Viessmann Combi Boiler Hot Water Goes Cold

If you’re experiencing problems with your Viessmann combi boiler and the hot water is goes cold or lukewarm, before getting hot again, you are not alone. This is a common issue with Viessmann boilers with the main issue being a blockage in the hot water heat exchanger. But there can be a variety of other causes.  Here, we look at the most common reasons.

Viessmann boilers
combi boiler hot and cold

Scaled up heat exchanger

A blocked up heat exchanger, partially or fully with scale, will cause hot and cold water problems. The boiler will struggle to reach temperature and will be more likely to overheat, which can lead to component breakdown.

If caught early, the plate heat exchanger can be removed and cleaned out. Otherwise, if completely blocked, it will need to be replaced, and that isn’t exactly cheap.

Sludge in the plate heat exchanger

This issue is relevant to Viessmann boilers. Similar to a blockage caused by scale, sludge in the plate heat exchanger restricts hot water flow through the component, at first causing intermittent hot water. Eventually, the hot water will stop and you will get only cold water.

Slow water flow

Another possibility is that there’s an issue with the water flow rate – if it’s too low, the boiler will struggle to heat up properly and will keep cycling on and off. Again, this is something that an engineer will be able to help you with.

Water flow is too fast

On the other hand, if the water is running through the boiler too fast, the system doesn’t get the chance to heat the water up enough. 

The boiler engineer will be able to adjust the valve after doing the necessary checks to adjust the flow rate through the boiler.  Slowing the water rate will heat it up much better.

Faulty thermistor

An issue common with Viessmann boilers is a faulty thermistor. Here, it doesn’t gauge the correct temperature of the water and so allows the water to get overly hot. Other sensors in the boiler pick this rise up and shut down the boiler, making the water go cold. This cycle then repeats often.

Faulty shower control thermostat

If your hot water suddenly starts going cold or lukewarm, check whether it is only at the one outlet or with multiple. Water being lukewarm or tepid at just one outlet, usually the shower, indicates a faulty thermostat on the shower. Check with a nearby hot tap to make sure.

Preheated water in the boiler

Viessmann combi boilers, taking water directly from the mains to heat up, will always have some hot water in the boiler, whether preheated or not.  So when a tap is turned on, you will get one or two seconds of hot water, then cold water coming through the pipes before the boiler plays catch up and delivers heated water.

Pressure relief valve

If your boiler is cycling on and off frequently, it could be due to a number of reasons. One possibility is that the pressure relief valve (PRV) is set too low. This can cause Viessmann boilers to overheat and trip the safety switch, leading to the boiler shutting down.

If you think this might be the case, you should call in a Gas Safe registered engineer to take a look.

Hometree provides engineers that give customers peace of mind with home care packages for your Viessmann boiler, central heating, plumbing, drains and home electrics all at an affordable monthly or annual cost. Click here to see why we recommend Hometree for home care cover.