Combi or not combi - help!

I note it says in the wiki that: "I need a new one, what should I get!" comes up in some form almost every day on uk.d-i-y, but couldn't find enough information to clench the deal, one way or the other.
My wife and I are living in a two bedroomed bungalow - there's nobody else living there except the cat! The central heating is old - if the pencilled date on the back of the controller I replaced recently is anything to go by it dates from 1996.
We had a quote recently from a local plumber who advocated a combi - which I've always been convinced from various things I've read (probably here) that they can be problematic - but he said that modern combis are much better than their predecessors and would be highly suited to our needs. He also pointed out that he has a combi in his 4 bedroom property and has no problems with it.
We then got another quote from someone who is clearly prejudiced against combis! Apart from additional strain on the old pipework - which we are well aware of - he said that the increased pressure can damage the non-return valve in dual fill washing machines (ours is currently a dual-fill Hotpoint) but that we wouldn't be able to run hot water from two taps at the same time!
This could be a worry if we couldn't wash our hands (or whatever) every time the washing machine was filling or the other is running a bath. Can this really be true?
Quote No. 1 includes an Ideal Logic+ 30kW boiler whilst No. 2 is for a Worcester Bosch 15ri boiler. No. 1 includes quotes for both combi and non combi options (but does give any indication of which non-combi boiler would be fitted) and No. 2, as might be expected, doesn't have a second option!
How do these boilers compare - assume No. 1 would use a non-combi version of the same boiler?
All comments gratefully received!
Terry
--

Terry

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On 20/05/17 13:35, Terry Casey wrote:

Does it work though?

He has a deal with comnbi supplires.

Well thats bollocks too.

Mate, the combi I have here currently wont even supply enough hot water to drive ONE hot tap, bath or shower.,

Bite the bullet and do the proper job, Mains pressure hot water tank fed from whatever boiler you have and if in a hard water area all fed via a pukka ion exchange softener.
Then never have a hot water shortage ever again
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On Sat, 20 May 2017 13:56:06 +0100

In East Anglia, it is stupid to not have a water softener. Other parts of the country can do perfectly well without them.
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Davey.

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If you already have a storage system which you're happy with, fitting a replacement boiler for that is not only going to save money, but be easier than changing to a combi.
The problem may be finding an installer who does think of your needs and likes, than just his own profit.
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In fairness, the first one has also included a quote for a like-for-like (but condensing) replacement which is £350 less than the combi approach.
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Terry

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Dave Plowman (News) wrote:

A personal observation going from a 1987 installed conventional system in a (then) new 4 bed detached.
A WB 34KW Greenstar boiler is more than adequate and shows no sign of struggling to meet the thermal demand for HW with multiple showers operating.
A lot of pipework & HW cylinder was removed and the airing cupboard recovered in full as storage. Roof insulation could now be improved as no need to worry about the cisterns freezing.
No issues with leakage from (now) pressurised 30 year old HW/CH system (8mm microbore). This was my biggest worry as the water pressure round here is highish (3 bar).
The thermostatic shower valves and newer taps now operate correctly with the higher pressure. It was a bit hit and miss before getting them started enough to kick in the shower pump.
There is a noticable issue with HW water flow rate upstairs & interaction between showers & filling the bath is a bit slow. This is probably down to re-using the original 15mm pipe work. It would have been better to have replaced this with 22mm.
Reliability OK so far 2 years in but needs paid annual inspection to maintain the 7 year guarantee - the old Potterton Netaheat boiler needed about £200 in DIY repair over 25 years (2 replacement fans), doesn't look possible with this one.
Been a noticable drop in gas consumption but not huge as the property is well insulated and the old boiler was not that bad (rated 69%).
HTH
Chris K
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On 20/05/2017 17:44, ChrisK wrote:

If your mains pressure and flow rate are adequate then 15mm should be fine. Sounds like you have another problem somewhere.
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Andrew wrote:

Probably not, it is quite a tortuous route the 15mm feed follows to join the upstairs HW pipework which is all 22mm. Not bad enough to do anything about it as it would involve taking up a lot of flooring.
CK
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On 20/05/2017 17:44, ChrisK wrote:

The mains water supply pressure has no real bearing on the primary heating side of the boiler - that is pressurised via a filling loop to typically a bit over 1 bar. It then rises somewhat as the water heats up. The only bit of a the boiler that will see the full mains pressure is the secondary heat exchanger that heats the water from the primary side of the boiler.

