Water Heaters and Water Pumps: Why a Hot Shower Feels Weak

Plenty of homeowners notice the same thing. The kitchen tap and toilet run fine, but once the water heater is switched on, the shower turns into a weak trickle. Sometimes no hot water comes out at all, or the temperature keeps swinging during a shower. Most of the time the heater is not faulty. The usual cause is the water pressure reaching the heater. This article explains how water heaters and water pumps affect each other in plain words, so you know what to check and what to tell your installer.

Two common types of home water heater

Before talking about pumps, know which heater you have. Both heat water, but each reacts to pressure differently.

Instant water heaters

An instant heater is the small unit mounted on the bathroom wall. It heats water as the water flows through it. Most of these units have a flow switch or pressure switch inside. The heating element only turns on when the flow is strong enough to trip that switch. If the flow is too weak, the heater stays off and you shower in cold water even though its power light is on.

Storage water heaters

A storage heater has a tank that holds hot water. The water is heated ahead of time and pushed out when you open the tap. The problem looks a little different here: hot water does come out, but weakly, because it is pushed out by the same household supply pressure. If the supply is weak, the hot water is weak too.

Why a hot shower can feel weak

In many terrace and double-storey homes, bathroom water comes from a tank on the roof. It flows down by gravity, and the pressure depends on the height difference between the tank and the shower head. The smaller that difference, the lower the pressure.

Upstairs bathrooms feel it most

The master bathroom is usually upstairs, only a short way below the tank. The vertical drop may be just a metre or two. That is enough for a basin tap, but often not enough for a comfortable shower, and even less for switching on an instant heater.

Several taps open at once

When one person is showering while someone else opens the kitchen tap, or the washing machine is filling, the flow is split. Pressure at the shower drops and an instant heater may cut out halfway through. That is why the water suddenly goes cold for no obvious reason.

Long pipe runs or a clogged shower head

A long pipe route with many bends, or shower head holes blocked with scale, also cuts the flow. Clean the shower head first. If the flow is still weak after that, the next thing to look at is pressure.

How a booster pump changes things

A booster pump is fitted on the outlet pipe from the tank and adds pressure to the water heading to the bathrooms. The pressure controller on the pump switches the motor on when a tap opens and off again when every tap is closed.

With an instant heater

With steadier pressure, the flow is enough to trip the switch inside the heater, so the element comes on more consistently. The temperature also swings less, because the flow no longer dips each time another tap is opened.

With a storage heater

The pump helps hot water come out with more force. However, every storage heater has a pressure limit set by its manufacturer. That is why the installer needs to know your heater type and model before choosing the pump size and pressure setting.

What to tell your installer

Clear information lets the installer recommend the right HP instead of guessing. Have these ready:

• The heater type in each bathroom, instant or storage, plus a photo of the label on the unit.
• How many showers and bathrooms you have, and which floor each one is on.
• How many people usually shower at the same time, for example in the morning before school and work.
• Photos of the roof tank, the outlet pipe from the tank and the spot where a pump could sit.
• Whether there is an old pump already, and what problem it has now.

A home with one upstairs bathroom usually needs a smaller pump than a home with three bathrooms in use at the same time. The final size is chosen after we see your photos.

A word on electrics

Water heaters and water pumps both run on electricity and sit close to water. Do not open the heater cover or touch the pump wiring yourself. If you smell burning or the breaker keeps tripping, switch off at the main switch and leave the wiring check to the installer.

Prices and next steps

Supply and installation prices for Tsunami and Grundfos pumps by HP are listed in the price section. Every price covers pump supply, installation, basic wiring and pressure and function testing. Extra work such as pipe modification is confirmed on WhatsApp before it is done. The Kuala Lumpur, Cameron Highlands and Kulai pages describe local housing, and the schedule for your own town is confirmed on WhatsApp.

Send photos of your heater, tank and pipes and we will suggest the HP and its price. WhatsApp us whenever you are ready.