Boost Switch Dilemma.

Drakon

Frequent Poster
Messages
435
My house has the usual hot water tank found in Irish houses, with two heating elements, “Sink” and “Bath”. It’s got the bog standard immersion switch, two toggles, one On/Off, the other Sink/Bath:
4025


I intend to add a boost switch to this set up, or ideally, replace the old switch with a new boost configuration. The boost switch I have in mind is one of these:
4028

One button that’ll give you four times (15/30/60/120 minutes), and “Off”.

However, I’m wondering if there is a dual version of this?

As the hot water tank has two elements I’d rather one dual boost switch rather than two single boost switches. The only alternative I can think of is to use the original switch with one single boost switch. Which would be a bit clunky.

As I write this it’s coming clearer in my head that maybe the best solution in the absence of a dual boost switch is to use two single boost switches side-by-side, labelled, “Sink” and “Bath” as appropriate.

I’m wondering if any other members have considered this switch? Or any other thoughts?
 

Attachments

Leo

Moderator
Messages
10,676
As the hot water tank has two elements I’d rather one dual boost switch rather than two single boost switches. The only alternative I can think of is to use the original switch with one single boost switch. Which would be a bit clunky.
Be careful here, both elements of the heater must never be live at the same time!

Given that it is a manual boost button you're looking at here, wire the contacts in parallel with those of the existing on/off switch. Then, when you want to boost, just make sure the sink/bath switch is in the position for whichever you want to heat. The boost will bypass the on/off switch then, so you can leave it in the off position.

If you are in any doubt about this, hire an electrician. These pull a heavy load, so poor connections can be a fire risk.
 

elcato

Moderator
Messages
3,307
The boost switch is purely a timer to allow current flow so if you put it into the circuit prior to the switch it will be straightforward. However then you will always have to have the boost on to heat water and you will need to switch both on every time. Why not replace the current switch with a timer type which always includes a boost/bypass switch for emergency heating as required without having to fiddle with the timer.
 

Leo

Moderator
Messages
10,676
so if you put it into the circuit prior to the switch it will be straightforward.
Yeah, if they wire it in series prior to the existing switch, then they can leave the existing on/off permanently on. Then they'd just hit the boost button to call for power. But wiring in parallel gives the option to do that, or still use the on/off switch to leave the power on for longer than the timer allows.

I haven't looked in a while, but last time I did I couldn't find a two channel timer that would allow you select between bath or sink elements.
 

Buddyboy

Frequent Poster
Messages
576
Correct me if I'm wrong (not an electrician), but the idea of the two elements in a sink/bath setup is that the sink element heats a smaller quantity of water, being a smaller element, that does not protrude into the cylinder as much as the bath (bigger) element. The bath element is longer and heats a larger quantity of water for a bath. There is a built in thermostat that prevents the element overheating/shuts it off when it's reached the desired temperature.

So, in theory, if you turn on the bath element for 15 minutes, you only heat enough water to fill the sink. The element itself is just as efficient as a sink element, i.e. either one of them turned on for the (small) amount of time will heat the same volume of water, up to a point.

So, why not ignore the sink element, wire the bath element to the timer and use it for 15 minutes if you need a small amount of water, or 2 hours if you want to run the bath?

I could be wrong, and the smaller element heats up faster and is therefore more efficient for smaller quantities. Leo, do you know (you seem to know a lot more about it than me)?
 

elcato

Moderator
Messages
3,307
So, in theory, if you turn on the bath element for 15 minutes, you only heat enough water to fill the sink. The element itself is just as efficient as a sink element, i.e. either one of them turned on for the (small) amount of time will heat the same volume of water, up to a point.
No. There are two elements. One at the top of the cylinder and one at the bottom. Hot water rises so the top will have all the hot water first so disabling the top element earlier.
When the Bath element (bottom) is on the water always rises so until the bottom reaches the desired temp the element stays on.
 

Buddyboy

Frequent Poster
Messages
576
So, my question still stands, why not just use the bottom element combined with the new timer? Set the timer for 15 minutes for a sink, and 2 hours for a bath?

(Unless I'm missing something).
 

Leo

Moderator
Messages
10,676
I could be wrong, and the smaller element heats up faster and is therefore more efficient for smaller quantities. Leo, do you know (you seem to know a lot more about it than me)?
The majority of older hot water cylinders contained a single entry point for the electric heater. Most immersion heaters installed in these had two separate elements, one bath, and one sink, both rated in the region of 2.5-3kW. The sink element was short, and so only reached into the top portion of the tank. So this element only heated the top portion of the tank, hot water rises, and water isn't a super efficient conductor, so the heat doesn't transfer down too quickly.

When using the bath element though, when you want a lot of hot water, this heats the tank from towards the bottom. As the water around the element starts to heat up, it rises up, and cooler water falls down to replace it. So the entire tank is much more gradually heated over time. This is fine if you want to heat a large quantity, but if you only want enough for washing dishes or to have hot water at wash hand basins, then it is very inefficient as you need to heat the entire tank to the desired temperature rather than just enough for the job at the top of the tank.
 

elcato

Moderator
Messages
3,307
So, my question still stands, why not just use the bottom element combined with the new timer? Set the timer for 15 minutes for a sink, and 2 hours for a bath?
No it will take longer to heat the water at the bottom because as it rises the cold water remains at the bottom. The bottom element will always be trying to heat cold water. If the top element is used it will heat up the same water when it's cold tepid warm and hot.
Actually my post is crossing with Leo's but his second paragraph explains it. (My tank has two elements at the side as oppose to one elemnt at the top).
 

SparkRite

Frequent Poster
Messages
1,254
@Leo 's suggestion of wiring is the one to go for , as it offers the best of both worlds, ie. timed or constant.

As already stated water is a bad conductor of heat in its literal sense, but is a great absorber of heat. Hence it is used in many applications for its cooling properties.
Heat movement within a body of water is primarily via convection so therefore the heat will rise, setting up a small current while doing so.
Minimum transfer of heat is via conduction.
In regards to the different sized elements, both psychical and power rated, the longer element is obviously deeper into the water and therefore has a greater surface area in contact with water and will heat up a bigger volume of water within a given time.
If both elements were the same rating it would follow that the same amount of heat would be transferred to the water, in the same given time. But because the shorter element is in contact with a smaller volume of water it would show a higher rise in temperature than the larger volume would over the same time.

So roughly:- 15mins for 'sink'.
30+ mins for 'bath'
 
Last edited:

NDynamite

Registered User
Messages
49
Hi there I coincidentally just bought the exact same booster switch and just found the thread. I am going to install it prior to the existing switch as we are happy with no constant. Seems an easy job. I wouldn't attempt parallel. How did it go for you in the end?
 
Top