Why Does My Shower Go Hot And Cold?

November 29, 2021

Have you noticed the temperature of your hot water fluctuating? Is your shower going hot and cold? Are you struggling to fill a hot bath?

If any of the above are familiar to you, your combi boiler may be at fault.

A combi, or combination boiler, is a single wall-hung gas appliance that provides your heating system with hot water for the radiators. It also supplies instant hot water to your taps without the need for large storage tanks in the loft or a hot water cylinder in the airing cupboard.

For more detail about the type of boiler you have, please click here.

Combi boilers are by far the most popular and complex boilers in London and require experienced; Gas Safe registered heating engineers to maintain and repair them properly.

In this article, we’ll give you a brief overview of how a combi boiler works, the pros and cons of a combi boiler, and answer why your hot water temperature is fluctuating and what you can do about it.

Why Are Combi Boilers So Popular?

Combi boilers are the most popular boilers in London, and for good reason.

They are the most efficient gas boiler, mainly because you do not need to heat a big tank of stored water. But, what sets the combi boiler apart from its rivals in London is the space savings.

London is one of the most compact cities globally, and space is a luxury that most of us don’t have. And what little we do have, we do not want to fill with storage tanks and water cisterns.

Enter the combi boiler. With its ability to heat hot water instantaneously and the distinct lack of water storage needed, the combi boiler could fit into the tiniest of London bedsits and provide the inhabitants with ample heating and hot water without taking up all the space in the airing cupboard.

How Does A Combi Boiler Work?

A combi boiler incorporates all the external components of a heating system, diverter valves, pumps, and expansion vessels inside one unit. It also takes over the hot water production. To do this, it needs to incorporate a secondary heat exchanger, also known as a plate to plate heat exchanger.

The hot water comes directly from the cold water mains supply and passes through the secondary heat exchanger to be heated for your hot water supply.

Note: It doesn’t matter what combi boiler manufacturer you have; the hot water performance will be the same.

It’s physics. A set amount of natural gas only holds a set amount of stored energy, so a 24kw gas boiler will heat water flowing at about 10l/min by 35k.

It doesn’t matter if you have a Baxi or a Vaillant. If the KW output of the boiler is the same, the flow rate of the water is the same, and the incoming mains temperature is the same, the temperature at the hot tap will be the same.

To heat the cold mains water by a sufficient amount to turn it into a perfect temperature for a shower, the secondary heat exchanger needs to be very efficient and transfer the heat it’s given into the mains water extraordinarily quickly. It does this by having lots of thin metal ‘plates’ tightly packed together. When a hot water tap is opened, the boiler will identify the demand for hot water, either by a flow sensor or by recognising the sudden drop in temperature as the cold passes through, and the main burner will ignite. The diverter valve will switch to hot water mode. The pump will circulate hot water from the primary heat exchanger through the diverter valve, around the secondary heat exchanger, and back to the primary heat exchanger. The cold water will pass through the secondary heat exchanger, separated from the water passing through the primary heat exchanger by the plates, allowing the heat to transfer almost instantaneously.

Pros And Cons Of A Combi Boiler.

Why Does My Shower Go Hot And Cold?

We’ve already gone over most of the pros of a combi boiler. They’re very efficient owing to the lack of stored hot water, they save a lot of space because there’s no stored water. They can give instant hot water. Because everything is inside the case, they’re generally quicker and cheaper to install. The manufacturer’s guarantee covers everything inside the case.

Cons. They are more complicated than traditional boilers and will need an experienced engineer to repair them when they break down. In addition, they need proper and regular servicing. Low mains water pressure will hinder the performance or make a combi unsuitable entirely. You might need to upgrade your gas supply as the instantaneous hot water uses more gas for short periods. When they break down, there is no backup hot water. Finallynt, the waterways need to be tiny and prone to limescale and magnetite blockages to make them so efficient. Especially the secondary heat exchanger.

Common Faults On Combi Boilers

The majority of faults on combi boilers are caused by poor water quality. For example, suppose you live in a hard water area and do not have an electronic scale reducer installed. As the cold water is heated in the secondary heat exchanger, limescale forms and attaches itself to the tiny plates.

The scale deposits harden and restrict the flow of cold water through the secondary heat exchanger. As we learned early, water flowing at a set rate will increase in temperature by a set amount, reduce the flow rate, increase the temperature. Due to the blockage and, in turn, the reduced flow rate, the cold water spends more time in the secondary heat exchanger, and so more heat is transferred to it. The boiler recognises this temperature rise and shuts off the burner. As the cold water continues to flow through and cool the secondary heat exchanger, the boiler sends the signal for the burner to ignite again, and the process is repeated. This will manifest itself as your shower going hot and cold.

Likewise, this process can happen on the heating side of the boiler. As the water circulates from the primary to the second heat exchanger, any magnetite deposits from the heating system in the primary heat exchanger will spread through the secondary heat exchanger and cause precisely the same symptoms.

The water from the primary heat exchanger won’t circulate to the secondary heat exchanger causing the boiler to reach its temperature and cycle on and off repeatedly, causing the water from your shower to go hot and cold.

Dirty water in the heating system is also the main reason for diverter valves and pumps to leak or seize and for switches and sensors to block up.

What Can You Do To Fix It?

Why Does My Shower Go Hot And Cold?

The cause for the shower going hot and cold will be due to a blockage and is usually repairable without needing new parts. However, be warned; in extreme cases where the system is in such an awful condition that other parts in the boiler are beginning to fail, it might be recommended to consider replacing the boiler rather than repairing it.

The secondary heat exchanger can usually be cleaned, but if the system is filthy, it will block up again pretty quickly, so you’ll probably need a powerflush too.

Because it’s a combi boiler, you’ll need a Gas Safe registered heating engineer to repair it.

There are a few things you can do to prevent the fault from happening again. Firstly you’ll need to get the system thoroughly cleaned. This will be an excellent time to install a magnetic filter and limescale reducer if you haven’t already.

While the powerflush and installation of the magnetic filter and scale reducer are things you can do because they don’t involve gas, we would still advise you to seek the help of a Gas Safe registered professional. Cleaning and altering the design of your heating system can be tricky, and you can make things worse if you don’t know what you’re doing.

You’ll also need to get it properly serviced every year. A thorough service should include testing the central heating water for the correct corrosion and inhibitor levels and adjusting them if necessary.

Lastly, you could try turning the temperature down a bit. I know it’s very counterproductive, but even a few degrees will make a difference, and you shouldn’t really notice it.

Energy savings aside, scale begins to form around 40 degrees and increases substantially as the temperature increases. And corrosion of the heating system takes place faster when the water is hot. In fact, the corrosion rate doubles with every 10-degree temperature increase.

So you can see how running your system at slightly lower temperatures will help you save money and help to prolong the life of your boiler.

We cover emergency boiler breakdowns across the whole of Surrey, Kent and south London. We can help you with your boiler repairs, heating services, central heating system repairs, combi boiler repairs, and other boiler breakdown and heating services problems. In addition, our heating and plumbing services can cover most servicing of gas appliances for homeowners and landlords, including issuing landlord safety certificates.

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