Can someone explain why pump overrun has any effect? Surely the boiler's heat exchanger won't get any hotter once the boiler and pump are switched off? There's no input of heat from the gas, so the boiler will stay at the same temperature as it was when it was running, so can come to no harm.
I can assure you it's needed. I suspect that all the sensors are set up on the basis that there is a wter flow. Stop that flow and there is a heat build up. It will trip the over-temperature sensor. Try it and see.
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For those of you who can speak English, it's a fair and honest question.
Build up from where though? There is no source of extra heat, so the water in the boiler must remain at the same temperature, then gradually cool. In fact it would even out, so the hot spots would get less hot. I can't try it as I have an old boiler which doesn't need it, but I was just wondering.
My boiler has the pump overrun and it is needed. If I shut the pump off at anytime the boiler is firing the thing bangs like hell for a few minutes after - as if the pipework is rapidly expanding and vibrating on a nearby floor joist.
The reason for the overrun, as explained by a heating engineer, is to take away the residual heat (which continues to build and heat the water for a few minutes after the gas is switched off) within that bloody great lump of cast iron in the Baxi system boiler known as a... heat exchanger.
Whether you think that explanation is fact or bull, the fact is that if the pump doesn't continue for around four minutes after the gas is shut off, then I get the noise as above. Those noises also occured in the old heat exchanger that was replaced in the above boiler some10 years ago (after a system flush) for the one there now.
For your info, the same thing also happened to the previous system boiler (Potterton), which the baxi replaced some 20 years ago.
Cash
OK, ask yourself how the water gets heated. Do the gas flames heat the water directly? NO. The gas flames heat a bloody big lump of metal which gets much hotter than the boiling point of water and which, in turn, heats the water as it passes over it. Because there is a constant flow, the water isn't in contact with it for long enough to overheat. But stop the flow, and the residual heat in the metal will boil the water which is in contact with it. Keep the pump running until the residual heat has been dispersed, and the problem goes away.
Simple!
The problem is much more acute in modern boilers which contain a relatively small amount of water but have a large mass of metal. Your old boiler probably holds much more water, and can cope without pump over-run. In the days of gravity HW systems, boilers *had* to be able to cope without pump over-run because the pump wouldn't be running in HW-only mode anyway.
The stupid dingo shot himself in the foot!
Ah, so there's metal that's above 100C heated by the gas that continues to push heat into the water and boil it over. Ok I understand.
But.... my Baxi system boiler (fitted previous to 2000) doesn't have an overrun and never complains. In fact it's never broken or been serviced since I moved in in 2000. The only thing it ever did wrong was not fire up once after not being used all summer. I simply switched it off and on several times and it worked - sticky gas valve or something. Now I fire it up every month or so to exercise it.
Ah, so there's metal that's above 100C heated by the gas that continues to push heat into the water and boil it over. Ok I understand.
But.... my Baxi system boiler (fitted previous to 2000) doesn't have an overrun and never complains. In fact it's never broken or been serviced since I moved in in 2000. The only thing it ever did wrong was not fire up once after not being used all summer. I simply switched it off and on several times and it worked - sticky gas valve or something. Now I fire it up every month or so to exercise it.
Yes that makes sense.
Why do modern boilers need the large amount of metal? I can see why they reduced the amount of water - for speed at heating up for combis and for efficiency. But why not reduce the metal too?
Yes that makes sense.
Why do modern boilers need the large amount of metal? I can see why they reduced the amount of water - for speed at heating up for combis and for efficiency. But why not reduce the metal too?
Maybe that yours is not a fully-pumped system as mine is - i.e. no working pump, then there no heating or hot water available. Also the boilers is wall-mounted and designed to fit inside a kitchen cupboard [1] with a balanced flue - so heat dissipation is fairly critical inside that cupboard.
[1] Which has caused some confusion to one or two lesser experienced of the British Gas fitters who come to service it annually - they only wanted to stick a prohibition notice on it before they even removed the cover.When, looking rather concerned at that and asked why, they stated that under the "regulations" the boiler shouldn't be fitted in such a cupboard and is "dangerous" to use!
