone can help.

I have an Ariston EuroCombi A/23 MFFI.

I live in an area with radical limescale build-up and because of this, I had the secondary heat-exchanger replaced about 9 months ago - flow rate was so bad that the diverter wasn't being activated.

Now, nine months later, the flow rate is getting pretty bad again. I can't afford to shell out another £270 for another replacement so I decided to descale the exchanger myself. Everything has gone OK: heat exchanger removed, descaled and refitted and the flow rate is back up.

But, now the water doesn't heat up enough - with the thermostat set to max., I can easily put my hand into the running water - I guess around 45C.

The new flow rate is not particularly high, and it doesn't make any difference to the temp. if I stem the flow rate by closing the tap, so I don't think it's a flow rate problem.

Anybody have any ideas. Thanks in advance.

 

[Cr8]

Anybody?

 

[Cr8]

Still no help. hmmph. What did I do wrong?

 

[ChrisR]

Everyon's out mending CH systems. I'm just ill.. again

What IS the flow rate? (litres per minute)
Do you have a thermometer?
How did you clean your secondary h/e
Is it a canister type or a plate type?
Do you have/can you use a multimeter?

Start with CH cold, run HW - does the CH flow pipe warm up?

 

[Cr8]

Thanks ChrisR - I was starting to wonder if it was something I said?

1) Flow rate = 7L per minute
2) Yes - I have a thermometer
3) Took out secondary HE, poured Oust descaler into it, let it bubble for about two hours, ran fresh water through it for a minute or two and then refitted it to the boiler.
4) Not exactly sure what you mean by canister or plate type - my guess is that it's canister as it's an elongated box shape and not flat like a plate.
5) Yes I do have and can use a multi-meter and yes I know that AC DC isn't just the name of a band!

Ch been on all morning so I need to wait for it to cool down..... I have checked this before and I don't think it's the diverter valve - but will check again later.

What next?

 

[Cr8]

Just checked if the CH flow pipe heats up when I turn on the HW tap and it doesn't.

However, it does heat up when I turn the tap off again, but from what I have read in this forum, that's normal - isn't it?

 

[ChrisR]

COnfused here - there are just too many aristons with similar names. Is yours the current one on the website:
http://www.mtsgroup.com/_doc/catalogo/brochure/newmicrocombi.pdf

?
If so you don't have a separate secondary heat exchanger, the hw is heated in the main h/e.

If you have a plate type one it will be flat sided - many plates in parallel iniside. Sounds like that's what you have.

SOmething llike

maybe without the brass bits.
Poor hw temperature is often caused by sludge/boiler scale on the boiler side(not tapwater side) of these. I don't know what Oust is but mild descalers won't touch it. I have used kettle descaler with some success but you often have to use citric acid or stronger, such as phosphoric or hydrochloric, which are dangerous.

Two of the holes are the tap water side, and the ones with black stuff in are the boiler side.

Next thing would be to check the resistance of the HW temperature sensor - hard without the manual. You could call Ariston tech and ask 01494539579. Have the GC number for the boiler handy, and make up a corgi number if they ask for one. Some poor soul must have 123456!

If you check the CW input temp and the HW output temp at 7 l/min you should be able to check with the manual.
If manual says 9.8l/min at 35C rise then you should get about

(9.8/7) * 35 C = 49C rise at 7 l/min. Expect a bit less.

Then you'd put all your HW taps on, and check the gas being used at the meter. 1 cu ft should burn in about 40 seconds.

that's normal - isn't it?

yes.

 

[Cr8]

No it's not the unit behind that link - it definitely has a secondary heat exchanger and it looks exactly like the one in your picture (minus the brass bits as you said).

Oust is just a particulalry powerful household descaler, and it sure bubbled quite violently when I did it. It must have worked bescause it did make a difference to the flow rate at the taps.

Is it possible that there is an air block somewhere on the boiler side, or does the power of the pump normally clear the system?

Edit: Sorry, for some reason your entire post hadn't been posted when I initially read it, I'll check and answer your further points shortly.

 

[Cr8]

Ok, got the answers to the questions:

I got the specs. for the thermostat and it's good - 1K to 10K ohms.

The change if temp. I am getting at the moment is a dismal 26C - while it the theoretical potential is 47.5C at 7l/min.

The gas consumption is around 0.014 cubic meters or 0.5 cubic feet per 40 secs - which is half what you suggest and very near the min requirements according to the manual.

So I guess it's obvious that I need to increase the gas flow rate - but something doesn't make sense to me: if I open the taps, the secondary heat exchanger remains too hot to touch while the water coming out of the taps is barely 40C. Surely, it would should cool down if there wasn't sufficient gas to heat up the system? Also, the temp. gauge on the front panel of the boiler still indicates 70-80C (on max. setting) even with the HW taps on?

The other thing I was wondering, is it possible to fit the heat exchanger the wrong way round???

All insight into this appreciated. Thanks.

 

[meldrew's_mate]

Gas rate is, as you say, about half what it should be, and the heat exchanger is too hot to handle, so the heat from the flame is not being transferred effectively to the DHW.

I suggest you have not de-scaled the heat exchanger thoroughly, enough to increase the flow, but still sufficient remains to limit the heat flow. Redo the descaling, and continue to top-up the Oust until the fizzing stops (=no scale left to dissolve )

Do it properly and do it only once

 

[ChrisR]

Agreed. It's the primary(boiler water) side of the h/e which is important for heat transfer, secondary usually only for flow rate. Your main h/e will be scaled too which will also reduce heat output but stick with the secondary one for now.
If you can see black stuff in the plate h/e that's go to go. If Oust doesn't do it (try it STRONG and HOT) then either get some DS40 form a plumber's merchant (similar to citric acid), strong as poss and hot. Or, if and only if you're confident, use HCl (Spirits of salts) diluted about 2:1 to 1/3rd strength. Only ever add acid to water. Take full precautions including goggles gloves, coverall and other measures necessary.

 

[Cr8]

Thanks - you guys are champs.

I'll redo the descaling (safety precautions noted) - with a vengeance this time, and then look at the gas settings if necessary.

Thanks again.