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Combi boiler and hot water tank

Discussion in 'Home and Garden' started by spluff, May 26, 2019.

  1. spluff

    Wise Guy

    Joined: Oct 19, 2002

    Posts: 2,031

    Hi,

    The flow/power of the 2 showers in house on first floor we just bought is terrible. The taps seem fine and have decent flow.

    We have a Baxi duo tec 28 erp combi boiler down stairs which feeds the whole house heating and downstairs hot water on tap.

    Strangely upstairs there is a hot water tank which is connected to control panel. When on it heats the hot water in tank from combi boiler. This water is used by upstairs for taps and shower.

    I asked a mate and he has no idea why they would do this and thinks it’s bad design. Wasting power to heat tank on timer twice a day when don’t use all water. He suggested maybe rip tank out and just connect to combi for whole house.

    Why would someone use this weird system and what’s best/cheapest way to have better shower power?

    Cheers
    SpLuFF

    https://ibb.co/Kxc6N7Q
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  2. Dis86

    Capodecina

    Joined: Dec 23, 2011

    Posts: 19,998

    Location: Northern England

    That's a very strange design. The combi should render the need for a tank as obsolete.
     
  3. RJC

    Don

    Joined: May 29, 2005

    Posts: 28,241

    Location: Kent

    That is weird design, I would do as you mentioned - remove the hot water tank and just use the Combi.
     
  4. spluff

    Wise Guy

    Joined: Oct 19, 2002

    Posts: 2,031

    I just checked the specs for the combi and it says ideal for 2 to 3 bed house with 7 radiators.

    This is a large house with 4 bedrooms and 15 radiators

    Maybe that’s why?

    I would guess we would need a better combi if we remove tank? Would there be issues removing tank?

    Thanks for your advice
     
  5. fezster

    Gangster

    Joined: Jan 7, 2007

    Posts: 428

    What kind of HW tank is it?

    A 28kw combi will give an ok shower experience, but will not be able to run 2 showers adequately.

    For larger houses, HW tanks are usually recommended if you have multiple hot outlets. Either a vented cylinder with a pump, or preferably an unvented cylinder.
     
  6. iraiguana

    Mobster

    Joined: Aug 24, 2005

    Posts: 2,568

    Either fit a shower pump to boost hot water output from the tank or look into megaflow mains pressure hot water system, should you be looking to upgrade things.
     
  7. spluff

    Wise Guy

    Joined: Oct 19, 2002

    Posts: 2,031

    hercal coil indirect cylinder - says 3.5 bar on label

    We will only use 1 shower at a time - just want nice power.


    Wouldn’t it be better to just get rid off tank and fit an upgraded boiler? Maybe 35 or 40 kw?
     
  8. RockLobster

    Wise Guy

    Joined: Sep 2, 2007

    Posts: 1,688

  9. pp111

    Gangster

    Joined: Oct 22, 2018

    Posts: 121

    It's not really a bad design. Combi boilers can be slow to generate a full bath of water and struggle to supply multiple devices all at once. Using a storage tank is a way to provide larger quantities of instant hot water.
     
  10. FlyingFish

    Wise Guy

    Joined: Jan 1, 2003

    Posts: 1,445

    Location: Derbyshire

    Yeah I've seen this design a few times over the years, normally where the water pressure isn't good enough to support UVHW or a stronger combi with higher flow rate, and there's a need to run two showers/outlets at once.

    With your setup you can run dhw from the combi and the cylinder at the same time without one affecting the other, as they are drawing from different sources.


    The boiler is a decent robust and efficient model.
    The hot water cylinder seems an old existing setup, left in place.
    The combi should do at least 10l/min (I haven't checked) which is adequate enough.
    The hot water cylinders performance is going to be down to it's age, head of water, and time given to be heated.
    If the head is low, then fitting a booster pump (correctly) will help immensely. But this has to be done correctly, balanced pressures and outlet suitability (shower valves) all need to be taken into consideration.

    Mick
     
  11. fezster

    Gangster

    Joined: Jan 7, 2007

    Posts: 428

    So a vented cylinder - the pressure and flow rate is usually quite poor, as it's gravity fed from a tank in the loft. You can, however, fit a pump onto this type of system to give a better shower experience.

