On the most common version of Powermax, the 155x, probably the most frequent fault I’m asked about is failure of the hot water supply while the central heating continues to work perfectly. The boiler lights and runs and heats up as normal, but the hot taps run cold.
There are several possibilities but the most common cause of this is contamination by water scale. This affects the hot water in two ways. To explain, firstly I’ll need to run through how the hot water works in the first place.
Unlike a conventional hot water cylinder, the Powermax water tank contains stored hot radiator water rather than clean domestic tap water. The hot taps are connected directly to the incoming high pressure cold mains water supply, which gets heated as it passes through the “plate heat exchanger” (inside the Powermax) on its way to the hot tap.
The plate heat exchanger is a stack of corrugated stainless steel plates and the cold mains water passes through alternate spaces between the plates. Through the other spaces between the plates, very hot stored radiator water is pumped, and the heat from the hot radiator water passes straight through the thin steel plates and heats the high pressure cold mains water as it passes through, resulting in high pressure HOT mains water coming out and arriving at the hot taps. The point of this rather complicated arrangement is to get the high mains water pressure at the hot taps as well as at the cold taps. Or more particularly, arrive at the shower (as old style low pressure hot tanks won’t run a shower).
Now water scale – from calcium dissolved in hard water – forms most easily on hot surfaces such as the inside faces of a plate heat exchanger. The scale progressively thickens as the the months and years tick by and this thin layer of scale slightly insulates and reduces the transfer of heat from the hot radiator water through the plates into the clean hot tap water. Eventually the thin layer of scale becomes a thicker layer and the insulating effect increases, which reduces the heat transfer enough to cause the tap water to be just warm, rather than plenty hot enough.
The second way in which scale affects the hot water is more obvious. Very obvious in fact, as it causes the hot tap to run completely cold. The pump which pumps the hot radiator water through the heat exchanger while a hot tap is on, relies on a flow-sensing switch in the cold water pipe into the heat exchanger. When a hot tap is turned on, the flow-sensing switch senses the water flowing and starts the pump which draws the hot radiator water into and through the heat exchanger. If the pump fails to run, the hot tap water will remain cold. Now the flow switch needs a certain minimum rate of water flow to operate. As water scale accumulates it not only causes the hot water to run less hot, but it restricts the flow rate too and eventually the flow rate through the flow switch and the heat exchanger becomes too low for the flow switch to operate, and turn on the pump. Cold water at the hot taps is the result.
Now this might sound as though it should be quite obvious to recognise happening and diagnose, but there is a complication that disguises the effect. There is a thermostatic blender valve fitted to the hot water outlet of the heat exchanger. This is for safety and limits the temperature of the hot water being sent to the hot taps to a safe temperature that won’t scald any hot water users. If the water from the heat exchanger rises above the preset temperature of 55c, the blender valve mixes in a little cold water (drawn from a ‘tee’ connection into the supply pipe before the heat exchanger) to limit the temperature to 55c. This blender valve tends to get damaged by water scale leading to it always mixing in a proportion of cold water into the hot, whether needed or not. This can lead to a misleadingly high flow rate of (cool) water coming from the hot taps, causing a boiler technician diagnosing a hot water fault to rule out water scale as the problem, as the symptom of reduced flow rate from the taps which usually indicates scale, is absent.
So a diagnosing technician needs to use a multi-meter to check whether the pump is running or not during hot water demand. If the pump is not running it is as likely to be caused by scale in the plate heat exchanger limiting the cold flow through the flow switch rather than a failed flow switch, and the heat exchanger simply needs descaling or replacing. (Replacement is better.) If the pump is running but still there is no hot water or luke warm hot water, the chances are the blender valve is damaged and incorrectly passing cold water through into the hot supply when not necessarily. Quite often both faults will be found simultaneously meaning a new heat exchanger AND blender valve are needed to complete the repair.
I’ll add some photos of the plate heat exchanger and the blender valve shortly.