I've just moved into a house that has hot water supplied by an electric
immersion heater. The small cold water tank is the top part of the hot
water cylinder. When running a bath the cold tank seems to empty far
quicker than it can refill from the mains. Consequently the hot water
starts to splurge out; still hot though.
Taking a shower which is fed from these tanks has woefully low pressure
and a piddly experience. I could put a pump on this but a shower would
then be short but sweet! And I could replace the cylinder with a loft
cold tank and standard hot water cylinder. There is also an electric
shower (10kW), but this too is rather poor.
So, I was considering a gas powered water heater to provide "continuous"
hot water at mains pressure. Several models seem to be available at the
£450 price point that have flow rates from 10 to 16 l/min. The central
heating is by warm air. Don't know how efficient these systems are.
My questions are (sorry, there are lots):
Do you need special gas pipework (wider) for these gas water heaters? I
couldn't find any reference to this in the installation instructions (or
didn't know what to look for). I will not be fitting it myself.
Are the following brands any good: Ariston, Rinnai, Main, Vokera. Any
others that may be a good choice?
Or can you use a normal combi boiler and simply not use the heating
part, or maybe just have a towel rail if it needs a minimum load.
Or maybe time to install a full radiator system. What's the current
opinion on rad location; under window or other wall?
Can you put a heater/combi on a stud wall? I think I read that they
might be noisy in this situation.
Thanks for reading and any comments.
On Tuesday, 3 May 2016 22:50:12 UTC+1, Grumps wrote:
The cold mains should be sufficient to fill faster than the hot draw-off - check the stopcock or ballcock. Or just only open the bathtap halfway.
Combi boilers or gas instantaneous water heaters are adequate for showers but not usually for filling a bath very quickly. Gas pipe size depends on kW heat input to the appliance, length of pipe and number of bends. 22mm is usual but you might need 28mm.
Fairly straightforward to use a combi boiler with the instant hot water for the shower, and the heating side S-plus plan to a vented indirect cylinder for a bath, and radiators if required.
Those combined cylinders and cisterns are rarely satisfactory.
Poor for different reasons though... It would be a worthwhile exercise
to measure the cold mains flow at the best tap (kitchen perhaps) to work
out what kind of flow rate you can get. You need at least 15 lpm before
being able to get decent performance out of any system that depends on
mains flow rate (i.e. combi, multipoint, heatbank, unvented cylinder etc)
The flow rate comes down to power - 24kW will give you the lower figure,
and you will need 35kW+ for the latter.
They have pros and cons, but if installed and maintained properly can be
Instant water heaters need power to work (if you want something more
impressive than an electric shower!) So that typically means at least
some of the pipe run will need to be in at least 22mm copper pipe
(unless the boiler is very close to the meter). Higher powers, and
longer pipe runs may even dictate wider.
Gas pipe sizing needs to be worked out for each installation normally,
although there are various rules of thumb that the less clued up fitters
will use in place of doing the actual sums.
BS 6891 Annex A ("Installation of low pressure gas pipework of up to 35
mm (R1¼) in domestic premises (2nd family gas)" will give you the basics
(you may be able to view a copy online using a library card number)
In summary though it comes down to find out the flow rate required (in
cubic metres per hour), work out the effective pipe run in meters (where
an elbow counts as 0.5m, and a swept bend 0.3m), Then look up in table
the maximum length of pipe of your chosen size that can discharge at
least the required amount of gas.
To give you an idea, a 35kW boiler will suck just over 3m^3/h
(multiply the power by around 0.09 to convert to m^3/h):
At that rate, the longest run of 22mm pipe you could cope with before
dropping too much pressure would be around 16 odd metres.
If looking at multipoints specifically, there is less choice that with
combis and other boilers.
You would need a minimal primary circuit typically (the combi normally
diverts this through one side of a plate heat exchanger when heating the
water (although there are a few exceptions that use the main HE for DHW
heating). You could also use a combi to heat a normal hot water cylinder
- using the mains pressure feed for a shower, and the tank for the rest
of the plumbing.
Either... or under floor heating.
