Boiler cycling when heating hot water cylinder

[vulcancontinental]

Sorry I'm not getting the point of bypassing the pressure switch - the pump runs without any problem whenever heat is being called for anyway.

The Vokera's generally need a half bar to activate a low pressure switch to stop the boiler firing if the system empties or pressure is too low. In some parts of Ireland they are fitting the boilers to the domestic cold water storage tank in the loft which is not high enough to provide .4 of a bar pressure in the boiler (what Vokera set the switches to in Ireland) so installers link the switch out so that the boiler runs.

How many tanks do you have in the loft?

It sounds as though the low pressure switch has been disconnected on your boiler.

With a NRV in the cold feed no matter whether there are one or two tanks in the loft you now have a sealed system as there's only one safe place for the pressure to be accomodated and that is the expansion vessel. If the vessel fails or loses charge the boiler will vent from the pressure relief valve into your house.

If for some reason a leak developed or the pressure relief valve lifted should the low pressure switch be linked out the boiler will fire even though there's no water in it.

God knows why they do it like that over there I'd never heard of it before until I went over there. I work for Vokera and guys I've spoken to say it's common in Dublin but I don't know why.

If I was in your situation I'd get the PRV run to a safe position and connect the low pressure switch then charge the system to over .5 bar.

 

[NuttySnake]

Thanks vulcancontinental,

There is only the one tank in the loft - the main cold water storage one. This has two 3/4" pipes from it - one feeds the bottom of the cylinder, the other the taps and the heating system (via a non return valve in the airing cupboard)

I've never seen the pressure read anything other than zero on the boiler (does make me wonder if the gauge is broken) even when the system is hot. This does make me wonder if the non return valve is stuck open, and the water is expanding back up the feed pipe. Am tempted to close off the gate valve above the non return temporally when the system is cold and see if that causes a pressure rise when the system heats. Is this a stupid thing to do?

Will have a closer look at the wiring on the low pressure switch - it doesn't seem to have been shorted out at the switch end, but I've not removed the circuit board cover to look at the board end of the wires. I suppose another possibility if these switches are "settable" is that they have turned the setting right down!

I'm looking online for a heating engineer now to get an expert eye cast over it and see about the pressure release pipe. We are in Kilcullen, Kildare which is not a million miles away from Dublin, so what you've heard is common in Dublin probably reaches out to us. Tricky thing is finding an engineer who knows their stuff - the guy who's serviced it up till now hasn't batted an eyelid, and he's ex-British gas apparently.

Would low pressure be explaining the cycling we are seeing though, or is that likely something completely different?

 

[vulcancontinental]

Thanks vulcancontinental,

There is only the one tank in the loft - the main cold water storage one. This has two 3/4" pipes from it - one feeds the bottom of the cylinder, the other the taps and the heating system (via a non return valve in the airing cupboard)

I've never seen the pressure read anything other than zero on the boiler (does make me wonder if the gauge is broken) even when the system is hot. This does make me wonder if the non return valve is stuck open, and the water is expanding back up the feed pipe. Am tempted to close off the gate valve above the non return temporally when the system is cold and see if that causes a pressure rise when the system heats. Is this a stupid thing to do?

Will have a closer look at the wiring on the low pressure switch - it doesn't seem to have been shorted out at the switch end, but I've not removed the circuit board cover to look at the board end of the wires. I suppose another possibility if these switches are "settable" is that they have turned the setting right down!

I'm looking online for a heating engineer now to get an expert eye cast over it and see about the pressure release pipe. We are in Kilcullen, Kildare which is not a million miles away from Dublin, so what you've heard is common in Dublin probably reaches out to us. Tricky thing is finding an engineer who knows their stuff - the guy who's serviced it up till now hasn't batted an eyelid, and he's ex-British gas apparently.

Would low pressure be explaining the cycling we are seeing though, or is that likely something completely different?

The cycling is something else I'm afraid; your boiler is producing more heat than it can dispose of and raising it's temperature so quickly it shuts off after a short while. If you're happy with the amount of hot water you're getting and the time it's taken and the system is working and not overheating personally I'd leave it. It's not ideal but it's working and the series of cycles will shorten the life of the switching components in the boiler but by how much, who knows. That might be a controversial opinion but you have to make a judgement on where you are and what cost it will be to get you to where you want to be. Do you plan to stay in the house a long while, how old is the boiler, are you planning extensions; stuff like that.

Turning the pump down simply slows the flow across the heat exchanger and may cause kettling or localised boiling.


