Heater Exchange Faulty or something else?

8 Jun 2006
Reaction score
United Kingdom
Just help in diagnosing a hot water problem.

Boiler is Ferroli Maxima 35c - Combi Boiler

Maximum Hot water temp on boiler is 55 which is more than hot.

I'm having problems with a mixer shower retaining heat but have now managed i think to localise the problem to the boiler.

The closest taps to the boiler are 1.8m from the boiler (actual pipework)
and are the 15mm copper ones.

Water Pressure is excellent

I can put the hot tap on full and the boiler will kick and the water temperature on the boiler starts rising but gets no further than about 36...luke warm to touch if that.....

If I reduce the flow from the tap by about 75% the water temperature gets very hot and the boiler quickly gets close to the 55 max that it can heat the water at but it's just above a trickle from the tap and takes ages to run a bath to say nothing of the gas consumption. If I increase the flow in the tap to full again it goes back to luke warm if that and the boiler temperature for the water again drops to around 36.

Is it a faulty heater exchange? or what else can it be?
Sponsored Links
You need to properly test the flow rate and temperature rise of the hot water.

There is something called the specific heat capacity of water. The amount of heat required to lift the temperature of the water based upon the flow rate of the water. You boiler cannot deliver hot water at 55 if the flow is too great. It is designed to lift the temp of the incoming main by 35C at a given flow rate (14.2 litres per minute)

You will be able to find out all this info in the manufacturers instructions for your boiler.

You need to test the following
The temp of the incoming mains water.
The flow rate at the tap and reduce it down to match the correct flow rate.
The temp of the hot water.

The hot water should be 35C greater than the incoming main at 14.2l/m.

Hope this helps
Is it a faulty heater exchange? or what else can it be?

May be a faulty Heat Exchanger (£££) but could also be a faulty thermistor (couple of £) or simply incorrect gas pressures (Gas Safe Engineer req)

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