It’s never good to turn on your hot water and find it only running lukewarm, is it?

And it’s even worse when it’s the same for your radiators – especially in the depths of winter!

For many of my customers, this means it’s time for panic stations, and I’ve been called out to this problem many a time over the years. It’s a particularly common fault with older combi boilers, although I’ve found that no two are ever the same.

Below, I’m going to tell you what to do if you’re only getting lukewarm water, and when it’s time to call in the professionals.

What’s causing it?

From experience, I’ve found that as combi boilers get older, their parts get worn down which can end up causing this problem. However, there are two sides of coin to think about: lukewarm water, and lukewarm radiators:

The main causes of lukewarm water AND/OR lukewarm radiators are:

• A faulty thermostat
• A faulty thermistor
• A blocked diaphram washer or heat exchanger
• A faulty diverter valve

Or, it could be that there’s nothing wrong at all.

If you turn on the tap and find your water is only running lukewarm, the first thing to do is to try restricting the flow to half on/half off and seeing if the water gets hot. However, this applies to combi boilers with a very low output. (24kw)

What should you do?

Unfortunately, if your lukewarm water is caused by some of the problems above, you may need to call in a Gas Safe registered engineer.

However, below I’ve put together a little tip you can try for yourself, first:

A possible quick fix

If you’re running your taps only to get lukewarm water, the first thing to do is to try turning the heating on as well to see if it gets any hotter.

You should also try turning your heating off and seeing if any of the radiators are getting warm when you’re running hot water.

If your radiators are working fine and don’t get warm when the water is running, the first thing I would replace is the hot water thermistor.

This is a small, inexpensive part that many people with a bit of DIY skills may be able to competently change. (If in any doubt, call a professional)

Faulty heat exchanger

When you turn on the hot water and your boiler fires up to heat the water, it should stay running the whole time the tap is turned on. However, if your boiler is cutting out and the hot water is only getting lukewarm – this is a sign the plate heat exchanger is blocked.

Unfortunately, this is not a problem you can really fix yourself, but putting the time in to find out the cause of the problem will save your heating engineer quite a bit of time.

Faulty diverter valve

Sometimes it’s the case that your thermistor is fine, and the diverter valve’s the real problem. It may be that it is simply jammed open in heating mode. A sure-fire sign of this is having to turn on the heating in order to get hot water to come through the tap.

On the other hand, if you have all the hot water you need, but no heating, ensure all your time clocks and controls are working. If everything’s fine, it’s almost definitely the case that the valve is not opening for the heating.

Read more about faulty diverter valves on our blog.

An engineer’s perspective

A reputable heating engineer will do their very best to diagnose the problem from experience. However, they may find it sensible to replace small, cheap parts first (like thermistors) that only take two minutes to replace.

This is NOT to run up the bill, but to see if it could possibly be these parts before deciding on a more expensive, time-consuming solution.

Occasionally, I’ve been convinced the hot water thermistors were the problem, so I changed these and still encountered the same problem. There were no usual symptoms of heating and hot water being affected, but I reluctantly replaced the diverter valve and the problem was solved.

Conclusion

It can be stressful when you suddenly find yourself with lukewarm water and/or radiators. It’s problem usually occurs in older combi boilers, and you may need to call a Gas Safe registered engineer, but it’s worth following my tips above first.

The main causes of lukewarm water AND/OR lukewarm radiators are:

• A faulty thermostat
• A faulty thermistor
• A blocked diaphram washer or heat exchanger
• A faulty diverter valve

Remember, if in any doubt, call a qualified heating engineer. Good luck!