Choosing a Combi Boiler

Discussion in 'Plumbing and Central Heating' started by steveharr, 2 Jun 2004.

This topic originated from the How to page called Problems associated with noises from the central heating system.

  1. steveharr

    Joined:
    2 Jun 2004
    Messages:
    52
    Thanks Received:
    1
    Country:
    United Kingdom
    I need a combi boiler for a house I'm renovating.

    Can anyone recomend a reliable Combi? (I don't need to hear that Combi's ain't reliable anyway, I just want to know of good ones)

    Also what output do I need? The property is a 3 bed semi with double glazing, concrete floors, brick cavity wall and insulated loft. I used the calculator on City Plumming website which gave me a value of 23,832 BTU and 6985 Watts for the whole house, but what does that mean in relation to a 24 kW boiler?
     
  2. Igorian

    Joined:
    30 Mar 2004
    Messages:
    2,736
    Thanks Received:
    63
    Location:
    Northamptonshire
    Country:
    United Kingdom
    To convert from KW to BTU, divide by 0.293. For your 24KW boiler, that works out to 24000/0.293 = Almost 82000BTU.

    Probably a good idea to get an installer to size the boiler for you.

    regards

    Ig.
     
  3. oilman

    Joined:
    16 Sep 2003
    Messages:
    7,965
    Thanks Received:
    26
    Country:
    United Kingdom
    No. Reliable and combi are a contradiction in terms. Anyone who wants to install one in a 3 bed semi that will probably be used one day by a family, should be given psychiatric help :D

    Combies are miles too big for the heating, just so they can allieviate the dismally slow hot water supply. Incidentally, have you checked the property's water supply pressure and flow?

    It's all acedemic when you have a combi involved. Never mind, I earn lots from fixing combies.
     
  4. bungarm

    Joined:
    2 Jun 2004
    Messages:
    16
    Thanks Received:
    0
    Country:
    United Kingdom
    Hiya..in my experience, a combi boiler will not cope with the hot water demands of a family who are likely to live in a 3 bed house...they can barely cope with hot water for a two bed terrace cottage type house with 3 people. Don't do it mate! :D
     
  5. steveharr

    Joined:
    2 Jun 2004
    Messages:
    52
    Thanks Received:
    1
    Country:
    United Kingdom
    Oilman, I know combis are not as reliable as a hot water tank but...

    I would just like to know which ones are the better ones, Please.

    The house has a kitchen and 1 bathroom with an electric shower.

    Even if there is a family in there, surely with only three hot taps and the washing machine plumbed cold it could cope, couldn't it?

    So the difference in power is just for the hot water? Does that mean 24kW combi easily copes with the central heating? My British Gas man had other thoughs on this!
     
  6. oilman

    Joined:
    16 Sep 2003
    Messages:
    7,965
    Thanks Received:
    26
    Country:
    United Kingdom
    With a combi you have only one hot tap. It can't cope with two. I have plenty of customers who ask "why did no one tell me that when I bought it?" Well maybe they would have listened, and maybe not. More than two people, and a combi is bad news (but then you're possibly not listening).

    If you have 22mm heating pipes from the boiler, you can only push 13kW down them (so it does make a bit of a joke of the hot water supply as that has 22mm pipes) So you can't get 24kW of heating. The boiler switches of and on using it's thermostat. It's a bit like buying a 170bhp car, mostly it's pointless except on a race track.
     
  7. kevplumb

    Joined:
    18 May 2004
    Messages:
    23,670
    Thanks Received:
    919
    Location:
    Georgia
    Country:
    United Kingdom
    nice one oilman do about two a week never thought of it in them terms

    i fit what the firm tells me to

    thanks for the insight
     
  8. ch4gas

    Joined:
    11 Mar 2004
    Messages:
    2
    Thanks Received:
    0
    Country:
    United Kingdom
    With a Combi, the boiler output is relevant in real terms to the hot water peformance. The higher the output, the better the hot water performance in terms of flow rate.

    A 32kw combi would be able to provide hot water at more than one tap at a time, as long as the incoming cold water main can provide adequate pressure and flow.

