If you’re experiencing problems with your combi boiler and the hot water is going cold or lukewarm, you may have a blockage in the hot water heat exchanger. This is a common issue for combi boilers and can be caused by a variety of things. Here, we look at the most common reasons.
Combi boiler hot water goes cold
Temperature fluctuations during your shower are likely caused by a blocked heat exchanger in your boiler. This obstruction inside the hot water boiler acts just like a clogged pipe, interrupting hot water flow. To fix inconsistent shower temperatures, the heat exchanger must be cleared of blockages by a qualified technician.
If you notice such changes in your hot water, it serves to remind you that there might be an issue with the heat exchanger that requires attention.
If a shower connected to a combi boiler is experiencing hot and cold water, the issue could potentially be related to the combi boiler. However, it’s essential to consider other factors that might also contribute to this problem, such as water flow rate and water pressure.
Troubleshooting no hot water issues from your combi boiler
Key Takeaways
- Hot water turning cold or lukewarm in a combi boiler may indicate a blockage in the hot water heat exchanger.
- The main reasons for temperature fluctuations are a scaled-up heat exchanger, slow water flow, or excessive water flow rate.
- Low water flow rate can hinder proper heating, while high flow rate prevents adequate water heating.
- A faulty shower control thermostat can cause lukewarm water at one outlet, usually the shower.
The main reason a combi boiler hot water temperature fluctuates is if the plate heat exchanger is blocked with scale and sludge. Other reasons include slow water flow through the boiler, or a pressure relief valve that is set too low. This results in hot taps and shower temperatures changing, going cold then hot again. Don’t worry, though! There are ways to fix this issue and ensure you have a steady supply of hot water from your combi boiler.
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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.
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.
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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
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 the boiler 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.
| Possible Cause | Solution |
|---|---|
| Air in the system | Bleed the radiators and check the pressure gauge |
| Faulty diverter valve | Replace the diverter valve |
| Blocked heat exchanger | Clean or replace the heat exchanger |
| Faulty thermostat | Replace the thermostat |
| Low water pressure | Check the pressure gauge and top up the system if necessary |
| Frozen condensate pipe | Thaw the condensate pipe or insulate it to prevent freezing |
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Faulty parts like diverter valves that can lead to cold water and expensive repairs
Summary
The main reasons for a combi boiler’s hot water temperature going cold are:
- A scaled up heat exchanger.
- Slow water flow.
- Water flow is too fast.
- Faulty shower control thermostat.
- Pressure relief valve setting.
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