Minimum flow rate required for unvented domestic hot water system

I think that it's marginal. The static pressure is reasonable but not exciting. Flow rate isn't brilliant but not at a level where the supplier is likely to do anything for nothing.

You could put flow restrictors on the feeds to the bath to protect the flow to the shower, and if you don't mind reasonable pressure but not high shower flow, you could do the shame with the showers and perhaps cut them to 10-12lpm.

At these levels I think I'd try to do something about it.

Don't forget that there are tanks available in different shapes and sizes from Polytank and that one can always link more than one together if it helps with the nature of the space available.

About the only other option is to look into having a larger diameter service pipe installed from the road main into the house. THis can be disruptive and not inexpensive of course.

Certainly a high flow type should be used..

Yes it can.

.andy

To email, substitute .nospam with .gl

Hi, Andy.

Unfortunately, I have already replaced the service pipe with a new 32mm diameter pipe. Cambridge Water tell me that they can't do anything about the flow/pressure "because the supply is fed directly from a reservoir".

Space in my loft is very limited, but I do have basement space so I could get an accumulator.

GAH make a "Dualstream" accumulator but you have to buy their rebadged Ariston unvented hot water cylinder with it. I don't know anything about the quality of Ariston.

For a 300 litre accumulator and 300 litre cylinder the list price is £2112 (less 40% from a distributor I'm told!). The 300 litre accumulator is 5'2" high and almost 2' diameter so it takes up a lot of room.

I suppose that I'll have to compare the Dualstream cost with a "cistern and pump" solution and take a view on it.

(Sorry - Posted this reply using my wife's newsgroup reader by mistake...)

Hi, Andy.

Unfortunately, I have already replaced the service pipe with a new 32mm diameter pipe. Cambridge Water tell me that they can't do anything about the flow/pressure "because the supply is fed directly from a reservoir".

Space in my loft is very limited, but I do have basement space so I could get an accumulator.

GAH make a "Dualstream" accumulator but you have to buy their rebadged Ariston unvented hot water cylinder with it. I don't know anything about the quality of Ariston.

For a 300 litre accumulator and 300 litre cylinder the list price is £2112 (less 40% from a distributor I'm told!). The 300 litre accumulator is 5'2" high and almost 2' diameter so it takes up a lot of room.

I suppose that I'll have to compare the Dualstream cost with a "cistern and pump" solution and take a view on it.

I assume you haven't bought any equipment yet. In your situation which I assume you have 1 bath and two showers, and I would go:

1. Combi boiler (to do the two showers only)
2. A "combination" cylinder with quick recovery coil. (cold tank and hot cylinder all in one space saving unit). This operates at low pressure
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   RCM will make a large cold tank section for you. They will ake the hot and cold section sizes to order.

The CH side of the combi acts as a normal system boiler heating the cylinder and CH. The cylinder does the bath and therest of the house to give good fillups. The water side of the combi only does high pressure showers. Something like an Alpha CB50, W-B Greenstar high flow condensing combi would do. The W-B Greenstar 40kW model can deliver 16 litres/min and you need to add cold to the showers, so a decent shower had if two on at the same time. This approach is cost effective and no power shower pump used.

Another approach is a "combinatiuon" cylinder witha quick recovery coil serving all hot outlets and cold for showers using a conventional power shower pump for then showers and a condesning system boiler. Use a Torbeck ball valve in the cold section and 22mm cold mains directly to it to give repaid fills ups. The Torbeck is either on or off.

Will you really have the bath filling and a shower on at the same time? A relative has had a single shower power shower pump serving two showers for

Given that you want to soften the water I strongly think that a pumped storage cistern approach will be best. I'm really surprised that 2.8 bar static on 32mm (25mm inside) MDPE pipe gives only 25lpm. This implies that the supply in the road is somewhat restrictive. Did the Water Company section in the road get replaced all the way to the main? or just to the meter? or just to the property boundary.

How was the 25 lpm derived?

The are all sorts of tanks for using the eaves and apex of roof spaces if that is an issue.

Is the stop c*ck a full one?

This rather suggests that their main is limited.

The best thing that you can do is to look at all the components after the service pipe. e.g. stop c*ck full bore lever ball valve, 22mm pipe internally to key points or 28mm for anything of any length, avoid too many bends.

This is an expensive hobby.

I think that IMM's idea of some storage above the HW cylinder is interesting, if you have the height. However, be careful about making the HW cylinder too small because you risk running out of hot water for the bath.