I think if the NRV was letting by into the DHW tank you'd notice a brackish taste or smell to the hot water at the taps. I think expansion is taking place into the vessel on the boiler. Our preferred method is fill the system, turn off the gate valve and the boiler then operates as a sealed system but get the PRV run to a safe place as that's your safety feature. I wouldn't say closing the valve is stupid, I'd class it as unnecessary and unwise.

You could ring Vokera Ireland and they'll probably have the name of an engineer in your area. Remember the boiler is how the area regs want it installed simply you should have the PRV terminated in a safer place.

 

[Agile]

As far as I know the boiler should modulate.

Ideally it should modulate down to the power which the cylinder can adsorb! Whether it will do that will depend on the amount of heat the cylinder can take and on the internal settings of the boiler.

Even if correctly set, the boiler output may well be more than the cylinder can take if its old or scaled up.

But you have not given us any details of this cycling. Can you time it for about 30 minutes giving the time on and the time off for each cycle starting with the cylinder pretty cold?

Tony

 

[NuttySnake]

Thanks for the replies. Have just done a run with the stopwatch with it heating up a fairly cool tank load. I started the stopwatch when the boiler fired for the first time.

On: 0:00 Off: 3:35 Burn: 3:35
On: 6:40 Off: 8:36 Burn: 1:56
On: 11:42 Off: 13:17 Burn: 1:35
On: 16:23 Off: 17:50 Burn: 1:27
On: 20:56 Off: 22:17 Burn: 1:21
On: 25:23 Off: 26:38 Burn: 1:15
On: 29:44 Off: 30:45 Burn: 1:01
On: 33:51 Off: 34:48 Burn: 0:57
On: 37:54 Off: 38:47 Burn: 0:53
On: 41:53 Off: 42:44 Burn: 0:51
On: 45:50 Off: 46:39 Burn: 0:49
On: 49:45 Off: 50:32 Burn: 0:47
On: 53:37 Off: 54:24 Burn: 0:47
On: 57:30 Off: 58:14 Burn: 0:44
On: 1.01:20 Off: 1.02:03 Burn: 0:43
On: 1.05:09 Off: 1.05:51 Burn: 0:42
On: 1.08:56 Off: 1.09:37 Burn: 0:41
Demand satisfied before it fired again

The decreasing burn time fits with a reducing temperature difference as the water heats, so the question is should it be cycling as much as this? From my reading of the boiler installation manual I can't see the word modulate anywhere in it, so I'm beginning to think that the boiler can't modulate and it's just a case of the cylinder not placing enough drain on the boiler.

Other things that were mentioned earlier - pump is a grunfos 15/50, but it has no speed control. I opened the pump electric connection box to reseat the cable entry gland that had come adrift - nothing bar a connector block and a capacitor in there (no hidden speed switch).

Pressure switch - no sign of electrical shorting of this at the switch or board end. If it has been "doctored" I suspect it's been "decalibrated" - looks like there is a calibration screw on it.

Pressure gauge still not moved off zero - the capillary tube doesn't seem to be broken, but I really think the gauge must be dead.

Cheers

 

[vulcancontinental]

Most of the answers are here regarding the boiler questions

http://www.vokera.ie/wp-content/upl...0e-installation-and-servicing-manual-copy.pdf

http://www.vokera.ie/wp-content/uploads/2011/08/mynute-12-16-20e-user-manual.pdf


It is not the pressure switch I've seen on other Mynutes and there does seem to be an adjustment and I wonder whether this has simply been wound out (I'll have to ask whether this can be done).

You'll see in the install instructions how it should be connected to the NRV and gate valve.

Like I said I'd be inclined to leave as is and get the PRV seen to (and personally the pressure switch also though that's not as important). Boiler is working OK and you have an annoying system problem, e.g. it can't get rid of the heat it's producing when in hot water only. There should be a bypass with an S plan system and by experimenting with that you may be able to extend off and on periods but I would think not by much.

I see nothing there about the anti cycle delay which we put on most boilers though I skimmed through them but the delay between firing on your timings indicate a consistent 3.05 sec delay in which the boiler temp must have dropped around 4-6 degrees in order for the burner to fire again.

The boiler does not modulate though the valve is adjustable between 12-16 KW at the burner.

How big and how old is your house? It might be possible to turn total output down to 12KW but there's no further adjustment I can see.

Remember Vokera produce boilers specifically for the Irish market so they will differ from those listed on the UK website, always check www.Vokera.ie

 

[NuttySnake]

Yes I've seen the instructions - the UK section on installation and filling is quite detailed, the Irish section is very vague...

