New tap, fluctuating water temp

[RandomUser666]

We had an old bath/shower mixer tap which worked well but looked bad, so we replaced with a very similar one, but now the water temperature fluctuates slightly from warmer to cooler about once a second, when using either the bath or shower. We have a combi boiler and it doesn't appear to be cutting out or changing temp when the tap is running. The shower head and bath tap both have eco restrictors but if I remove the shower head the temp still fluctuates. Mixer taps on sinks in the house don't have this problem.

Nothing with the plumbing has been changed except the tap, so I'm wondering, could it be one of the following reasons:

- Flow rate of the new tap (10L/min) is different from the old one and that's causing issues
- The old tap had one handle to adjust the water temp, the new tap has two separate handles
- The new tap is faulty or badly designed in some way?

FYI, this is the new tap: https://www.screwfix.com/p/swirl-ca...ap/190pg#product_additional_details_container

Thanks for any advice!

 

[Johntheo5]

(If) Can you set the DHW temp to say 40C and just open the hot tap and see if the temperature changes.

 

[RandomUser666]

Assuming DHW means the temp of hot water on the boiler, I have done that and the hot water tap gives a solid temp!

 

[CBW]

Could be a faulty tap. Isolate the cold feed for hot water under the boiler and run the hot tap on the bath, if it runs you have crossover and likely a faulty tap.

 

[RandomUser666]

Hmm, no nothing comes out of the hot tap when I do that

 

[ianmcd]

is your new tap fitted with flexis ?

 

[RandomUser666]

Yes, the same ones the old used in fact

 

[ianmcd]

Yes, the same ones the old used in fact

Well if the old setup didnt have a problem with them they should be fine, new ones are very restrictive it sounds like your tap set up, I know it says 10L/min, but that is the combnined flow rate, of the hot and cold together so if the temp set at the boiler is high, then the actual flow rate of the hot will be very low, try turning the boiler setting (simply as a test) to 45 degrees, and only open the hot side of the new tap and see if it runs constant then , you have found your problem, insufficient flow rate of the HW through the tap when using both the hot and cold inlets

 

[Johntheo5]

Assuming DHW means the temp of hot water on the boiler, I have done that and the hot water tap gives a solid temp!

So if you then open the cold water tap as well to give a cooler mixed flow temperture; the temperature fluctuates?, if so you may have two different pressures at the hot/cold, if you open them separately does the flow seem more or less equal from either?

 

[RandomUser666]

Yes, if I open them separately they seem pretty much the same, cold could be slightly faster but hard to tell. If I try the boiler at 45 degrees, run the hot, then add in a bit of cold the temp seems to fluctuate less than when the boiler is at 55. Hmmm

 

[Johntheo5]

What make/model of combi and output, how old.

 

[ianmcd]

Yes, if I open them separately they seem pretty much the same, cold could be slightly faster but hard to tell. If I try the boiler at 45 degrees, run the hot, then add in a bit of cold the temp seems to fluctuate less than when the boiler is at 55. Hmmm

read what I said set it at 45 and ONLY open the hot side not the cold side

 

[Johntheo5]

What make/model/output of boiler and age.

 

[RandomUser666]

@Johntheo5 It's a Ideal Vogue Max Combi 40, 4 years old

@ianmcd Yes I did try with hot only and no fluctuations

Thanks!

 

[Johntheo5]

Yes, if I open them separately they seem pretty much the same, cold could be slightly faster but hard to tell. If I try the boiler at 45 degrees, run the hot, then add in a bit of cold the temp seems to fluctuate less than when the boiler is at 55. Hmmm

Your boiler should be able to modulate down I think to around 6.1kw, if so, then you should be able to get down to a flow of 2.9LPM at 55C alone through the hot tap which you apparently can achieve or a a mixture of ~ 0.75LPM at 15C mixed with 2.2 LPM at 55Cto give you around 2.9LPM AT 45C so unless your combned flowrates are less than around this which is unlikely then IMO a boiler fault, perhaps thermistor or flow switch problems.