DIY Doctor

Cool hot Water on DHW System Combi Boiler

Postby hairyjockman » Fri Feb 01, 2019 10:06 am

Hi folks.
Some help with how water system on Ariston EuroCombi SX20. Ttter ye not folks! 20 Years old this month and generally trouble free all that time.
Parts replaced since new: 3 x pumps (1 failed in warranty), 2 x diverters, 1 x pcb and a new gas valve 6 months ago – not bad eh?
Current issue – water not hot enough – never been a prob before and yes the thermo is turned up full where it was set for last 20 years at 40% - beyond that it was scalding esp in shower.
System set up as follows – all on 1 floor and in a straight line. Boiler is 12 feet from kitchen sink where water will literally melt your skin off. The bathroom sink is a futher 6 feet, again water will melt your skin but only if flow is very low. Bath & shower (next to each other) a further 6 feet away. Shower has thermostatic valve and was always scalding now it’s at best luke warm and you can forget about running a bath – especially with these outside temps in mid winter.
Initially only had a prob with heating and turned out previous fitter who changed the gas valve had blanked off vac tube in gas valve and reading were well off – gas pressure was sky high. After correctly setting up to the factory specs (it’s now got British Gas maint contract) the heating system now hits 88c but takes 3 hours to get there.
Whist, with me watching on, setting up I heard the fitter phone his mate and ask how toturn the pump down to no 2 it’s at 3! As far as I am aware, rightly or wrongly, you cannot change pump speed – only regulate flow via diverter valve – correct?
So my thinking is the boiler is running ok but the flow of hot water is not fast enough to overcome the mains pressure of the cold water hence when it gets further from boiler the more the cold water overrides the hot water temp? When tap running at “normal” flow in bathroom sink & shower the temp gauge on boiler shows ub 60 degrees. Slow the flow and it shoots to 88+ in sink (takes an age to get there) but not in bath or shower.
If so, hot to get it back up again – increase the gas pressure???? Or the flow from divertor (if so the blue fitting on top of divi is it turned clockwise or anti clock to increase?) or is there something else I am missing?
Help much appreciated folks as the shower is bloody uncomfortable…
hairyjockman
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Postby ericmark » Thu Feb 07, 2019 12:46 pm

Not fully sure why you now have a problem? I am assuming it was always there, but hidden by a second problem?

I looked for your boiler, think this one http://mrboiler.notjustjargon.co.uk/wp- ... MFFICE.pdf I was rather surprised if 20 years old, as it seems to be a modulating boiler?

For the flue gases to condense, the return water from central heating has to be under a set temperature, so to maintain that the boiler is able to turn itself down from 21.3 kW to 8.8 kW to match the demand, so one radiator with the lock shield valve wide open, can cause output to reduce, the same applies to any blockage in domestic hot water.

My really old boiler has a problem, if the shower head is not big enough the boiler turns off, shower must be used flat out, or water goes cold.

Any kink in a pipe, or valve turned down can reduce flow to a point where boiler turns down or off.

This may also explain slow central heating, each radiator should be set with around 12 deg difference when TRV all turned full on between in and output, if that is not the case, boiler will turn down before it should.
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Postby hairyjockman » Fri Feb 08, 2019 9:08 am

Yes it's a EuroCombi SX20 MFFICE modulator model 1999.
The heating we can live with - fully balanced took bloody hours to set right.
Water is the serious main prob - this morn, based on boiler gauge, we only got to 55 deg in the shower. Should mention at this point when I stated rise I meant head/lift.
It's a basic a boiler as it get yet shortage of answers means either the old boys are retured or, other than yourself ericmark, no one has any experience of it?
I've measured using flow cup at bath tap / 22mm pipe (expensive for what they are...) it gets a max 10lits/min regardless of the adjuster on the divertor valve - tried it in all positions.
Further check of divertor confirms that no hot water is bypassing to the heating system.
There are no changes to the pipework or damage and last year had a full flush/inhibitor and 3 of the 12 rads replaced.
As I said we can live with the heating but need the water.
So the big question: how on a short straight run can we get scalding water at 1st tap then luke warm at next 2 taps and literally "looks warm" at the bath???
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Postby ericmark » Sun Feb 10, 2019 3:07 pm

I am not a Plumber, I am an Electrician, you would not believe how many times I have gone to fit a new thermostat or similar and have found plumbing faults. So over the years I have learnt how to correct basic faults, like the lock shield valves all being wide open.