    All combi's modulate; ie they automatically adjust their output based on the temperature of the water flowing through the boiler. When the boiler initially fires for central heating, it will fire at full rate and then quickly reduce it's output as the water flowing through it's heat exchanger increases in temperature.

    The advantage of this is that the heating system (radiators) increase in temperature quickly which helps in heating the house more quickly.

    I would agree, however, with some of the previous posts concerning the installation of a combi in a three bedroom house. Ideally, you would be better going for a mains pressure hot hot water system (if the cold mains pressure and flow are adequate) with a system boiler (condensing of course).

    If you still want to go the combi route, the choice will be purely personal. However, as a company, we install the Ideal range of combi's.
     
  9. Grunt

    Joined:
    3 May 2004
    Messages:
    44
    Thanks Received:
    0
    Country:
    United Kingdom
    I think that only gas combis' modulate - oil models just cycle. Kindly correct me if I am mistaken.

    Some combis' have an internal storage tank that is heated up before hot water can be drawn off meaningfully and although you do not get instantaneous hot water initially this gives better performance afterwards when two or more taps are operated simultaneously.

    Agree/Disagree.
     
  10. ChrisR

    Joined:
    24 Jul 2003
    Messages:
    23,654
    Thanks Received:
    1,156
    Location:
    London
    Country:
    United Kingdom
    Steve - Baxi 105, the aristons are ok, best may be the big Vaillant er 835 is it? Condensing options are a bit riskier - it's down to luck.

    VG water supply to house is v important. Measure your pressure and flow. BUy a £10 gauge to check pressure WITH A TAP OPEN elsewhere say at 5, 10 , 15 litres a minute. open ALL taps to measure total flow - add them up. Less than 25 l/min, or less than 2.5 bar static pressure (o flow) and you're in trouble. 3 bar with 10 litres/min then go ahead. Lots of bigger houses have combis. They do have "features" which need getting used to, but people do - given electric shower and decent suppply.
     
  11. emin-j

    Joined:
    3 May 2004
    Messages:
    31
    Thanks Received:
    0
    Country:
    United Kingdom
    Hi,I cant boast of having technical experience like the guys above but i have had a Vailant Combi for 15 years ( a 240 i think ) in a three bed house and brought up three kids and all the washing and baths that go with em ! I have found the economy and conveniance of these boilers far outweigh any hot water supply problems and i have not noticed any,until now! my boiler has stopped giving hot water,someone said a valve is probably faulty.
    Is it worth repairing or should I replace the boiler as i dont want to spend money only to find something else goes a couple of weeks down the line.
     
  12. DP

    DP

    Joined:
    10 Dec 2003
    Messages:
    7,128
    Thanks Received:
    899
    Location:
    Glasgow
    Country:
    United Kingdom
    The only reason why the Vaillant is broken is because it has not been serviced. These boilers were built to last a lifetime. Lifetime for this boiler is not over. If you are removing this boiler and are located near to where I am, I would take it in and strip this boiler down and overhaul it to work fore ever once more.

    If someone suggests that the boiler be replaced, chances are they do not know how tthis boiler works. Just rescued a client who was told his Vaillant (two square gauges on right hand side) cannot be repaired
     
  13. ChrisR

    Joined:
    24 Jul 2003
    Messages:
    23,654
    Thanks Received:
    1,156
    Location:
    London
    Country:
    United Kingdom
    YES
    Oh sorry that's what DP said :)
     
  14. emin-j

    Joined:
    3 May 2004
    Messages:
    31
    Thanks Received:
    0
    Country:
    United Kingdom
    Thanks DP,I live in Bristol and from your comments I had better call in the repair man,I hope I can find a good un !
     
  15. ChrisR

    Joined:
    24 Jul 2003
    Messages:
    23,654
    Thanks Received:
    1,156
    Location:
    London
    Country:
    United Kingdom
    I bet 4 out of 5 would suck their teeth and say it's had its day........£2500 for a new boiler. Might be worth asking on the phone if they'd replace a vaillant (exact model number ) diaphragm or diverter valve.
     

Share This Page