After I had a bloody great laugh at that, I politely point out that the pre-privatised British Gas company fitted it - and then show them the big blue and white sticker that they "hadn't" noticed on the front of the boiler stating that it WAS designed for such situations - they went red-faced and changed the perennial subject of....trying to get me to fork out for a new Combi boiler.
The privatised BG's fitters of today seem to have a fixation on trying to get me to buy a combi for some reason - I wonder why? *vbg*
Cash
Not sure what you mean. Mine is not in a kitchen cupboard, I'm not sure if it's designed to allow for that or not. The front of it gets as warm as a radiator if it's on for a while. It only has one in and one out pipe (although I see you can attach another out). The out goes through a pump then to valves which go to the radiators and or the hot water tank's heat exchanger. The pump and boiler are electrically wired in parallel - i.e. the pump and boiler always run at the same time, whenever the room stat or the hot water stat call for heat.
I've spotted your problem, getting your boiler serviced annually. I've lived here for 14 years and have never once had my Baxi boiler serviced. It works just fine. Servicing something that works is just throwing away money. How much have you spent in 14 years of servicing? I bet it's more than the new boiler I haven't had to replace.
Let's agree to disagree here Peter as it obvious that my system is a little more complicated than yours.
As for the cost, I could go back and work that out precisely but I'm too bloody tired now) as a rough average of the premiums though, the cost would be around ?2000 for those 14 years of yours - and I've had a quote for a new boiler (not from BG) of around ?1500 - ?2000 fitted (to include some removal and reinstatement of cupboards for access) BG would be more, but again the value of the extra cost would be that their quote would usually include all the works necessary - and no bloody quibbling if they have to come back to resolve any problems (unlike some low-quoting private contractors).
As for the VALUE of those maintenance costs - lets say that I had a repair done on my central heating pipework and rads (not boiler) at about this time last year which took over 18 hours to resolve and that, with a few spares cost well over ?1800 at BG prices with the 'gas man' in attendance within four hours of the phone call.
In the middle of December of last year, the boiler fan decided that it had worked hard enough since it was installed and decided to retire in a rather dramatic way thus stopping the boiler igniting. Rang BG at about mid-day and the gas man was telephoning me to say that he was on his way at about
2:00pm. He was here at the time stated, looked at the boiler and ordered the part and left after about 15 minutes - to return late the following morning to install the fan.All that little lot above, including the annual service, cost me the grand sum of ?220 annual premium - in my opinion, not a bad outcome for last year.
Yes, it costs me every year and hopefully I won't need to claim, but that's what you pay insurance for [1] - to be there when you need it and without some fecking idiot telling you that even though you have paid your premiums, they won't do the work because it was either because a) I didn't follow to the letter one of their obscure T&Cs or b) the repair would cost more than what was agreed again in their T&Cs - and the boiler and heating controls get checked/serviced annually.
[1] All the insurance's that I have (house, car, heating, burglar alarm) are with reputable companies who are certainly not the cheapest, but don't quibble when I need to use them. I've been with the same car insurance company for over thirty years for both business and domestic use and made a claim off it about six times and have never had a problem with those claims.Ditto when my house was burgled at the end of the last century, the house insurance company paid for everything that was taken without a quibble (around ?4500 in total IIRC) - even though I told them that I thought I had left the house unsecured by mistake - and the car insurance paid for the car they took out of the drive and wrote off, along with lifting some rather expensive sound gear (and I'm still with both companies).
Cash
They don't have excessive metal - it's just that they hold very little water - so the metal to water ratio is higher than older boilers.
You really are a clueless muppet aren't you! The boiler has significant thermal mass and the outer side was recently in contact with a flame. The boiler material has a thermal gradient across it and the outer surface is hotter than the boiling point of water. If the water flow through the heat exchanger stalls then there is a risk of flash boiling.
You can see the same effect if you boil an electric kettle poor all the water out of it and then quickly add a small amount of cold water to the empty kettle. It will flash boil to steam - thermal shock doesn't do the kettle much good but it might improve your understanding.
Without overrun all the residual heat in the boiler and the water contained in it, will just be lost out of the flue. With overrun its dissipated in the house.
Modern boilers have greater surface area of metal in contact with a smaller amount of water so they have a more efficient heat transfer than your old Baxi.
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