combi with existing tank question

Firstly the link to Ed's boiler page in the FAQ seems to point to a dud url.
I recently did some voluntary gardening work for a retired lady and whilst I was there she had a new gas cooker fitted with a new bayonet connector installed. After the gas was switched back on the gas boiler failed to relight.
This is a baxi type gas fire with a conventional gas boiler behind and a pilot light ignited by a piezo electric igniter. It looks like the igniter hadn't been used for years and the pilot light had been burning all that time.
A gas safe firm came round and complained the boiler was in a bad state (never looked at for 30 years, about the same time she abandoned the garden), parts for the igniter were not available so it was beyond economic repair. He cleaned the fire and said it was fit for continued use.
I barely know the lady so have to be wary of advising her but it seems to me that the boiler behind the fireplace could be capped off and abandoned. The gas fire still being a source of warmth in the only sitting room.
A 24kW combi will fit in the kitchen and as there is no running water upstairs, plumbed to the adjacent bathroom. There are only 5 radiators in the house so the valves can be replaced by TRVs on all except the one in the living room where the thermostat can be.
What she doesn't like the sound of is losing her supply of DHW from the vented tank with its secondary heating by immersion. I have suggested using a local firm offering a 7 year warranty on the combi which may well see her out, at which stage the property would be a candidate for a rebuild.
What I did wonder is rather than remove all the vented system could a separate zone from the combi run through the DHW water tank coil, the tank and cold water feed in the loft left in place? Then a separate DHW tap in the bath would provide hot water back up, the combi just being plumbed to the shower head?
Cost is a significant issue.
AJH
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snipped-for-privacy@loampitsfarm.co.uk wrote:

    Put up the type number of the fire. I bet the parts are available if you look hard enough.
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I'll look next time I'm there. In fact I suspect it is just the wire from the piezo igniter to the spark gap that's broken but I'm not going to touch the fire front to get access.
AJH
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On 06/11/2017 12:57, snipped-for-privacy@loampitsfarm.co.uk wrote:

Why a combi? If the present DHW tank has an indirect coil and provided this and the radiators are not corroded to death, use a system boiler and make it unvented. This avoids the possible problems of running a shower off a combi, and keeps the immersion DHW backup. And you are not seriously proposing using a combi with vented radiators, are you?
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On Mon, 6 Nov 2017 17:31:31 +0000, newshound

Plumbing is not my thing and I thought system boilers had to go with pressurised tanks, like the megaflow, which I took to be more expensive.
I'll look at the system boiler options. Combi seems to be the cheapest deal.
No I was going to make the rads a sealed system, risking that they may already be corroded inside but they look fine.
AJH
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On 06/11/2017 17:42, snipped-for-privacy@loampitsfarm.co.uk wrote:

I think you will find a system boiler is cheaper than a combi, for a given capacity, if you are re-using the DHW side.The plumbing cost will rather depend on the distance between the existing fire/boiler and the new location, and/or whether current flow and return pipes run near the new location. And how handy it is for the incoming gas (might need 22mm pipe).
System boilers also have fewer parts to go wrong.
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On 06/11/17 17:42, snipped-for-privacy@loampitsfarm.co.uk wrote:

No, but that does give you nice high pressure hot water...
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On Monday, 6 November 2017 12:57:56 UTC, snipped-for-privacy@loampitsfarm.co.uk wrote:

It can be. It needs to be properly emptied and vented and notices attached inside the firefront.
However, given the need for a combustion vent in the room causing howling d raughts up the chimney, far better to take the ghastly thing out altogether and replace it with an electric firefront with no possibility of CO poison ing and much much less likely to cause burns/fire if she falls against it.

Put a radiator in for the old dear. And her feet will be warmer with no col d draught across the floor from aforementioned combustion vent.

st

Combis are usually cheaper than system boilers to buy and a straight swap t o combi / unvented operation (if the radiators will take it) usually the ch eapest install option. She saves the possibility of loft tanks freezing or leaking, and unvented radiator circuits also have limited possibility of fl ooding.
Splitting the bathroom plumbing between cylinder (bath) and shower (combi) may involve dismantling bath panels etc. Not sure if it's permitted to inst all a new boiler to an older Part-L-non-compliant cylinder so a new cylinde r may be required too.
Also, is she likely to continue using a bath for much longer? My mum had a thermostatic shower on a vented system which passed through causing the CW tank to overfill and bring the lounge ceiling down. When she went to a wetr oom she got a Mira disability electric shower (which gave a very good perfo rmance when I tried it).
Owain
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On Mon, 6 Nov 2017 09:45:49 -0800 (PST), snipped-for-privacy@gowanhill.com wrote:

There is a radiator in the sitting room, she likes the gas fire.

That is as I thought, I can get one fitted for under £1500.

Yes I know of these benefits in having a sealed system.

Thanks for that titbit, can anyone confirm that it would or not be allowed under part L? If not at least it solves a problem.