The way boilers modulate has produced some odd faults, in this house we found hot water at kitchen taps was intermittent, with boiler in Eco mode often no hot water, turn off Eco mode and we got hot water.

The fault was simply the short flexible pipe just before the tap was kinked, so not getting full flow. We live in a soft water area, so our kettles can last 30 years, and changed because they look tatty rather than anything wrong, but when in Suffolk with hard water, there were many problems from the build up of lime in the system.

It seems there is a temperature where the lime starts to leave the water and build up inside pipes, taps, and heat exchangers, seem to remember my Bosch boiler has a fixed domestic hot water temperature of around 55 deg C where the central heating water can be varied, the latter returns to boiler at between 30 and 60 deg C but since that water is not replaced it does not matter if it gets too hot other than wasting energy.

So two independent problems, central heating and domestic hot water, the latter you say 10 litres per minute and instructions say 11 litres per minute to get a 30 deg C temperature rise, so it would seem there is a restriction some where.

Central heating I think likely return water too hot, output water is between 35 to 85 deg C you can set it, and that water needs to cool before it returns, so we have to limit the flow speed through each radiator using the lock shield valve, if the gas man has changed the pump speed then also the lock shield valves will need resetting to match.

If the lock shield valves are open too much, the water will return hot and turn down boiler, if not enough then the radiator will not get hot. Also with your boiler it seems the by-pass valve is not built into the boiler, so that also needs to be set so as the radiators TRV close it opens allowing water to continue to circulate. If the by-pass pressure is too high the pump will be damaged, if too low then it will open too soon allowing hot water back and so turning down the boiler before it should.

I tried to set the lock shield in this house, and it is not as easy as it first seems, reading it says from memory 15 deg C temperature drop in coming to out going water. However what I found was the TRV takes time to open and close, so if the flow is too high, radiator gets hot before valve can close, specially as the plumbers have fitted the TRV on out going pipes on some radiators.

With electronic TRV heads one sets the actual temperature say 20 deg C not just *123456 so when the room reaches 25 deg and valve set to 20 deg you know it's not the TRV at fault but the lock shield valve, what is happening is hot water fills the radiator then room heats up, then valve starts to close however there is that much hot water in the radiator it continues to heat the room. I have found throttling back the lock shield valve gives the eTRV time to close. However with wax type you would be unaware about what was happening.

We do find changing set temperature in a room from 16 deg C over night to 20 deg C in the morning from 16 to 18 is quite fast maybe 1/2 hour, but 18 to 20 can take 1.5 hours as the anti-hysteresis software kicks in, so I cheat and set it 16 to 22 deg for 1 hour then down to 20 deg, and then I do get to temperature within the hour.

However clearly one has to ensure the wall thermostat does not turn off boiler too early, we should in this house use a wall mounted Wave modulating thermostat, however they only set the temperature in one room, so we have a simple off/on thermostat who's job is to turn off boiler to stop it cycling when we get warm weather, it does not control room temperature, the TRV does that.

It took me nearly a year to get settings right. In this house morning sun is main problem, if shining the living room gets very hot, if not then cool unless the TRV are set correctly.
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Postby hairyjockman » Wed Feb 13, 2019 12:23 pm

Many thanks for that excellent reply ericmark - must have take you ages to write.
As it stands the heating is prob best left alone now - I re-balanced all the rads the other day 1st by setting inlet/out temps on each one but max difference I can achieve is 5-8 deg c! I then opened all up again and set each to 1/4 turn from closed and las one in line fully open - it was the only one showing cool. Checked all temps again and diff was still 5-8 deg.
Boiler on heating, being a modulator, set with manual thermo at full runs at max 88deg. Rads run approx 69/70 deg at top of them and 55 at bottom. All run the same bar 1 of them that runs colder. What we have noticed over the years at every now and again a rad will literally throw heat into a room which you can feel on your face. Sometime but not often - another gremlin...
The hot water is biggest concern and after yet another (10th in 2 years) they finally aggree it's the heat exchanger being blocked! We are now running 3lit/min on max temp with a flow of 10/min max at both (less than luke warm).
Then they now have the balls to say ah1 but you are not covered for that part!!!! Despite 2 companies and 10 visits costing us got knows how much in lost wages and untold costs in paying for gas to heat nothing!
D&G at their best - no customer service whatsoever.
Guess I'll have to stump the £90 and fit it myself.
hairyjockman
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