Choosing pipe sizes carefully can minimise interactions a bit. Although a 15mm pipe will likely be able to supply as much hot water as most combis can produce. Also a smaller pipe will give you less dead leg of water to drawer off at each use.
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John Rumm wrote:

Of course, I knew that :-(, must engage brain before touching keyboard... It is running at about 1.5 bar.

Agree, it's not a problem, just a noticeable side effect & the 15mm pipe run is quite long with a lot of bends in it.
CK
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On 20/05/2017 13:35, Terry Casey wrote:

Should be OK unless you have hand washing OCD. Combis are adequate for small or medium flats/houses unless you have more than one bathroom.
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Do you have a suitable size gas pipe to the location? I needed to re-pipe in 22mm with minimal bends. A combi needs a lot of gas and a good flow. My earlier conventional boiler took less - for longer and 15mm pipe was ok.
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On 20/05/17 18:24, DerbyBorn wrote:

no gas at all here
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snipped-for-privacy@Nearhome.com says...

The gas pipe does indeed need upgrading for a combi and that is included in the quote we have.
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On 20/05/17 17:51, Max Demian wrote:

Given you can get combis up to 35-42kW (WB) they should be adequate for 2 bathroom properties. WB even do a floor standing combi with insane DHW flow.
Some combies also have a small water store that they keep hot that helps with occassional demand.
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On 20/05/17 18:29, Tim Watts wrote:

a 40kw combi costs a lot more than a 10kw system boiler with a mains pressure tank

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On 20/05/2017 19:26, The Natural Philosopher wrote:

Vaillant ecoTec 612 £916 Vaillant unistor 120L £605
Vs
Vaillant ecoTec 838 combi £1497
Odd definition of "a lot more"...
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I resisted a new combi for years but they are now quite good...you will save a fortune ....just go for it ....
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On Sat, 20 May 2017 13:35:45 +0100, Terry Casey
[snip]
I installed a combi boiler (Worcester Bosch) last year and I am a bit disappointed with it.
The water takes a while to heat up when using Eco mode. I mostly run without Eco mode (pre-heat is capable of being programmed) so I assume I am using extra energy to save water.
The bath takes ages to fill. This may however be related to having an unusual thermostatic mixer tap arrangement that serves the bath and the shower.
The radiators warm up even when the heating is supposed to be off. I understand this is because the boiler uses the heating as a heat sink when it is pre-heating the hot water. Again, I assume this means wasting energy.
I find the extractor system noisy for a domestic application, but this may be common to all condensing systems.
The hot water even at its maximum setting is not hot enough. It's okay for filling sinks and baths in the first place but for a top-up it is fairly ineffective.
The washing machine is cold fill so this is not an issue. I think nearly all washing machines are cold fill, so probably your next one will be. Could you not disable hot fill if this would help?
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On 20/05/2017 18:38, Scott wrote:

24kW by any chance?
(makes for dismal bath filling - but will do a "twice as good as the best electric" shower)

Not much value in ECO mode really - it only fires fairly infrequently to temper its small store of hot water.

Or an underpowered boiler...

That sounds more like a faulty diversion valve.

Extractor system?
(do you mean a fanned flue? If so, a standard feature of any boiler you can buy today - although some are louder than others)

Depending on how well the plumbing is done you can get temperature interactions even from cold fill only - especially of the WM cold feed is taken from the pipe before the cold feed to the combi. Really it wants to be plumbed with that being the first thing to be taken off from the incoming main.
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