Gas water heater

[Grumps]

Hi All 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.

[spuorgelgoog]

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.

Owain

[tabbypurr]

only way to fix that is a bigger HW cylinder. Or just live with wartime bat hs.

but wouldn't solve the issue of low mains water flow. I'd look at aeration or pulsejet shower heads first, they'd entail far less cost & work than any thing else.

depends what you have now, but generally instant HW does mean higher power

Vaillant & Worcester Bosch are the top 2, the rest a fair way behind on rel iability

is the warm air system problematic?

both work fine

If you did the combi would use the wall as a sounding board.

NT

[Grumps]

The mains pressure seems fine in the kitchen and upstairs bathroom sink. The bath runs from stored hot+cold and hasn't sufficient head for a decent shower (it's a shower mixer tap thing).

I'll check them out. Ta.

No. It's fine. Quick to warm up.

OK. Outside wall it will be if I go that route. Ta.

[John Rumm]

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 good.

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.

[johnjessop46]

Strip and check your ball float valve hasnt got a piece of crud in the internal "jet" before launching into capital expenditure.

[spuorgelgoog]

"The boiler's got a 15mm inlet so a 15mm pipe will be okay, regardless of kW or length"

Owain

[harry]

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 inside. 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.

[Tim+]

It's not clear whether you're quoting a "less clued up fitter" or contradicting John.

Tim

[Andrew Gabriel]

But just to be clear, it's not true.

My Keston has a 15mm gas inlet fitting, but needs 28mm most of the way from the gas meter due to being at other end of the house.

[Andrew Gabriel]

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).

[newshound]

+1

[John Rumm]

Yup, that would be one!

(I have also met the "must be 22mm!" - for a 15kW boiler 2m from the meter)

[John Rumm]

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.

Indeed.

Its quite likely only a 15mm pipe as well...

Pressure at the shower will still be crap.

[DerbyBorn]

[e-mail] wrote in news: [e-mail]:

+1 - it may even have the wrong size nozzle for the pressure rating.

[Grumps]

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.

[John Rumm]

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.

[harry]

The water will still only come out as fast as it goes in.

[Grumps]

Thanks. 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.

[John Rumm]

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.