Vaillant ecotec plus cycling on HW only demand

[scoobydo123]

Got an eco tec plus that I have recently put onto HW constant mode due to change in tennants hot water demand. This was done during the cold spell we had at the start of the year. Obviously I have noticed an increase in gas usage but put this down to the cold weather and also the change in use. However this weekend I was round there changing the cooker and noticed that the boiler was short cycling ( only on for around 7 seconds). It is the open vent version and it feeding a 145L megaflo. Got the missus to go round there today to do some testing and she had the CH on for over 45 minutes without the boiling cycling and the display having a very slow ramp up to temp (where as on HW demand it ramps up very quickly). Obviously with the HW how it is I would imagine that the cylinder is taking ages to heat up and costing more money and also not good for the longevity of the boiler. D0 is currently set to 11.

Not sure if this has always been like this.

 

[scoobydo123]

Starting to think that the coil within the Megaflo is severly scaled and not releasing the heat to the water. Anyone ever de-scaled a megaflo? Was thinking of removing the immersion and having a peak inside? Do you think if heavily scaled it could cause these severe symptoms? Megaflo I believe is a MK2 so that may give a clue to its age.

 

[GuessMan]

If the coil is buried in scale, you could be on the right track. Drain, remove immersion and aqua vac the contents out. If not, is the safety two port opening fully? And has a balancing valve been fitted on the primary? Open fully if this is the case.

 

[scoobydo123]

Cheers Guessman. Have moved manual lever on 2 port and it seems to be moving but obviously not sure if it is actually opening the valve fully. Also had a twiddle with the gate valve but no change. If the 2 port is not opening fully would I still expect the flow and return to come up to temp and would it be chucking it down the bypass guess not as then I wouldn't have the issue as it would be dumping the heat into the radiator circuits? Have you ever seen scale that bad to cause these symptoms?

Obviously need to sort the issue but in the meantime is there a way to get the anti cycle working when only the DHW thermostat is calling for heat? Anti cycle currently set to 20mins but after firing has completed then the countdown is only displaying 2mins and is not blocking subsequent firing sequence.

Just wanted to add as well can limescale stick to the coil if so have searched online and found the attached product that seems very reasonable in price if the coil does have heavy deposits I wonder if this product would dissolve it?

http://www.chemicalproducts.net/phos-88

 

[GuessMan]

Have a look at D40 & D41 if the Δ T is low, it would suggest low heat transfer to the cylinder; pointing to a scaled or buried coil. If it is high then it would be a circulation issue; gate valve or two port issue, possibly locking the boiler in S53. D2 set at 20 is a bit complicated, if the problem is low Δ T try dropping this to 10 or even 5.

 

[scoobydo123]

Hi, whilst I was round there on Sunday I was pushed for time but did have a look at d40 & d41 from memory it was 75 and 64 but cannot confirm as cannot get round there at the moment. Would this be classed as delta t low or high?

Can get round there tomorrow after work and collect some data. Anything else you can advise to collect whilst I am there

Really appreciate your help.

 

[GuessMan]

Subtract D41 from D40 this is your Δ T It should be around 16 ° C

 

[Agile]

All those figures seem within the normal range.

But I dont think you have said what power your boiler is.

Could be you have low flow and low transfer.

Do you have an auto bypass? What setting is that on? record it and test with 0.1 Bar higher and see what difference that makes. Does it seem to be passing?

You really need to time the on/off periods as well as monitoring the flow and return temps at the point it goes off!

I had to desludge a heating coil on one of those last week! There was virtually no flow at all and the immersion was on 24/7 when he bought the house.

Tony

 

[scoobydo123]

Tony was hoping that you would pop in for a look. Boiler is 428 which is grossly over rated for the load, long story but when it was purchased the house was going to over double in size and have a megaflo and gravity feed system alas local planning would not allow the conversion from bungalow to house. So got stuck with an over rated boiler. Yes it does have a auto bypass but dont know make or current setting but will pay visit tomorrow. Have been having a read up and it appears like the boiler fires at 75% max for the first minute, this I reckon could be the issue and not sure if anyway round it but that is where you guys come in with your years of experience.

Interesting regarding the de-sludge was really thinking it might be this until I learnt the above 75% theory. Would really like to have a peak inside but not sure if I want to risk not getting the immersion back in and re-sealed.

Is it worth me tomorrow draining off say 30L of hot water then getting the times of firing and D40 D41 readings and reporting back rather than when it is trying to top up the cylinder that is mostly full of heated water?

Thanks

 

[Agile]

Right!

We now have your confession that the boiler is grossly over rated for the application.

They can be fiddled with but excessive cycling is inevitable in that situation.

Tony

 

[scoobydo123]

Can you confirm if Valliant 4 series do run at 75% load for the first minute? If this is the case then I am thinking along the lines of putting a timer relay between HW 2 port and CH 2 port that will open the CH circuit for say 1 minute on intial fire of the boiler. The boiler will then be able to modulate down to 5KW which hopefully the cylinder can absorb. Can I easily test this by having the boiler fire whilst doing its cycling stuff and manually open the CH valve on the lever holding for say 1 minute then releasing it hopefully this may prove my theroy?

Obviously still could be sediment but what do you reckon on the above?

Also is there any mileage on changing to return sensing trying to get my head round what impact that would have. What other fettles can you suggest.

 

[The831Bunny]

You can't adapt the operation of the HW MV as its a safety device on a UV cylinder. A 145 has a coil rating of about 18Kw, but as you realise a 428 will fire on 75% load for 50secs at start up. Personally I don't think the coil is scaled. I'd set flow temp on boiler to 70C, make sure pump is a 15-60 set to 3, open fully any gate valves+pump valves, set Megaflow HW temp to 3 on stat (about 55C). You could also increase pump over-run (D.1) to 7 minutes.

Don't know if you have checked, but make sure pump is wired back as MI to boiler.

As mentioned 15-20C delta T is about right on a cool cylinder, narrowing as the stored water is heated and the boiler modulates.

 

[scoobydo123]

Thanks Bunny 831, pump isn't 15/60 so could try this if you feel it will have a positive effect. So it looks like problem is related to the 75% of max on startup. Can this setting not be changed, have a Glowworm ultracom that trickles then ramps where as the valliant seems to work in reverse unfortunately. Do you mean is the pump wired up correctly as per the instructions as in does pump overrun work as I have this set currently at 10 minutes.

I think my main issue is that as there is not that much draw off most of the day it is sitting there and is struggling to reach satisified due to the 6 seconds on cycling. I think it will cope if it was not for the 75% issue.

 

[The831Bunny]

I'm afraid the 75% startup load is set in stone. The 28kw heat exchanger is quite resistive, and requires 20 l/min flowrate through it to prevent the large difference between flow and return temperatures which can cause most problems on systems with this boiler. Usually a 15-60 pump is a minimum requirement, you may also need 28mm from the boiler too.

The pump should be wired to the pump terminal block on the boiler pcb to work correctly. Maybe an idea to check the pump installed is not blocked slightly in the impeller too. Your main course of action is to ensure you can do all you can to dissipate that initial ignition phase by giving the correct flowrate. Even if the boiler approaches the flow temperature set point in the first 50 seconds it will not modulate, only go straight off with a wait and pump over-run. Welcome to my world

 

[scoobydo123]

Ok looks like I am in for some fun. Will have a word with the tenants to see I f I can somehow get it back on timer. Will have a play around with it this afternoon to see if I drain say 30L of hot water off then call for heat if the water around the coil will cause enough heat dissapation to get past the critical 50 seconds then modulate down. What a pain