Again a good point, I'll ask. Fitting a 10kW electric shower of decent quality was my fallback position.
AJH
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On 06/11/17 17:45, snipped-for-privacy@gowanhill.com wrote:

https://www.mrcentralheating.co.uk/potterton-gold-218kw-condensing-system-boiler-only
under 500 quid for 18kW is bloody cheap.
In a small house with 5 rads you probably only need about 10kW
reuse existing rads and pipework - pressurised.
Reuse existing DHW tank, and run the primary pressurised as well
Might need acopule of motorised valves, radio thermostat and a strap on thermostat for the water tank
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On 06/11/17 12:57, snipped-for-privacy@loampitsfarm.co.uk wrote:

Was it a Baxi Bermuda at the back? The was one at Father-in-law's house, and the pilot light started playing up shortly after he died. It was a bugger to light, but once lit, the boiler was fine. AFAIR, there was a trick in getting it to light, but I can't remember the method!

Out of interest, why are you considering a combi (or even a system boiler) rather than a bog-standard one? I just wondered if a pressurised system might find weaknesses in the pipes which a conventional boiler might not.
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Jeff

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On 06/11/17 19:11, Jeff Layman wrote:

You dont perssurise a boiler primary even to full mains pressure - just about 1 bar.
If that leaks you needed a fix anyway.
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On 06/11/2017 12:57, snipped-for-privacy@loampitsfarm.co.uk wrote:

Which link?
I re-hosted most of Ed's FAQs on the wiki some while back after his site went down:
http://wiki.diyfaq.org.uk/index.php/Boiler_choice_FAQ

Yup you could do that. Alternatively just fit a system boiler instead of a combi. (its basically like a combi with internal pump and all that, but lacks the DHW bit).
(you can even get traditional vented "heat only" boilers if you wanted something closer to a drop in replacement)
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On Mon, 6 Nov 2017 20:27:05 +0000, John Rumm

http://wiki.diyfaq.org.uk/index.php/Central_heating_design#Boilers

Yes I now favour this...

...over this because it gives the benefits of a sealed system whilst keeping the DHW tank. I initially liked the combi approach because the pipe work from the kitchen looked so much simpler.
Taking the point TNP made the bottom of the system is already 20ft below the F&E tank so if the system pressure stays below 2 bar it's not a great increase.
Thanks for the help all.
AJH
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On 06/11/2017 22:03, snipped-for-privacy@loampitsfarm.co.uk wrote:

Ta, fixed!

Indeed. My feelings on this are if something springs a leak on conversion to sealed, it was going to be pretty close to failure anyway, so at worst you have probably only brought it forward a few months.
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We had a Baxi back boiler and over the years when the boiler would not ligh t it usually boiled down to two things the pilot thermocouple or the pilot light jet. The thermocouple was the most common issue these can fail in ser vice but we also noted that if it had been in a long time and probably near its fail point turning off the gas usually caused it to fail thus not sust aining the pilot light when you tried to re-light it. The pilot thermocoupl e is quite easy to change and quite a number of third party suppliers exist if Baxi no longer supply them. The pilot jet is a small disposable item co sting penny's again easy to change they can get clogged up over time causin g a weak pilot flame. The only tricky thing with either replacement is gett ing the thermocouple correctly aligned so the pilot flame hits it.
The chances are if the boiler has not been serviced in a long time it will be due a clean. Remove the front panel on the heat exchanger just a couple of wing nuts if I remember correctly. There are a couple of baffles below t he flue which slide out to clean. The heat exchanger needs the vanes cleani ng out with a bottle brush type implement and finally a good Hoover all rou nd especially around the burners and controls at the bottom try not to get crap down the pilot nozzle, dismantle and clean/replace if you have. For a complete job the burners need to be removed and dismantled but that is a a bigger job but not beyond the capabilities of a competent DIYer.
Richard
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On Mon, 06 Nov 2017 12:57:53 +0000, news wrote:

Ten years or so ago but we bought a house with an old Baxi back boiler. It had been turned off (house unoccupied and gas turned off) and wouldn't light from the piezoelectric thingie. Fortunately the gas person we located understood the boilers (standard fitting on the estate) and was able to fix it. We ditched it soon after for a combi but those horrible old inefficient things seem to be reasonably bomb proof.
It might be worth asking around to see if there is a traditional gas fitter who deals with Baxis. Anyone who isn't an expert won't want to touch it, quite reasonably.
As others have stated, it is quite possible that the pilot light can be relit using real flames (IIRC that was the stop gap for us)and it may then burn on for many years (or until the gas is next turned off).
According to web search there should be two buttons, one to get the gas flowing and the other to make sparks. A very long match (cook's match?) may be able to reach the pilot light. Alternatively one of those very long nose gas lighters for lighting candles and the like.
If you can get the pilot light going that might solve all the issues (although a service by someone who knows the model would be money well spent and cheaper than a new boiler).
Cheers
Dave R
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Yes the local council fitted lots of these some 30 years ago, the lady bought the house as a sitting tenant and had no maintenance done after that.

This is the rub, people qualified to touch it won't without taking it all apart to clean it, their labour cost then becomes a significant portion of a replacement cost.

I understand this, the gas button has to be held for long enough for the pilot light to heat a thermopile which then holds a solenoid on to maintain pilot gas. Whether I should after a gas safe person has deemed it unwise to relight without cleaning is a factor.

Still looking for a gas safe person who will charge less than £70/hr and do the work in a day. Most younger ones haven't got the right qualification for the fire. Yes this should be the cheapest option as I don't think the increased economy of a modern boiler will pay back her outlay.
The house is an ideal "doerupper" and volume could be comfortably increased by 75%. I realise this as the cottage adjoining mine is over double the size after two rebuilds and worth a packet.
AJH
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