    Possibly. Most people with larger houses go for an unvented cylinder. However, if you are not bothered about using more than one hot outlet at any one time, a combi may be preferable. They are usually more complicated than heat only boilers and therefore more prone to failure.

    Also worth noting that the flow rate quoted for combi's (eg. 11.5 l/minute for the baxi duo tec 28) is for a 35 degree rise. This means in the summer, when the incoming water mains is around 10 degrees, this will give you a half decent shower at an average temperature of 38 degrees. But in winter, when the incoming mains is -1 degrees, you'lll get a far lower flow rate to achieve the same 38 degree shower.
     
  12. FlyingFish

    Wise Guy

    Joined: Jan 1, 2003

    Posts: 1,445

    Location: Derbyshire

    Was there a ninja edit fez? ;)
     
  13. fezster

    Gangster

    Joined: Jan 7, 2007

    Posts: 428

    Haha. Didn't Google the cylinder name - just assumed 3.5 bar meant unvented. Which it is not.
     
  14. spluff

    Wise Guy

    Joined: Oct 19, 2002

    Posts: 2,031

    Cheers guys for all the replies - I can understand a bit more now :)

    I still dont understand how the previous owners could find the trickling shower acceptable it takes ages to shower as its so slow (its 20oC outside).... We will only every use 1 shower at a time but option to use 2 would be a nice but not essential.

    Hot water flow upstairs from taps is fine and bath fills quickly...……… Its just the showers...…..

    So just wondering - is this my options and whats the best

    1. Get a pump - this will be noisy and may not fix problem?
    2. Use existing Boiler and pipe it to the upstairs instead of tank - not sure how complicated this would be / would the existing boiler be able to cope (the existing boiler is about 5 year old it seems)
    3. Buy new boiler and pipe that into upstairs instead of tank - again not sure how complicated would be / cant use 2 showers at same time?
    4. Get an unvented Cylinder?​
     
  15. b0rn2sk8

    Mobster

    Joined: Mar 9, 2003

    Posts: 4,166

    You can only get an unvented cylinder if you have high enough water pressure and would require a significant change to your internal pipe work.

    Have you checked your shower heads to make sure they don’t have flow restrictors installed? It’s odd that taps and bath have decent flow but the shower doesn’t even though they are from the same source. Check this first.

    Realistic options are hook the shower to the boiler if it has enough power or pipe the shower into a new more powerful combi boiler. Either way you’ll probably need to take the floors up to pipe upstairs into the boiler.

    You might even save money in the long run given how much cheaper it is to heat water from gas than it is a resistive electric heater. Sounds like it’s time for a spreadsheet.
     
  16. fezster

    Gangster

    Joined: Jan 7, 2007

    Posts: 428

    Most mixer showers have integral strainers - they may be full of dirt/debris, restricting flow.
     
  17. the-evaluator

    Wise Guy

    Joined: Sep 24, 2015

    Posts: 1,445

  18. spluff

    Wise Guy

    Joined: Oct 19, 2002

    Posts: 2,031

    Had a plumber come around and he said one mixer is from the 80s and the other more modern one is designed for combi boiler and us more restrictive....

    I suggested changing shower mixer but he seemed to think we would be disappointed by the power. Is this true?

    He checked the pressure down stairs and was 4 bar. The flow rate 14. He mentioned a Baxi duotec 40 is more powerful but our flow rate is a bit low for it. Plus we would need to take floorboards up in bedroom. The current boiler is only 3 year old!!

    He doesn’t do unvented but said that would be the least mess as would just need work in the airing cupboard and a pipe going to outside.

    Both options would involve taking tank out from loft.

    He did mention the same pump as in here.

    He said if it was him he would go for the unvented solution (even though he couldn’t do it)

    What do you guys think?
     
  19. iraiguana

    Mobster

    Joined: Aug 24, 2005

    Posts: 2,568

    Unvented would yield the best solution, but not the cheapest.
     
  20. Maccapacca

    Don

    Joined: Apr 13, 2010

    Posts: 16,207

    Location: Sunny Sussex

    If all you're trying to solve is a poor shower install an aqualisa quartz and leave the rest alone :)