My gas hot water system is working fine. I have just had a shower panel installed in the existing shower alcove. The tap handles have been removed and the hoses are connected directly to the tap outlets,these hoses are then connected directly to the new shower head. The previous shower outlet has been plugged. For some reason, the hot water will only run for 2 minutes then it goes lukewarm, this is happening with all the hot water outlets in the house. Australia

It is quite possible your water heater dip tube has broken off or deteriorated. Best test is to let the system sit for about 60 minutes and then put your hand on the two pipes connected to the top of a traditional water heater. Have someone turn on the tub, shower and a sink hot spigot. One hand will feel very cool/cold and the other very hot. Over the next 2-4 minutes if the hot hand cools then my suspicion gets stronger. Then open the drain spigot and allow some heater water to drain to a bucket or floor drain, if this water is still hot then the problem is for sure your dip tube. Did anyone do any soldering on the pipes on top of the heater lately?

My gas hot water system is 6 months old, the tank is heating fine. I ran the water as you suggested and the h/water pipes remain hot. No soldering has been done to the system. I have noticed that if I turn a cold water tap on in the house I will get (boiling) hot water through the hot taps until I turn the cold tap off, then it will revert back to lukewarm. The hot and cold pipes in the interior of the wall in the shower are both scalding hot, is this normal?

Are you in USA? Never heard of modification that you describe but sounds like you have some kind of pressure balancing valve in your bath faucet that is being tricked by the modification.

I live in Australia. I have not had any probs until the panel was installed. No other modifications have been carried out. The panel has 6 spa jets, a permanent shower head and a hand held shower hose/nozzle. The panel installation is DYI and fairly basic. The water mains were turned off, the old shower outlet was plugged. The hot and cold taps/spigots/faucets were then removed, the cold hose and hot hose on the panel were then connected. The hot and cold hoses then merge into one hose that is connected to the mixer. I don't understand how that can make a difference to all the hot water taps in the house nor why the hot water will continue only if I run a cold water tap elsewhere in the house.

Footnote; both hot and cold pipes that had the original shower taps on (now connected to the hoses of the panel) are scalding hot to the touch when not in use, is this normal? Shouldn't the cold be cold and the hot be hot to touch?

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Shouldn't the cold be cold and the hot be hot to touch?

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Yes. Some how hot water is flowing from the hot through the new panel to the cold pipe when the panel is not in use.

Is the hot and cold pressures equal at other points in the house?

If the hot water pressure is greater than the cold (shouldn't be) then hot water will pass from the hot incoming pipe to the cold, through the body of the old mixer.

This would especially be true when cold water is being used else where. Any cold water use at another faucet would cause a lowering of pressure on the cold side at the mixer valve.

What happens at the mixer valve if hot water is used somewhere else? Does the mixer turn cold?

What is the temperature of the cold water at other faucets? Is it normal of is it warm?

Edit:
Just reread your original post.

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this is happening with all the hot water outlets in the house

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Does that mean that you get warm water from the cold side at other faucets when first turned on?

Sounds like you have hot water from the hot side passing through the mixer valve to the cold side, going back to the cold water input to the water heater and being recirculated. In effect you have a recirculation line to the bath. Great for having instant hot water at the bath but not good for what you want. It also appears that when you use your new panel a mixture of hot and cold water is coming in the cold side and a mixture of hot and cold is coming in from the hot side.

The hot and cold water is getting mixed in the old mixer valve. Don't see how the panel could work unless some provision is made for keeping the hot and cold separate inside the old mixer valve.