Ravenheat Little Star Hot/Cold Water Issue and Pump Speeds

[jamiegleave]

Hello Everyone!

I have what appears to be two (possibly linked) problems which I would appreciate advice on if anyone could help. I have a Ravenheat Little Star Gas Combi Boiler, approx 4 yrs old, running 12 radiators. Pump speed currently on setting 3 (max).

Problem 1 - Hot Water keeps cycling hot/cold if the temperature dial is turned anything past the 1pm position due to boiler burner cut-out and restart. I have read a couple of posts already on this and it sounded like a thermistor problem. I replaced the correct thermistor and it hasn't made any difference. The manual and other posts points to potentiometer; however I am slightly reluctant to go purchasing more parts if this isn't to be the likely cause. Any ideas?

Problem 2 - Noisy radiators. None of my rads have TRVs and are all correctly balanced to give good output. Aside from the usual odd bang when they come on and expand/contract, there is an intermittent pipe tapping noise when the heating is on from the odd radiator. This can actually be felt by hand on top of the flow pipe offending valves as a pushing movement. All radiators have been bled time and again. The noise wakes the house up which means many cold mornings! I have also checked the pipes for rubbing; no obvious signs as I don't have the place fully carpeted.

The link between the above problems came to me when playing with the pump speed setting. I noticed if I turn it down to setting 2 (medium) the noise in the radiators reduces considerably , and they remain just as hot. However; upon doing this the hot water side of things then cuts out much, much sooner which results in the intermittent hot/cold cycling happening more frequently . The cycling starts to kick in after about 30-60 secs, the boiler shuts down, restarts and then does the whole thing again. I then tried pump speed 1 (low) out of curiosity and the hot water then cuts out after about 10-15 seconds!

There is some issue here between the two. I think water is cycling around the radiators far too fast due to the pump speed being too high (on setting 3); hence the noise and vibrations. However addressing this makes the water cut out problem a lot more prevalent.

Apologies for the long post, but I needed to fully explain the current situation as far as I'd taken it. Any help is really appreciated on this as it's driving me and my partner mad!

Jamie

 

[gas4you]

Which thermistor did you replace? It should have been the ch one most likely.

Could also be the stat faulty its self. I have one on my patch like this, only l/lord refuse to have it replaced so they have to keep stat turned down to where it works

Also could be scaled up so overheating.

Combis MUST have the pump set on full speed for the hw side of the operation

 

[jamiegleave]

Thanks,

We replaced the one on the hot water pipe (dry clip on type) as the main problem we assumed lies with the water cycling and the temperature sensing of the water in the pipe, causing it to cut out. I have another brand new thermister which I can put into the CH pipe if necessary for a try?

HW cutout occurs at all pump speeds if temperature is set to max, much less of an issue on pump speed 3 (one o'clock setting as previously described above).

Jamie

 

[gas4you]

The ch thermister controls the overheat for ch and dhw. As you already have one try it, if only to eliminate it

Do you live in a hard water area? Is your system water black, rusty or sludgy?

 

[jamiegleave]

Thanks again,

I will give this a try and see if it makes any difference. I am curious as to the potentiometer replacement which is suggested in the manual too. Any thoughts on this being a possible issue?

On bleeding the rads the water has a very light brown tinge to it. The rads etc were changed about 1-2 yrs ago throughout and all valves are new.

Jamie

 

[richw]

My RSF84 is similar. I have to leave the DHW temperature control potentiometer at approximately 1pm. Any higher, and the boiler will overheat and cut-out. There are some other posters on this forum with Ravenheat boilers and the same scenario - the consensus seems to be to leave the control as high as you can without it cutting. Does your DHW work OK at the 1pm setting? (mind does)

It's possible that you have a sludged up system, or that the gas valve min/max settings are wrong. In my case, I know these are not the case, so I'm happy to leave the control where it is. If anything, my DHW is too hot!

 

[jamiegleave]

Well, I changed the thermistor on the CH, refilled and bled system and there is no change to the situation. Wasn't a complete waste of time as stripping the boiler casing down revealed a small leak on the diverter valve nut which had a perished seal, now replaced .

Any other suggestions guys on what to try next?

Richw - The system is working; however I could do with the hot water being slightly hotter.

Jamie

 

[gas4you]

If you're in a hard water area I would suspect the dhw side of the heat ex to be scaled up possibly.

 

[jamiegleave]

Ok will have a look at the DHW heat exchanger this week if I can and see if this needs attention.

I have noticed that over the past fortnight the overheat reset button has popped out twice for no real reason - once running HW and once on CH startup in the morning. Both times nothing had been altered tmperature wise. Could the main overheat thermostat be a possible culprit?

Jamie

 

[Slugbabydotcom]

Firstly 12 rads is too many for a little star Your pressure vessel may be struggling although you don't mention any pressure problems so this is aside from the main topic.

Your max and min gas pressures need checking. Corgi job I'm afraid.

Is a system bypass fitted in accordance with the manufacturers instructions. You have balanced your system down. If as a result there's too much resistance around your circuit your pump will leave the water hanging about in your heat exchanger for too long where it could overheat.

