A girl needs help! Cold/weird showers with unvented hot water tank/megaflo (3 plumbers baffled

Discussion in 'Plumbers' Talk' started by CA Mackenzie, Apr 14, 2020.

  1. CA Mackenzie

    CA Mackenzie New Member

    I moved into into a new house with my husband and all 3 mixer showers running luke warm. Get very hot water out the sink and bath taps with good pressure. But showers not hot. The only way I can get them to go hot is if I run the cold sink tap in that bathroom and then the shower goes a lovely temperature (at the expense of running cold water from other outlets). The other thing that works (oddly) is changing the shower head function to a high pressure jet which causes the temp of the shower to go from 37.2 degrees up to 41.5 degrees (a difference of 4.3)

    Boiler working. Heating working. I have a megaflo unvented indirect hot water cylinder upstairs where all my showers are. I repressurised the megaflow which has made no difference. No water dripping from the tundish. Plumbers have come and gone with no solution.

    Any solutions/thoughts/diagnosis as to what’s causing this would be greatly appreciated! I can't help but think its the megaflo. Have attached photos of bits around my megaflo at the back if that helps.
  2. Mike83

    Mike83 Screwfix Select

    What is the actual temperature of the shower??
    The showers may just need calibrated.
    If the temp is really low I’m wondering if the hot and cold could be reversed.
    Hfs likes this.
  3. one of the showers non return valves not working correctly mixing hot and cold. Or thermostats gone on all showers.
    CA Mackenzie likes this.
  4. CA Mackenzie

    CA Mackenzie New Member

    The temperature is 37.2 degrees at its max temp setting (scald guard adjusted time give highest temp). If I turn on cold tap in the sink OR change the shower head function to a high jet/pressure setting, then the shower water heats up to 41.5 degrees. Happens on all showers
  5. CA Mackenzie

    CA Mackenzie New Member

    It would be such bad luck that all thermostats have gone on all the showers. If this were the case would me running a cold tap in the sink make the shower hotter?
    How do I know if it’s a non return valve? Is there a way of checking?
  6. I have seen it happen, especially when unvented cylinder was set up incorrectly (to high) damaging all thermostats in the property. The way to check for faulty non return valve is to turn cold supply to the hot water cylinder off and turn hot water tap on (kitchen or basin) - on unvented system it will run for a while (expansion vessel or expansion bubble discharging) but eventually it will stop. If it doesn't stop - you know you have a non return valve passing somewhere.
  7. You may also have unbalanced supply to the showers. That would explain non-thermostatic mixers working correctly and the behaviour of your shower when you turn the tap on.
    The Teach likes this.
  8. Looking at your previous post re temperature on the showers (38 degrees) - it is actually a maximum setting on most thermostatic shower valves! Ignore my other posts. All working as it should.
  9. andy48

    andy48 Screwfix Select

    The following is one theory, not a guaranteed correct answer!

    If its a relatively new build house, there may be thermostatic mixing valves (TMVs) on the inlets to the showers. Likely to be hidden away underneath bath for example. Have a look for something like Screwfix 96288. These would regulate the hot water temperature to the maximum set on the TMV, and altering the shower temperature setting would have no effect. They work by mixing cold with the hot water, which might explain why turning on another cold outlet increase the temperature, because it robs the TMV of cold.
  10. Mike83

    Mike83 Screwfix Select

    I had though about this but if it was causing the low temperature it would also affect the bath tap.
  11. Joe the Plumber

    Joe the Plumber Screwfix Select

    Most showers come factory set at a suitable temperature for a care home. If they're all the same model, it may be they were fitted as they were out of the box and not adjusted for normal domestic use.

    If you can find the instructions, there may be an adjustment you can make to each to increase the maximum temperature.

    Running a cold tap will be starving them of cold water and allowing the hot flow to "dominate", but it does sound like they need setting up.

    Any chance of a photo of them please?
    WillyEckerslike likes this.
  12. Mr Rusty

    Mr Rusty Screwfix Select

    I'm no plumber, just a DIYer who does some plumbing occasionally. The bar mixers (assuming they are) that I have come across have the ability to set the temperature by removing the adjusting knob, setting the temp and then putting the knob back on. As they all seem the same, is it possible that the temp was set by the original installer at 38degrees, then the knob put on at max instead of against the temp override stop? Have your plumbers actually tried this? Perhaps download the installation guide for your shower units and see what it says.

    Ha, Joe simultaneous posts! :D
    WillyEckerslike likes this.
  13. The Teach

    The Teach Screwfix Select

    Did they mention anything ? Oftenthere is a reason why plumbers dont want to engage in work !

    That is something that needs to be found out,
    the hot shower inlet pressure may be to low to mix with the cold shower inlet and turning on a cold water tap reduces the cold flow to the shower mixer resulting in a satisfactory blended temperature 41˚C is a comfortable blended temperature.

    Which shower mixers & shower heads do you have ?

    Whats is the bath hot water temperature ?

    Has the unvented cylinder and its associated components been serviced ? filter clean,everything in general good health etc.

  14. I wonder what the incoming cold mains pressure is. The Megaflo will regulate the hot to 3 bar and if the cold for the shower hasn't been plumbed from the combination valve but direct from the mains it could be pressure imbalance. Hence running the cold tap. If you turn off the feed to the Megaflo does the cold to the showers stop also? Can you measure your incoming mains pressure?
  15. P and H

    P and H Member

    Have seen something like this before and it was unbalanced supplies, the cold feeds to the showers came direct from the mains so higher pressure than from the cylinder.
    Had to put a pressure reducing valve on the mains supply so balance things up
  16. 2dowhat

    2dowhat Member

    Blimey this is giving me flashbacks. I remember a job with similar symptoms and eventually the cause was nailed down to a faukty blending valve. Once replaced, all was well again.
  17. PlumbyGas

    PlumbyGas Member

    Pressure issue. See it on new builds alot, nice new water main in the road, nice new 25mm supply to the property, great flow rate and pressure on cold. Sepressure re from cylinder. "Drowns" out the mixture at the shower with cold water. Test to check is run a cold tap at see if water gets hotter as you have already found.

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice