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Do you have a combi boiler?

(38 Posts)
Battlefront Wed 12-Dec-12 16:30:44

If so what happens when you turn on another tap/flush a toilet if someone's in the shower?

In our house you MUST NOT turn on any other tap while someone's showering - the shower instantly runs freezing cold.

Is this normal or was mine fitted by cowboys? What happens if you have more than one shower - the point being that more than one person can shower at a time?

nextphase Wed 12-Dec-12 18:23:57

thats what happened at my old house.
Tho people seem to be fitting them in larger and larger houses, so maybe things have moved forward.

M&D have 2 showers, one runs from the hot water tank, and one from the cold mains (electric shower), turn on a tap, and one shower goes hot, the other cold!

nocake Wed 12-Dec-12 18:33:23

nextphase, the reason is that the water pressure reduces to the electric shower so the water that's coming out gets hotter (less water for the element to heat) while the other one goes cold because the tap is taking hot water from the tank which would have gone to the shower.

I don't know why your shower goes cold, Battlefront. Ours stays the same temp but the flow reduces.

PigletJohn Thu 13-Dec-12 21:27:23

if the shower goes cold when a WC is flushed, it will be because the WC cistern is taking flow from the cold supply. The remaining flow available to pass through the boiler is insuficient to operate the flow switch in the boiler which detects a demand for hot water and sets the diverter.

Fill a bucket from a hot tap, with a cold tap running somewhere else, at the highest flow that doesn't turn the boiler HW on, time it, and see how many litres per minute it is. It might be that your flow switch needs replacing, or it might be that you have unusually poor water flow. If necessary the cistern valve could be tinkered with to reduce the amount of flow it takes.

It might also be that your shower head is an economiser one, or clogged up with limescale, reducing its flow. Run it into a bucket and calculate litres per minute.

If you can find the make and model of your boiler we can probably look up its flow switch setting.

PigletJohn Thu 13-Dec-12 21:30:41


you might be able to guess why I am not keen on combis in houses that have more than one bathroom and more than one person wanting to run taps at the same time.

Battlefront Fri 14-Dec-12 17:50:56

Thanks Piglet, I wish someone had explained this would happen before we had the boiler fitted!

wendybird77 Fri 14-Dec-12 20:00:08

Sorry for the hijack, but Piglet - what do you suggest then? We are about to have our old conventional boiler replaced and moved to another room. We will eventually have two bathrooms and I foresee more than one person wanting to run taps at the same time! (3 beds, old house, replacing radiators as well)

AnnieLobeseder Fri 14-Dec-12 20:02:28

We have a combi and you can do anything with toilets/sinks and the shower isn't affected. Only one shower so can't tell you anything about two showers running.

I can tell you that a constant hot water supply was a godsend to fill my birth pool!!

PigletJohn Fri 14-Dec-12 20:55:46


fill a bucket at the kitchen cold tap, time it, calculate how many litres per minute you get

Annie, it sounds like you get a good flow.

wendybird77 Mon 17-Dec-12 20:28:24

Sorry Piglet, I've just gotten around to doing this. We get 12 L / min from the cold kitchen tap. However, the stop cock seems to be in the bathroom. Shall I re-time it from there?


PigletJohn Mon 17-Dec-12 20:35:40

12 litres p m is usually considered the minimum you need for a combi. It is not very much so you will get flow reduction when somebody turns on another tap, or flushes a WC, or even the washer starts taking in water. It will be OK for a shower but you will find it rather slow to fill a bath.

try the current bath taps and see if the flow is satisfactory.

wendybird77 Mon 17-Dec-12 20:51:09

The flow on the bath is ok actually - much better than our last house. Or did you mean calculate L / min? When someone is in the shower (currently just a hand held from the bath tap) it does get cold if someone turns on a tap / flushes, etc. Obviously we don't want this if we're going to the expense and hassle of replacing the boiler. Also don't want to waste money on a super fancy system if we don't need it.