3A.6.6 FILLING POINT
A method for initial filling of the system and replacing water lost during servicing etc. has been provided. You should ensure this method of filling complies with the local water authority regulations.

Doesn't say what the method they provided is! The UK method with the non-return valve shows a separate feed tank above the valve - we are just running off the main cold water tank.

There is no external bypass - but the boiler has an automatic one built into it. As you say though I can't see an external one doing much to help - all the bypass will do is route hot water straight back to the boiler without even giving the cylinder a chance to take some of the heat.

House is about 6-7 years old - thrown up during the property boom and my now wife bought it off the initial owner before we were together (I'm English and moved over having sold my UK house).

We're in negative equity up to our eyeballs, so are going to be staying put here for the medium term at least. It's a 3 bed house so will do us for a while yet. With that in mind I'm trying to get things running as efficiently as we can. We're in the process of putting in a pumped shower in the ensuite (shower the builders fitted never worked) and I'm aware that the hot water cylinder isn't very big - 90 or so litres I think. If we do end up replacing it with a larger one I was going to get one with a fast-recovery coil in it as it's my understanding these are better at transferring the heat from the boiler - sounds just like what we need!

 

[vulcancontinental]

Yes I've seen the instructions - the UK section on installation and filling is quite detailed, the Irish section is very vague...

3A.6.6 FILLING POINT
A method for initial filling of the system and replacing water lost during servicing etc. has been provided. You should ensure this method of filling complies with the local water authority regulations.

Doesn't say what the method they provided is! The UK method with the non-return valve shows a separate feed tank above the valve - we are just running off the main cold water tank.

There is no external bypass - but the boiler has an automatic one built into it. As you say though I can't see an external one doing much to help - all the bypass will do is route hot water straight back to the boiler without even giving the cylinder a chance to take some of the heat.

House is about 6-7 years old - thrown up during the property boom and my now wife bought it off the initial owner before we were together (I'm English and moved over having sold my UK house).

We're in negative equity up to our eyeballs, so are going to be staying put here for the medium term at least. It's a 3 bed house so will do us for a while yet. With that in mind I'm trying to get things running as efficiently as we can. We're in the process of putting in a pumped shower in the ensuite (shower the builders fitted never worked) and I'm aware that the hot water cylinder isn't very big - 90 or so litres I think. If we do end up replacing it with a larger one I was going to get one with a fast-recovery coil in it as it's my understanding these are better at transferring the heat from the boiler - sounds just like what we need!

Yes, you're right I suppose they are vague but you are allowed to fit the way Irelands water regs require it. The reason I mentioned the bypass is that the internal bypass is to protect the boiler from overheating, we always recommend an external bypass as well on an S plan to stop noise if there are a lot of thermostatic radiator valves on the system that might be closing at the same time (gives rise to system noise). If one were fitted then it might simply elongate the on-off periods a little to give you some respite but it's a gesture.

There are a lot of new properties over there and I'm amazed when I visit. There seems to be many sites at the moment half completed and at a standstill also.

If I lived there and had the same problem I confess I'd deal with it when something else cropped up and not bother at the moment but I'm notorious for 'if it isn't broken you can't fix it'. What you're doing is switching the boiler more often than it really needs to which will eat into the operating cycles lifetime. Should your DHW demand require X amount of KW per day it is only very slightly different when it is supplied in 5 bursts rather than 15.

If you have a powered shower fitted and you both like your showers then upgrading the cylinder might be the time to act because it's possible you'll start running out of hot water with a long shower.

PRV is the thing to fix.

Good luck

 

[blackfriars]

hi nuttysnake,
if you are still struggling with your installation, i have all tech specs for vokera products
i can give you the name and number of the technical engineer from vokera if you like , he lives in dublin, but works from vokera in kilkenny
my contact info is in my profile

 

[Agile]

Unfortunately your boiler does not modulate and it has a built in anticycle delay of about three minutes.

The effect of that is going to be little on the efficiency but it will increase the reheat time.

It may be possible to increase the firing time by reducing the heat output setting to the boiler's minimum. But that has to be done by a gas reg engineer.

Tony

 

[NuttySnake]

Thanks for all the replies and advice.

I'm going to get the boiler serviced soon enough so will talk to the engineer about cycling and see if the boiler can be turned down any more. Will also get them to connect up the PRV.

New shower is coming on a pace now, so shortly we'll know for sure how long our hot water will last and we may decide to change the cylinder for a bigger one - which should help anyway.

Vulcan you are are right about the houses - there are loads sitting empty I hear!

cheers,