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New boiler - combi or unvented, cylinder size and make and model


Our boiler has stopped working and I've had 2 quotes for a new one but they're both recommending different options.

We have a 3 bed house and there are 5 of us.
We currently have a 900x400 cylinder, tank and boiler.

Water pressure is 14/15 litres per minute.

1 bathroom, currently with electric shower but will be replaced with thermostatic when we upgrade and downstairs WC.

One engineer is recommending unvented system with 120l tank. I'm concerned with this option as Google is telling me I need atleast 150l + . And with 3 kids who like baths and showers and will all soon be teenagers I don't want to be waiting for it to reheat. This guy is set against combi saying they will start to break down after the years and have max 10 year life span. Plus side is we will have an immersion but cost to put is in higher.

Combi is cheaper, we only have one bathroom but I'm concerned that it'll take ages to run a bath in winter when the water temp will be starting off lower. But I like the idea of keeping some cupboard space and having instant endless hot water.

Unvented is recommending the brand Joule mid range ? I have never heard of them and on going can only find cylinders and no boilers

Other guy is recommending valiant.

Also has anyone got experience with condensate pumps? Are they really noisy?
One us saying to go up into loft and go into soil pipe other is saying to go straight out and have a soakaway dug?

Any advice gladly welcome because at this point I am very confused

Comments

  • 1. Can the existing boiler not be fixed?
    2. With 5 of you and imminent teenagers, I'd be reluctant to go the combi route.
    3. "Water pressure of 14/15 lpm" isn't pressure, its flow.  You'd need:
    3a. Dynamic pressure of 1.5 bar and flow rate of around 15 - 20 lpm for a combi.
    3b. Dynamic pressure of 2.0 bar and flow rate of 20 - 25 lpm for an unvented cylinder.  
    4. Your existing hot water cylinder is about 115 litres.  If you have the space, 150 litres would be more appropriate for a family that size, and 210 litres would be even better.  However, you can get fast recovery cylinders which will re-heat a whole cylinder full of water in 20 minutes, provided the boiler can supply the heat.  You'd probably need around 25 - 30 kw boiler.
    5. Combi is more complicated than a heat only or system boiler, so more susceptible to breakdown.  However, given a decent make like Vaillant (not "valiant"), Veissmann, Intergas or Worcester Bosch you should be OK.  The quality of installation and subsequent maintenance has much more impact on the life of a boiler than the make.
    6. I don't have direct experience of Joule, but they have a generally good reputation. They don't make boilers, but any boiler can power any indirect hot water cylinder.  (Indirect = water heated by an external source, not immersion heater only).
    7. Condensate pumps aren't particularly noisy.  
    8. Putting the boiler in the loft means boarding the loft between hatch and boiler location, with a handrail round the boarding.  Also need to fit lighting and a loft ladder.  If not properly installed, you may get some people refusing to service it.  Also, you would need to go into the loft to check pressure, clean filter etc.
    9. Condensate best run into an internal drain.  Soakaway would need marble chips to neutralise mildly acidic condensate, and these would have to be changed when exhausted.  Not a big job or particularly expensive, but another chore.
    10.  There are three types of boiler:
    10a. Combi, which you already know about.  No tanks / cylinders needed.  No back up hot water if boiler out of action.
    10b. System.  Similar to a combi but doesn't heat the water directly, rather heating water in a cylinder.  Has the pump inside the boiler and has a sealed circulation system - i.e. no small tank in loft.  
    10c. Heat Only (sometimes called "regular").  The boiler just heats water.  All controls and pump are external,  Can use small tank in loft, but can also be sealed.  Advantage is that controls and pump can be changed without buying boiler manufacturers specific parts, enabling you to have the best pump, the best motorised valve etc.  
    11. There are three types of hot water cylinder:
    11a. Vented cylinders.  These have a big pipe which goes up to and over the cold water storage tank so that any expansion of the water caused by heating vents into the tank, and does not cause excess pressure.  Can have immersion heater fitted for back up in the event of boiler breakdown.  
    11b. Unvented cylinders.  These do not require a cold water storage cistern but heat water at mains pressure.  To avoid the vent pipe they have three levels of protection against excess heat and pressure, and require a pipe to allow excess pressure to escape.  This pipe has quite strict controls over its size and placement.  Unvented cylinder may only be put in and serviced by a G3 registered person, and should be serviced annually.
    11c. Thermal stores.  These provide hot water at mains pressure by passing water from the mains through a coil inside the cylinder, the coil being surrounded by very hot water.  They are not terribly popular, can be a source of problems in hard water areas, and there aren't many engineers who know how to install or service them.  
    12.  Any of the three types of boiler can heat the water in any of the three kinds of hot water cylinder, although it is rare for a combi boiler to be used in this way.  
    13. What you choose depends on what you need and have room for.  My personal favourite is a heat only boiler on a sealed system, with a vented hot water cylinder, cold water storage cistern in loft, and a pump for the shower. Old fashioned, but reliable and individual components can be replaced as and when required.  
    14. If you are desperate for space, then the combi route would be OK, provided you get a big combi (30+ kw), have the supply pressure and flow rate required, and accept that you can't have more than one shower at a time.  You would find a bath slow to fill ( 7 to 10 minutes).
  • grumbler
    grumbler Posts: 58,629 Forumite
    Part of the Furniture 10,000 Posts Name Dropper Photogenic
    IMO, if the shower is over the bath, not separate, you don't need a cylinder at all. What you need is just a boiler with sufficient power.
  • JohnB47
    JohnB47 Posts: 2,637 Forumite
    Part of the Furniture 1,000 Posts Name Dropper
    You can get combis now with 10 and 12 year guarantees.
  • BUFF
    BUFF Posts: 2,185 Forumite
    Part of the Furniture 1,000 Posts Photogenic Name Dropper