You can - assuming the fluing options allow for sufficient run to an
outside wall. The wall needs to be non combustible as well (which
plasterboard normally is)
Slightly more perhaps - but with a decent quiet boiler, not dramatically
so. A solid wall is easier though - especially for fixing.
They have a common fault in hard water areas. The pipe from the
top cistern to the inlet at bottom of the hot water tank often runs
through the hot water tank. That heats the cold water as it flows
down that pipe, and over the years, scale builds up in that pipe
and restricts it. If your cistern is still full when hot water
stops flowing, it's a good pointer to this having happened (or
something else restricting flow in that pipe - remains of a drowned
mouse or similar).
[email address is not usable -- followup in the newsgroup]
On Tuesday, 3 May 2016 22:50:12 UTC+1, Grumps wrote:
Think about it.
The limitation you have is due to water not filling the tank fast enough.
If you get a gas instantaneous heater it with be fed from the same source.
Ergo it will be no faster though the pressure may be higher (for showering)
So, you need to look into the cause of the slow mains water.
Look for closed valves.
Look for those silly ball valves that only have a very small water passage
If the mains pressure is low, you might be able to change the float valve (
jet)in the header tank to one of lower pressure (ie a bigger hole)
The istallation you describe dates from the 1960s
You may find your incoming mains pipe is filled with crud.This means the pr
essure will still be there but not the flow.
If this is the case, the only solution is to replace it.
Things will gradually get worse.
So the first thing is to determine if you have low mains pressure or the in
coming pipe is partially blocked/valves obstructing flow.
There's no point spending any money until you have found out which and deal
t with the matter.
When you have fixed this problem, you'll likely find the existing installat
ion works perfectly well.
One can't say that with any certainty. It could just be a poor or
inadequate ball valve that's limiting flow rate - the combi (or
whatever) is not going to be fed from the same ball valve.
Also running a bath (though low flow resistance taps and pipework), may
still permit a flow rate higher than that which the mains can supply -
that does not automatically mean that a lower mains driven flow rate
would necessarily be too low.
(i.e. say the main can deliver 20 lpm, and the current setup can fill
the bath at 30 lpm. Swapping to a combi which can deliver 15 lpm will
result in slower bath fills, but the combi will still have a more than
adequate flow of water from the mains)
or one fitted with a "high pressure" flow restriction.
Just a little extra info.
I was talking with my neighbour and told him about the splurging hot
water. He said his was the same. Was always like that from new!
I measured the flow rate from the kitchen cold tap (mains). I get a
better shower from that tap spraying all over the place than I do from
the one in the bathroom! It measured about 31 l/min. The bathroom cold
tap (also from mains) measured about 22 l/min.
So I'm still inclined to replace the hot/cold tank with an instantaneous
gas water heater.
31 lpm is a good basis for any mains pressure system.
It will work, but you may find the flow rate for bath filling will be a
bit disappointing. (i.e. probably 15 lpm at final temperature - no scope
for adding extra cold at the same time). But, its simple and relatively
cheap. A simple gas system boiler and unvented cylinder would give much
better results - but take more space and cost a bit more.
You say some profound stuff sometimes.
If it goes in at 31 lpm, then it can come out of an unvented cylinder at
that - and at a higher than final use temperature as well. With a combi,
it won't be able to heat it fast enough to gain the full advantage of
having a supply capable of 31 lpm.
For comparison, I have another house which has a loft water tank and
vented cylinder. In the bathroom the bath single-lever mixer tap can
deliver the following flow rates (all pipework seems to be 22mm):
Cold(from loft):11 l/min, Hot:12 l/min, Mixed:15 l/min
Filling a bath does take a bit of time, but it's not too bad.
So, (yeah I know I'm banging on about gas water heaters) an
instantaneous water heater could provide a flow of 12 l/min (delta T
35C). I was also planning on using a thermostatic mixer tap with shower
attachment. The hot would come from the heater, the cold would be mains.
With a winter min cold temp of 5C (apparently) then theoretically the
heater should be able to provide a 40C shower at 12 l/min (larger values
would be possible at a higher heater cost).
Haven't looked at unvented cylinders yet.
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