Unvented hot water cylinder

[pmgd]

I've read many of the threads on this subject, but cannot find simpl

answers to the following questions on which I'd be very grateful fo some advice:

1. I want to install an unvented cylinder (about 200l). I will run i from a 45kW condensing boiler which has control circuits which are H priority. Thus I want a very fast reheat time, utilising as much of m

45kW as possible.

Keston have a 'spa' cylinder which looks like it is the most efficien around. BUT - they only give a 5yr warranty and they are only distributor anyway.

Does anybody have any experience with keston spa cylinerds? Does anybody happen to know who really makes them?>

Does anybody have any favourite other very high efficienc suggestions?

1. I'd quite like to hang whichever cylinder I get on a wall. Any experience of this anyone?

1. Again because of the bath + 2 showers problem and the hous ecoolin down while waiting for the HW to recharge, I wondered about putting small bleed betweent he HW coil and CH circuits. Unvented cylinder always come with a 2 port shutoff valve anyway and so when CH is on th bleed will not be able to leak into the HW circuit, but when HW is on bit will keep the CH going.

Any thoughts? Stupid idea?

Peter

PS Yes I do have plenty of mains flow and pressure. No - I dont want a thermal store or a combi

-- pmgd

[Doctor Evil]

I think it was Ariston, not too sure now.

You will need strong supports to keep 200 litres up there.

Firstly do not dismiss a heat bank. A heat bank can , and will outperform an unvented cylinder.

Unvented cylinders have the pressure reduced between 2 and 3.5 bar. If you have 5 bar mains pressure it is dropped. A heat bank can go up to 10 bar.

Unvented cylinders are potential bombs, storing water at high pressure. Heat banks are at super low atmospheric pressure.

Unvented cylinder do not have full quick recovery coils. Getting a 200 litre unvented that will take all the output of a 45 kW boiler will be diffficult, if at all.

Reheat times:

No. of Kilowatts = Quantity of water in litres x temp rise in degrees C x Specific heat of water / No. of seconds in one hour.

Specific heat of water = 4.2

So to heat a 200 litre in 1 hour will take:

200 x 50 x 4.2 /3,600 = 11.66 kW

In 1/2 hour = 23.33 kW In 15 mins = 46.66 kW In 7.5 mins = 93.33 kW

So, if you have a cylinder that will take all the boilers output, which I doubt you will get, it will reheat in about 15 mins at a 50C temperature rise, which is about the worse case example. In reality you are talking at near 1/2 hour due to the restricted coil, which is geared to around 20kW in most unvented cylinders. Some makers are cagey at giving reheat figures for unvented cylinders. They don't like very fast recovery of unvented cylinders as the pressure inside rises too rapidly and opens up the pressure relief

A "direct" heat bank "will" take all the output of a 45kW boiler as there is no coil between the cylinder and boiler.

A heat bank will take all the flow and pressure the mains throw at it. It will cope with, up to about 10 bar. High flows can be had by doubling up plate heat exchangers. You can add plates as a retrofit. Try adding a larger coil to a cylinder.

Because of the quick recovery of the store there is no need to have any heat pumped into the CH circuit.

So, there you have it. The heat bank will:

- re-heat super quick

- give higher pressures

- give higher flows

- easy to de-scale, as all you do is take off the plates to clean.

- safer as it is at atmospheric pressure

- square versions available for wall mounting.

- eliminates boiler cycling. (a smallish unvented cylinder coil will not)

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[9100DN OWNER]

I have an OSO 210 litre indirect fitted last summer. It can be wall hung and OSO do the brackets as an extra. The coil is 26.5 KW, we run it from a standard 27 KW oil boiler old five bedroom house . While there is a tempreture controlled valve to open the coil circuit it never shuts of the CH. I have never ran out of hot water and we have two showers neither has a friend with two kids in his house and the same set up. Thye hose does not go cold at all when it is heating the cylinder.

Last night I changed the fixed 2.1 bar pressure reduction valve on the cylinder for the OSO supplied circa £55, adjustable pressure reduction valve and spent 15 minutes measuring the flow from the showers ( You have to prove that the £55 was well spent). Then 10 mins showering in one of the showers and another 10mins in the other to give the system a good test and feel the improvemets in the flow.

Measured the temp of the water coming out a tap at the end of that nonsence and it was still around 58 C.

Eddie

----- Original Message ----- From: "pmgd" Newsgroups: uk.d-i-y Sent: Thursday, February 03, 2005 5:08 PM Subject: Unvented hot water cylinder

[Doctor Evil]

26 KW is about the best you will get.

What is the cylinder rated at? Usually 2 bar copper, 3.5 bar steel. Don't go above what the makers state.

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[9100DN OWNER]

The OSO is stainless steel, max working presure advised by the manufacturer is 8 bar. The supplied adjustable pressure reducing cartridge from OSO the cylinder manufacturer as an extra cost option is adjustable from 0.5 bar to

6 bar on the input side of the cylinder.

At 3 bar I'm getting 19 litres of water at one of the showers. At the speed and the quantity of that the water is comming out at it's getting to be uncomfortable. I'm thinking of turning it back to 2.5 bar.

Eddie

[Doctor Evil]

You have a good, and an "expensive" one too.

What is the pressure at the stop c*ck?

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