Bathtub capacity, combi sizing, Ecobollocks

If you have one of those pointless "shower baths" then a combi is the last thing you want. The baths bulbous shape means that even a standard hot water cylinder (36" x 18") may not be sufficient.

David submitted this idea :

I'm an impatient sod, I have an open vented cylinder and (assuming hot water is ready) I can be in the bath in a few minutes. From cold, my boiler heats up a full cylinder in around 20 minutes. None of this waiting for the HW to begin to come through, or only being able to run HW or CW. I have a 9.5Kw electric shower and having tried it before, it can take forever to fill the bath and be barely warm enough by the time it is filled.

You choose. Your advantage is you know what you're getting. Personally I wouldn't put much depth of water in a bath, I grew up with annual well shortages, which made for shallow shared baths of brown water, presumably iron stained. How times change!

NT

We've got rid of the bath and only have a shower now, but when we had the bath it was used for bathing the children and for deep, hot, long relaxing baths for the adults. For the adults (and later for the children were old enough to use the shower), the bath was never used for getting clean, the shower was for that.

Under those circumstances, I'd say you want a fast bath fill for regular use, but if it is only going to be used ocassionally for relaxation, a slower fill is acceptable.

SteveW

This is correct.

When my house was built, the heating engineers calcualetd it needed 10KW of house heating. You cant even run a shower off that at any decent flow rate. But 150 liters of preheated water at 2.1 bar does very nicely!

In germany my sisters old house, insulalted to the hilt and triple glazed had a MASSIVE - I mean the size of the average toilet - sized combi to provide hot water. Id guess around 50KW+.

It is the omly combi that I've ever seen that could provide hotw water to teoo places simulatenously. It might have had a heatbank - I dont know

You can stil get peat stained water in some locations.

I've never worked out how one is supposed to get clean, that's clean off *real* dirt in a shower. How on earth do you scrub your feet or knees clean in a shower, or even for that matter scrub your hands clean. A shower is fine for rinsing oneself quickly but for actual removal of ground in dirt it's not very good.

The answer perhaps depends on how much heat you need to make all the radiators hot. I suspect you need 25kw or more to have an adequate supply of hot water. The boiler I have is rated at 20-25kw. It does not take 10 minutes to fill a bath. It would be much to hot if I did not add cold water.

Make sure the overflow isn't blocked. It takes so long, you /will/ forget you have a bath filling.

Cheers

Chris Green presented the following explanation :

Nothing to stop you from just sitting on the bottom of the bath or shower base to scrub your feet and knees. Get wet, shower off, sit and scrub, rinse off with shower.

Which is *very* unlikely, so I reckon that the figure of a shower using 62 litres in 8 minutes is wrong.

Without doing the maths, a bath being only the size of a car fuel tank sounds wrong. If a bath is very roughly:

width 0.8 m length 1.5m (nowhere near long enough for me, at 1.8m to lie full length) depth 0.3 m (very approximately - that is the most variable dimension because it depends on preference)

and if you assume as a rough approximation that a bath was straight sides

then volume is 0.8 x 1.5 x 0.3 m^3

so a bath is 360 litres.

That sounds far too large - it's about 4x the figure that the document quotes.

I imagine a realistic figure, taking into account the tapered sides of a bath, is going to be somewhere in between.

Next time I have a shower, I'll time how long it takes me and measure how much water flows (into a bucket) in that time.

Isn't your logic the wrong way round? If you have a very large bath, then a standard hot water cylinder will not be enough, so you *will* need a combi (heat-on-demand) rather than a stored-water heating system.

The oil-fired combi boiler that we had at our last house could run hot water that was too hot to bear your hand under, at full flow from mains cold water pressure (ie not from a header tank), and it could keep that up indefinitely.

Yes, it took a little while for the water to run warm, but not much more than it takes for hot water from a cylinder to run warm, having displaced all the cold water in the pipe from the cylinder to the tap. Most combi boilers have a small internal tank of water which is pre-heated and kept warm, to provide initial hot water until the heat exchanger has got up to full temperature and is producing full-temperature water.

The boiler was sited outside and was about (WxDxH) 1.5 x 1 x 1 metres. I'm sure a lot of the size was due to the extra insulation needed for withstanding freezing temperatures outdoors; a wall-mounted indoor one may well have bee a lot smaller.

I've no idea what power rating it was.

I'd always thought that combi boilers had a restricted flow rate - ie that if you turned the tap on too far, the water would be cooler. But that certainly wasn't the case with our oil-fired boiler. Even with the tap full on, fed from mains pressure, the water was too hot to bear your hand under. It took as long to run a bath of hot water as if you'd filled it from the cold tap - no longer.

You should generally size for the hot water demand... Note that ive you go for particularly powerful combi boilers (>= 35kW) they will often only output their full power when heating DHW, and not CH.

Note that those figures are usually quoted at a 35 degree temp rise, so in the winter with the incoming main at 5 deg that's going to be a pretty tepid bath by the time its delivered.

Go for a minimum combi size of 35kW if bath filling is something you will do often.

Modern boilers will load match and modulate when running the CH, so you don't have the same problems as one did in the past with an oversized boiler.

In a past property I fitted a 35kW combi, and it was just about tolerable for a bath fill (you can always get in before its full!), but it was very good for one shower, and ok for two showers at once. (IIRC it only delivered up to 24kW when in CH mode)

I would argue its the only point worth considering with a combi, unless its a storage type with an integral unvented cylinder.

If you have a large bath then you need a decent sized cylinder (although keep in mind the cylinder can store water at much higher than final use temperature, and if the controls are well setup the boiler can fire to start reheating long before its actually depleted).

Even a 105L cylinder or water at 60 deg, will allow delivery to a 150L bath at final mix temperature; a more respectable 220L cylinder significantly more.

A basic shower will be say 7 to 8 lpm, so that sounds about right.

(although I can spend more than 8 mins in a shower quite easily)

A more luxurious shower could easily be 10 to 15 lpm - especially if it has a "soaker head" or body jets.

Remember that a fair bit of that volume will be taken up by the bather. Note also that the height the overflow is positioned at will "steal" a fair bit of theoretical capacity.

You forgot Archimedes surely. When you get in the water level rises so you don't actually need 360 litres.

It also depends on how hot the water is in the tank. For a bath you may use 50% hot water and 50% cold water from the mains to get the required temperature.