Choosing a hot water cylinder






Choosing a standard hot water cylinder

1) When choosing a hot water cylinder the first decision that needs to be made is whether you choose a traditional cylinder with a separate header tank or a combined unit.


			Traditional cylinder The traditional hot water cylinder is used in many family households in the UK and is suitable for a variety of heating systems e.g. electric,coal oil and gas fired heating systems. The cylinder is fed by a cold-water storage cistern usually located in the attic, which provides a greater head of water, thus a higher flow rate at the taps, than combination type cylinders.


		Combination cylinder A combination water storage cylinder has its own cold-water header tank fitted on top of the hot water cylinder. They come in a variety of sizes, 115 litres of hot water storage being the most common with 20 -115 litres of cold storage on top depending on the circumstances Intergrating a cold water cistern and a hot water cylinder in a compact all-copper unit, are an effective way of providing adequate supplies of hot water when storage space is limited, it also enables a 'dry loft' thus lessening the risk of freezing. This system is ideal for rented accommodation due to is low maintenance requirement, or partially used properties where frequent draining and re filling by the owner is required.


2) The next step is to determine which mode of heating you will use to heat your water, this will determine whether a heat exchanger will be required for central heating use.


	Direct Hot water from the boiler circulates directly through the cylinder. Alternatively used for all electric immersion heater operation. For further information click here


	Indirect Hot water from the boiler circulates through a heat exchanger (usually a coil) within the cylinder. Avoids the problems of mixing boiler water with the water in the storage vessel. For further information click here


	Self priming Incorporating a patented heat exchanger designed to provide two airlocks to prevent water mixing. This system also removes the need for a separate expansion and feed system for the primary circuit. For further information click here


3) The next decision is to determine the overall size and water storage capacity of the cylinder.

Hot water usage varies with the type of building, it uses and the number of occupants. Occupant usage may vary depending on their level of activity, age, and water usage by appliances etc. In modern homes hot water usage can vary between 35 and 45 litres per person per day, with hot water at between 60 - 65 degrees centigrade. A wash hand basin tap will use 0.15 litres per second at 40 degrees C A Kitchen sink will use 0.20 litres per second at 60 degrees C An average bath can use 60 liters of hot and 40 litres of cold water Modern showers can use 11 litres of blended water per minute. Standard copper cylinders are usually specified in the following sizes:

Standard Cylinders
British standard t ype ref	External dia excl foam insulation	External height over dome	Min storage capacity
British standard t ype ref	External dia excl foam insulation	External height over dome	Direct	Indirect
	mm	mm	Litres	Litres
0	300	1600	98	96
1	350	900	74	72
2	400	900	98	96
3	400	1050	116	114
4	450	675	86	84
5	450	750	98	95
6	450	825	109	106
7	450	900	120	117
8	450	1050	144	140
9	450	1200	166	162
9e	450	1500	210	206
10	500	1200	200	190
11	500	1500	255	245
12	600	1200	290	280
13	600	1500	370	360
14	600	1800	450	440

Combination Cylinders
Overall height	External diameter	Minimum storage capacity
		Hot	Cold
mm	mm	Litres	Litres
900	450	85	20
1050	450	115	20
1200	450	115	40
1400	450	140	40
1850	450	210	40



4) Which grade? Copper cylinders are manufactured with varying thicknesses of copper. When specifying a cylinder it is important to select the correct grade for the job.


		British standard specification cylinders are currently provided in four grades, with grade one being the superior. Amendments to the specification are currently under consideration and the prospective purchaser may wish to consult the new draft specification. Note in particular that it is proposed to remove grade 4 cylinders from the British specifications. It is particularly important to ensure that the grade selected is adequate for the working head of the cylinder, (i.e. the distance between the bottom of the cylinder and the level of water in the cold water feed cistern. The better the grade the better the resistance to copper corrosion and the better the longevity of the cylinder. Note the thickness of copper sheet before forming on the chart below, just over half a millimetre in some cases. View proposed specification for British Standard cylinders

