Is this how Combi boiler systems work, or I have cross cnxn?

[rizsher]

My property is on rent now and the new tenants have reported a strange problem which I and 2 previous tenants never noticed in the last 5 years. They first complained the boiler was only working intermittently. Luckily I had extended warranty with Viessman and they came and checked it out and found no fault. Then the tenants brought their own plumber and contended the house has a cross connection between the hot and cold water pipes (for domestic water, not heating, I hope). This is the test they said was done:

"It seems like there’s a crosslink between a hot water pipe and a cold water pipe somewhere in the property. The engineer suggested it might be from one of the showers or the bidet.
We had the kitchen tap on running hot water and the engineer cut off the hot water pipe going from the boiler. At this point the water should’ve stopped but instead, cold water started running. This means that the hot water is mixing with the cold one and it will not be scorching hot. I do believe it is hot enough though for normal household use"

Is this how combi boilers piping is supposed to be, or is it a closed loop system where closing the hot water supply would force cold water through the hot water circuit?.

Thanks.
Riz

 

[32pandh]

Hi there!

The hot water pipe circuit shouldn't be directly connected to the cold main circuit only via mixer showers, taps etc... if you turn off the cold water inlet isolation to the boiler then this should turn off the supply to all hot water outlets in the property.

Jay

 

[rizsher]

Thanks for the response Jay.

I guess this means there is either an interconnection (unlikely as after the new boiler was installed in 2010, I lived in the property for over 2 yrs and never had a problem with hot water temperature), or likely one of the mixers, showers or tap, have started passing over the years... could this be the problem??

 

[TeaTime]

The tap he used, was it a mixer tap or was it purely a hot tap only?

What specifically caused them to think that there was an issue?

Shower too cold?
Taps too cold?

 

[32pandh]

Thanks for the response Jay.

I guess this means there is either an interconnection (unlikely as after the new boiler was installed in 2010, I lived in the property for over 2 yrs and never had a problem with hot water temperature), or likely one of the mixers, showers or tap, have started passing over the years... could this be the problem??

Its hard to diagnose without being there but if you isolate the cold main supply on the boiler then all hot outputs should stop. To trace the reason why the hot tap still runs with the boiler cold main isolated you need to look at any part of the cold main circuit meets the hot. Mixer showers, mixer taps etc.... you should be able to hear and pinpoint where the water is mixing.

Has there been any kitchen, plumibng etc... work done recently?

 

[Madrab]

What type of taps are all the taps in the house? Is the kitchen tap a monobloc? Aree they all or are their taps sets too?
Does this just happen in the kitchen? I would be looking to see if it happens on all hot outlets when each isolation valve is turned off.
Is there a thermostatic mixing valve somewhere on the hot supply?

Lots of test to narrow down cause and location but their plumber, if that's what he was, should have done all of this.