If you’re experiencing problems with your Vaillant combi boiler and the hot water is going cold or lukewarm, before getting hot again, you are not alone. This is a common issue with Vaillant 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.
Combi Boiler Hot Water Goes Cold Causes
A combi boiler’s hot water can go cold if any of these occur: A scaled-up heat exchanger inside the boiler that is blocked so water cannot be heated, water flow is too slow or even too fast so isn’t heated up, the pressure relief valve is set too low or it could even be the shower thermostat isn’t working.
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 particularly relevant to Vaillant 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 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
Vaillant 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 Vaillant 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.
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Summary
Causes of a boiler where hot water goes cold:
- Scaled up heat exchanger.
- Slow water flow.
- Water flow is too fast.
- Faulty shower control thermostat.
- Preheated water in the boiler.
- Pressure relief valve setting.
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