When you fitted the CH thermistor did you use a thin smear of thermal paste? The end product of these thermistors and potentiometers is a voltage of up to 15v at the modulating coil on the gas valve. What voltage range are you getting there?

pump speed being too high (on setting 3); hence the noise and vibrations

The pump should be on 3! Noise and vibes points to a possibly dodgy pump if you think the pump is where its coming from.

I've had a mini spate of overheat stats on ravenheats in general lately so it is a possibility that all this overheating has killed it.

There's other stuff to go through but my strongest instinct is that your pump is the main cause of your current troubles. It is possible that crud in your system has shortened the life of this component

 

[jamiegleave]

The 12 rads are well within max BTU outputs for the boiler as most are small. No pressure problems as far as I know with CH or DHW.

Bypass is a gate valve which is closed off as none of the rads have TRVs fitted. The current setup on the rads is they are all fully balanced - the noise and vibes are coming from the flow pipes, not the returns. The noise is not apparent in the boiler or pump; however I have noticed the knocking comes from about 3 different places on the heating circuit. There is also a water whoosing noise from the first rad on the circuit nearest the boiler.

Bled all rads again this morning and got a tiny bit of air out; also noticed it had tripped out (again for no reason) this morning - this is now becomming much more frequent, as is the knocking, banging and vibration in the flow pipes.

Thermal paste was not used on fitting the thermisters. No obvious deterioration or improvement on boiler performance upon fitting these.

Pump is on 3 as this is the only way I can get the DHW to work and stay on for any duration of time; however this is increasing flow speed and noise.

Also noticed something else following a review of the installation manual. The burners kicks in fairly frequently when the CH is on, and this is definately more regular than the 2.5min burner fire up delay that this boiler is supposed to have. Sometimes it comes on within 1 minute of switch off.

Jamie

 

[Slugbabydotcom]

The 12 rads are well within max BTU outputs for the boiler as most are small. No pressure problems as far as I know with CH or DHW.

I was referring to system pressure not btu. As you say rads are small so you will probably be ok on this.

Bypass is a gate valve which is closed off

Open it !

Thermal paste was not used on fitting the thermistors. No obvious deterioration or improvement on boiler performance upon fitting these.

Yet !

Pump is on 3 as this is the only way I can get the DHW to work and stay on for any duration of time; however this is increasing flow speed and noise.

Is your auto air vent cap loose or indeed functioning?

The burners kicks in fairly frequently when the CH is on, and this is definately more regular than the 2.5min burner fire up delay that this boiler is supposed to have. Sometimes it comes on within 1 minute of switch off.

As I was saying about the Ch thermistor paste! Could be lots of other stuff but that needs eliminating first.

 

[jamiegleave]

Ok guys thanks for the help up to now.

I got a Corgi Heating Engineer out who is a friend of the family who tested the gas pressures - no problem there. He tested something else (not sure) with some kind of instrument and said no problem there either. Somewhat foxed, he rang Ravenheat's techies and they said bung a load of Fernox Superconcentrate cleaner into the system for a week.

We did this last week and I am about to drain it after 7 days, as per manu. instructions. Noise in system remains (if anything it's got worse over this week).

How much do I open the gate valve by, and what effect is this likely to have on my system if no rads are fitted with TRVs? This is fitted under the floor beneath the boiler - would this not rob the circuit of hot water and make my radiators cooler if opened?

The only other thing Ravenheat said to him is they think it may be a blockage in the pipe (which could mean anything!).

I still suspect a problem with sludge (which no doubt I will see when I drain and refill the system tomorrow) and possibly the HE. I will check the auto air vent on this after draining the system this weekend to check all this out.

Any comments appreciated on the above update, particularly the bypass point.

Jamie

 

[jamiegleave]

Hi everyone.

Update from last post. I drained and flushed the system twice yesterday and refilled to ensure all the Fernox gear was out of the circuit.

Bled the radiators and let the system run for a while. Noticed boiler noise is louder than ever now (I'm sure next door can hear it too it's that bad). The noise appears around 2 or 3 points on the circuit. I get huge knocking and pipe vibration on the first radiator off the boiler, along the main flow pipe serving the downstairs rads, and on two upstairs rads. Noise seems to get worse the longer the heating is on. I get intermittent tapping which starts fast and slows down upstairs, and sharp (loud!) thuds downstairs.

Tried running the system both with gate valve bypass open and closed - it makes absolutely no difference to the noise.

I am tempted to bund a load of DS40 in as a last resort before I start to investigate other options (i.e. new boiler and powerflush). Anyone think this would be worth doing as I now suspect the residue may be in the pipes; hence constricting water flow and causing a lot of resistence?

Jamie

 

[Slugbabydotcom]

So have you put some thermal paste on that thermistor yet so that it can get a true reading of the water temperature and not overheat it?
Have you tried running the heating on a setting other than MAX to compensate?

Bypass gatevalve should be open about one turn.

Sometimes systems dont work without some expansion and contraction noise especially if the pipes are run through tight notches. Spread talc about wherever the noise comes from.

There is also a water whoosing noise from the first rad on the circuit nearest the boiler.

By restricting the flow when you balanced it you will get some noise as the water squeezes through the valve. Opening it up a bit could help.