    14. If you are desperate for space, then the combi route would be OK, provided you get a big combi (30+ kw), have the supply pressure and flow rate required, and accept that you can't have more than one shower at a time.  You would find a bath slow to fill ( 7 to 10 minutes).
    If you don't mind a floorstanding combi & have the appropriate mains flow/pressure then a Worcester Highflow can be a good option (but not a cheap combi) offering up to 25l/m for DHW depending upon model https://www.worcester-bosch.co.uk/products/boilers/directory/greenstar-cdi-highflow
    No doubt Viessman, Vaillant etc. can also offer something similar.
  • grumbler
    grumbler Posts: 58,629 Forumite
    Part of the Furniture 10,000 Posts Name Dropper Photogenic
    edited 17 September 2021 at 9:05PM

    14. If you are desperate for space, then the combi route would be OK, provided you get a big combi (30+ kw), have the supply pressure and flow rate required, and accept that you can't have more than one shower at a time.  You would find a bath slow to fill ( 7 to 10 minutes).
    And if the flow rate is lower than required, I don't think that an unvented cylinder with any boiler will be able to make it higher.


  • Bendy_House
    Bendy_House Posts: 4,756 Forumite
    1,000 Posts Second Anniversary Name Dropper
    edited 18 September 2021 at 8:27PM


    Our boiler has stopped working and I've had 2 quotes for a new one but they're both recommending different options.

    We have a 3 bed house and there are 5 of us.
    We currently have a 900x400 cylinder, tank and boiler.

    Water pressure is 14/15 litres per minute.

    1 bathroom, currently with electric shower but will be replaced with thermostatic when we upgrade and downstairs WC.

    One engineer is recommending unvented system with 120l tank. I'm concerned with this option as Google is telling me I need atleast 150l + . And with 3 kids who like baths and showers and will all soon be teenagers I don't want to be waiting for it to reheat. This guy is set against combi saying they will start to break down after the years and have max 10 year life span. Plus side is we will have an immersion but cost to put is in higher.

    Combi is cheaper, we only have one bathroom but I'm concerned that it'll take ages to run a bath in winter when the water temp will be starting off lower. But I like the idea of keeping some cupboard space and having instant endless hot water.

    Unvented is recommending the brand Joule mid range ? I have never heard of them and on going can only find cylinders and no boilers

    Other guy is recommending valiant.

    Also has anyone got experience with condensate pumps? Are they really noisy?
    One us saying to go up into loft and go into soil pipe other is saying to go straight out and have a soakaway dug?

    Any advice gladly welcome because at this point I am very confused

    Rachel, apart from the boiler being kaput, does your system actually work well enough? Are you happy with how it fills your bath, for example?
    NFLO has given a superbly detailed overview of the pros and cons of the available systems. As has been pointed out, though, with a mains flow rate of only 14/15 litres per minute - that's a bit pants - I would suggest you simply rule out any system that relies on this for its delivery - that means no unvented cylinders or combi boilers.
    Unless you can get the mains supply comfortably over 20lpm and backed by a solid near-3bar+ pressure, then you'll be forking out a lot of money for a disappointment.
    (There are ways of increasing this, depending on the cause. If it's a narrowed mains pipe to your house, you can replace this after confirming that the street pressure and flow will make it worthwhile. The other method is to install a pumped mains accumulator tank at an additional ~£1k.)

    Qs:
    1) How many storeys do you have?
    2) Where are the baths and showers located?
    3) If you run the existing bath tap - hot and cold at the same time - into a bucket, how many litres come out in 10 seconds?
    4) The CWS tank in the loft - how high up from the loft floor is it, and can it be lifted further, even by a foot or so?



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