However......

I don't subscribe to the notion of a combi with small store running two showers and certainly not a shower and a bath for three reasons:

- The amount of stored energy is small and once it runs out, the performance is very poor.

- The hot water is mixed with very little cold in the winter, so the run time will be quite short.

- You are still governed by the total delivery of water from the main.

I think that I would try to look for different ways to store cold water high up before getting into accumulators.

If it really doesn't add up, then this may be the sensible option though.

.andy

To email, substitute .nospam with .gl

And you were doing so well up until now. 5/10

That's correct.

Sorry, I'm misunderstanding something here. I thought that the total flow rate was limited by the available flow rate at the incoming service pipe. Or is hot water stored in the combi boiler and pumped to the shower on demand? Apologies for my ignorance.

I would like to budget for a bath and shower that are on simultaneously. I have a large family.

Stored water in the combination cylinder for all the house giving a good flow, which is not on the mains. The combi cylinder is the bottom right.

That is fine. Some people tend to oversize water systems. If one bath and one shower simultaneously then the combination cylinder with a quick recovery coil and a combi boiler doing only the showers is the way. I assume the bath will be below or on the same level as the cylinder? If so, then no problems.

Better than an accumulator?

I wonder if the water meter could be resticting the flow significantly.

The 32mm MDPE pipe was laid by me from the water meter point at the boundary of my property to the main stopcock in my house (a distance of 45m).

Unfortunately I wasn't present when the Cambridge Water engineer came to measure the flow rate, so I don't know how the figure of 25 lpm was derived. My own test was very empirical - I switched off all water supplied to the house and turned on the garden hose. The jet of water did not "feel" very substantial, particularly if is to be shared among appliances.

Thanks. I'll look around.

Combination cylinders:

The large cold storage means you can have a the cold section supplying the hot and cold in the house and a 15mm pipe filling the unit. The showers are via the combi.

I think Ed means a basement located tank with a pressure pump doing all the house, hot and cold. I prefer the combination cylinder.

They sometimes do. Make sure you have a full bore 1/4 turn stop c*ck.

The garden hose is usually at the end a run in 15 mm pipe running through double check valve. If you can, take a large bore hose directly from the stop c*ck and measure that. If it is substantially better then run 22mm to any mains fed appliances.

You cannot be seriosu.

Any main that feeds more than a couple of houses will be large enough to feed one at high volume.

I think I am at about 3 bar hgere and water simply gushes out.

My main restriction is internal pipework and the water softener.

Totally agree here.

My outdoor tap delivers water at less than half the rate that I can get at another tap internally. I have no idea why this is since I didn;t fit the tap and I really don't need lots of water delivered quickly in the garden.

However, you might find that your outdoor tap is similarly slow compared to your internal, and you might be in better shape than you think.

Neil

Hi,

Before going for an expensive solution it may be worth checking that they don't mean 25lpm _at_ 2.8 bar.

Also a garden tap is quite restrictive, a better measure would be to open all direct mains taps at a time when the mains water pressure is likely to be lowest.

One way round could be to use the hot tank as a heat bank with a tanked supply for the bath and direct mains for the showers, both heated via separate plate exchangers.

cheers, Pete.

The initial problem is that he "thinks" his mains water supply can't handle demand. A "full" mains pressure system appears out of the question. He needs to assess the proper flowrate and pressure of his supply. He may be OK.

If the mains is fine then a heat bank would be fine. One plate should be fine with one bath and 2 showers.

If the mains is not that good then an element of stored water is required. He could use a cold tank/cylinder setup heated to 70-80C with the draw-off blended down, via a blending valve, to 50-55C. A bronze pump could pump water out of the cylinder into a plate heat exchanger which will provide mains fed showers.

I did that for a neighbour, who wanted a power shower. It was about the same price to buy the plate and pump, etc, at the time.

In this case, if the mains can't cope, I would most likely go for a:

- combination cylinder for all low pressure hot water

- high flow combi for mains showers.

Divide and rule. A combination cylinder is about the same size as an unvented cylinder but on low pressure, so no BBA approved fitter or large overflows, pressure safety devices, etc.

Well, the garden tap does have its own stopcock and a double check valve in line, so you could be right.

I'll do a test to see how long it takes to fill a large bucket from the main supply stopcock where water comes in to the house...

Just out of interest, is your water softener a high flow type? Does it feed a vented or unvented hot water system?