PigletJohn Mon 17-Dec-12 21:00:36

litres per minute.

you might find that the existing loft tank and hot cylinder can fill a bath faster than your potential new combi will do. Mine does. You can also run the hot tap and the cold tap at the same time without affecting each other.

wendybird77 Mon 17-Dec-12 21:13:55

Ok, 25.8 L / min with the cold bath tap on full.

Sadly we have to replace the boiler as we've been told no plumber will move it (I need it moved from kitchen wall for kitchen refurb) as it is so old and they won't want liability. We're in the process of replacing the old radiators as well. Also, I need the hot water tank out of my bedroom or else there isn't enough room for wardrobes for our clothes. I am creating a large cupboard in the utility room where I'd ideally like all utility type things to live.

wendybird77 Mon 17-Dec-12 21:20:09

As to the loft tank, etc. Yes, I am questioning whether a combi is the best option. Happy to get another conventional boiler if that will work best.

PigletJohn Mon 17-Dec-12 21:23:03

I got a conventional boiler. It is much less complicated so I reckon less likely to go wrong.

ElectricSheep Mon 17-Dec-12 21:35:51

YOu can install a thermostatic valve for showers that will prevent burning if a tap is switched on elsewhere in the house.

I prefer a hot water tank personally (though have a combi at the mo). With hot water tank you've got options eg solar thermal panel/woodburner that you can link to your system.

wendybird77 Mon 17-Dec-12 21:38:02

Yes, I remember that from another thread and I looked up the one you had. I was wondering if something like this would work or if a megaflo system might be appropriate.

My concern about the conventional boiler is how many people can shower without the hot water running out. We will eventually have 2 showers, a bath and two teen boys. wink

PigletJohn Mon 17-Dec-12 21:46:04

that Vitodens is a very fine boiler, but like a Megaflo, or indeed a combi, it can't deliver water to the taps any faster than it comes in from the mains.

If you only get 12 lpm at the kitchen tap, you may not get any more from the boiler.

If your cold bath tap measurement of 25.8lpm was from the main, and not from the cold water tank in the loft, then it would be enough, but you may need to have new larger pipes and stopcock fitted all the way from the incoming main to prevent it being constricted.

do you live in a modern property with a blue plastic incoming supply? what is the external diameter of the incoming pipe? It should be visible coming into the stopcock.

you can get megalflos to hold 250 litres, which is a pretty long shower, and a big boiler can heat it about as fast as a shower (but not a bath) can use it.

wendybird77 Mon 17-Dec-12 22:05:00

Ok, here is where I get publicly stupid. In my defense, we've just moved in.

It is not a modern property (victorian cottage) and there are no plastic pipes as far as I can tell. There is no stopcock under the kitchen sink - which leads me to believe it is in the downstairs bathroom which is a 70's extension (with blue basin and bath!). The largest pipe in there, which has a spigot on it and is coming in from under the cement floor is about 22mm.

There is also another pipe coming up from the floor which is about 24mm which leads to the bath. The other pipe leading to the bath is smaller, about 15mm and is fed from the pipe with the stopcock on it.

Forgive me if none of this makes any sense. blush

PigletJohn Mon 17-Dec-12 22:11:50

does the cold bath tap come out at mains pressure, or does it seem to be supplied from the loft tank?

does the hot bath tap deliver the same flow?

wendybird77 Mon 17-Dec-12 22:36:24

Thanks Piglet -

I don't know is the answer. There are no pipes coming from above - both come in from under the floor, the extension is a small single story extension that is obn the opposite side of the house from where the tank is located in the loft.

The hot water does deliver the same flow.

PigletJohn Mon 17-Dec-12 23:30:05

put your thumb over the spout of the tap (easier on the basin)

if you can hold back the flow, it's from a tank, If you can't, it's mains pressure.

compare it to the kitchen sink tap, which is almost certain to be direct from the main.

wendybird77 Tue 18-Dec-12 00:02:18

Ah, I can stop the basin tap with my thumb, so tank fed then. Kitchen I cannot, so mains fed.

wendybird77 Tue 18-Dec-12 00:03:57

Also, sorry OP for total thread hijack. I hope this is useful to others too!

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