Cross flow issues

Joined
17 Oct 2021
Messages
39
Reaction score
3
Country
United Kingdom
Hi,

I had a new vaillant combi boiler installed a year and a half go, powerflushed etc. Water temp was fine then could have a hot shower. Recently in the last couple months the hot water in the shower has become lukewarm. In kitchen tap the temp fluctuates from 39-45 degrees Celsius. This is with the boiler set at 65 Celsius.

The boiler is under warranty and the engineer came down and replaced the heat exchanger. First he isolated the hot water but water was still coming out the hot water kitchen tap. He said its an issue with cross flow where cold water is making its way in to the hot water pipe and get a plumber to take a look. The old heat exchanger was fine no gunk or anything on inside.

The shower was also replaced the same time as boiler during an extension.

Please can someone advise how I can isolate which tap is causing the issue? All are mixers in the house. I'm just afraid that floorboards will need lifting up etc don't want to go through all that pallava.

Thanks
 
Sponsored Links
Do all taps and mixer shower valves have isolation valves on their supply pipes ?
 
You need to have ISO valves to each of the outlets and then you need to shut everything down, have the HW valve at the kitchen open and then open each outlet in turn, you will then see which one is letting by
 
Do all taps and mixer shower valves have isolation valves on their supply pipes ?
I've got three mixer taps in the house and they all have isolation valves on both cold and hot. The shower thermostatic valve I'm unable to check as it's concealed behind tiles.
 
Sponsored Links
You need to have ISO valves to each of the outlets and then you need to shut everything down, have the HW valve at the kitchen open and then open each outlet in turn, you will then see which one is letting by
Sorry not sure I follow. Shut everything down at each tap and the mains stop cock? The shower valve is concealed so I'm not sure if it has isolation, even if it did I'd have to rip out tiles.

Wouldn't it just be a simple case of closing the hot valve on each tap and seeing which still flows?
 
Close both isolators on all taps.
Run shower and see if hot temperature Is now ok . If it isn't the cross flow can't be from any of the isolated taps.
 
If shower still lukewarm with H&C isolators shut on all taps you might consider this.
Put the shower to your normal showering temperature and flow, measure this flowrate and the temperature rise in the shower will be (boiler DHW output in kw)/860/60/LPM. With a mains cold temp of (my) present 8.7C the shower temperature should be 8.7+calculated temperature rise.
IE, if the DHW output is 30kw and your meaured flowrate is 15LPM then the temperature rise will be, 30X860/60/15, 28.7C and the showering temperature, 8.7C+28.7C, 37.4C.
 
Sorry not sure I follow. Shut everything down at each tap and the mains stop cock? The shower valve is concealed so I'm not sure if it has isolation, even if it did I'd have to rip out tiles.

Wouldn't it just be a simple case of closing the hot valve on each tap and seeing which still flows?
AS suggested ... You need to go through a process of elimination if you want to find out which outlet is causing it, that can only be done by isolating every outlet, open up the hot at the sink then open each taps supply in turn and see if one affects the kitchen tap.
 
So I had a chance to do the following today:

Close all isolators hot and cold on all three taps. Turned on the shower and it made no affect. I tried turning the thermostat in all directions slowly and it barely made a difference to the temperature. So it looks like the shower thermostat needs replacing? Only issue it's behind tiles so I was really hoping it wouldn't be the shower. Would this cause a cross over between hot and cold pipes? Not sure how.

The other method I tried was close all valves including hot and cold on the taps then turn on the hot valve one by one to see if there was any increase in the temp. Unfortunately not.

So likely its the shower valve? You can see the shower valve in the pics.
 

Attachments

  • 20210819_183300.jpg
    20210819_183300.jpg
    351.8 KB · Views: 44
  • 20210825_184121.jpg
    20210825_184121.jpg
    241.2 KB · Views: 46
Can you isolate both feeds to the shower, could possibly be the boiler??
 
I'm afraid I can't isolate the feeds to the shower. The engineer said if I fix the cross over issue it will fix the hot water. He said boiler is all fine even after replacing heat exchanger.
 
Just because the hx is OK doesn't mean there isn't a boiler fault, could be dhw sensor or whatever. The only 100% way to prove it is to disconnect the DHW outlet at the boiler, connect a short section of piping with a valve on the end and run it into a large container and measure the flow/temp.

If the boiler has (I think it has) isolating cocks then if you isolate the cold mains at the boiler and at the boiler DHW outlet, when you remove the DHW outlet then if no Xover.... no water should backflow through the now disconnected pipe?. You can then carry out the hot flow check.
 
Last edited:
Another thing just noticed now. If I open the shower which now is barely lukewarm after trying different thermostat settings, the bathroom tap hot water is scalding. Sadly it doesn't work the other way. Either way I think the thermostat is faulty on shower. So would this cause the cross over if open at the same time the tap closest is scalding?
 
Can't answer that I'm afraid, if you open the other two hot taps do they also get hot?
Would still suggest consider doing at least the first part of my suggested test above.
Can you take a photo of the pipework under the boiler and post the exact vaillant model and we can then see if this test is easily carried out or not.
 
Last edited:
With all taps isolators closed,and the kitchen tap only ,has its hot isolation valve open,does the kitchen tap give hot water at consistent temperature ?
(And shower not running either).
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Back
Top