			Min nominal thickness of copper sheet before forming
			Grade 1			Grade 2			Grade 3			Grade 4
British standard type ref	External dia excl foam insulation	External height over dome	Test pressure 3.65 bar			Test pressure 2.20 bar			Test pressure 1.45 bar			Test pressure 1.0 bar
British standard type ref	External dia excl foam insulation	External height over dome	Max. working head 25 m			Max. working head 15 m			Max. working head 10 m			Max. working head 6m
			B	T	B	B	T	B	B	T	B	B	T	B
	mm	mm	mm	mm	mm	mm	mm	mm	mm	mm	mm	mm	mm	mm
0	300	1600	1.2	1.2	1.6	0.9	0.9	1.4	0.7	0.7	1.2	0.55	0.6	0.9
1	350	900	1.2	1.2	1.6	0.9	0.9	1.4	0.7	0.7	1.2	0.55	0.6	0.9
2	400	900	1.2	1.2	1.8	0.9	0.9	1.6	0.7	0.7	1.2	0.55	0.6	0.9
3	400	1050	1.2	1.2	1.8	0.9	0.9	1.6	0.7	0.7	1.2	0.55	0.6	0.9
4	450	675	1.6	1.6	2.0	1.0	1.0	1.6	0.7	0.7	1.2	0.55	0.6	0.9
5	450	750	1.6	1.6	2.0	1.0	1.0	1.6	0.7	0.7	1.2	0.55	0.6	0.9
6	450	825	1.6	1.6	2.0	1.0	1.0	1.6	0.7	0.7	1.2	0.55	0.6	0.9
7	450	900	1.6	1.6	2.0	1.0	1.0	1.6	0.7	0.7	1.2	0.55	0.6	0.9
8	450	1050	1.6	1.6	2.0	1.0	1.0	1.6	0.7	0.7	1.2	0.55	0.6	0.9
9	450	1200	1.6	1.6	2.0	1.0	1.0	1.6	0.7	0.7	1.2	0.7	0.6	0.9
9e	450	1500	1.6	1.6	2.0	1.0	1.0	1.6	0.7	0.7	1.2	0.7	0.6	0.9
10	500	1200	1.8	1.8	2.5	1.2	1.2	1.8	0.9	0.9	1.6	0.8	0.9	1.4
11	500	1500	1.8	1.8	2.5	1.2	1.2	1.8	0.9	0.9	1.6	0.8	0.9	1.4
12	600	1200	2.0	2.0	2.8	1.4	1.4	2.5	1.2	1.2	2.0	1.1	1.2	1.8
13	600	1500	2.0	2.0	2.8	1.4	1.4	2.5	1.2	1.2	2.0	1.1	1.2	1.8
14	600	1800	2.0	2.0	2.8	1.4	1.4	2.5	1.2	1.2	2.0	1.1	1.2	1.8

B=Body T=Top

Combination cylinders

Because the cold water supply tank is an integral part of the unit the hot water storage component is not subject to great pressure and a single grade is all that is deemed necessary by the British Standard This provides for the following specification.

Component	Type	Capacity	Minimum nominal thickness of copper sheet before forming
Component	Type	Capacity	Body	Top	Base
Hot water	cylindrical	up to 145 L over 145 L	0.55 0.7	0.6 0.7	0.9 0.9
Feed cistern	cylindrical	up to 145 L over 145 L	0.55 0.7	- -	0.6 0.7
Integral primary feed and expansion cistern			0.55	0.55	0.55
Primary heater			0.55	0.55	0.55




5) Tapping Connections Cylinder connections vary depending on the size and capacity of the cylinder.

The secondary circuits are the cold feed and hot water draw off, whilst the primary circuits allow for the boiler connections. Connections do vary though the table below gives an indication of the standard sizes used in cylinder manufacture. Additional copper fittings will be required to make the final connections, though if a made to measure cylinder is manufactured, compression fittings can be built in removing the need for any additional fittings.

Direct cylinders - tapping connections
Cylinder size		Height of connections			Size of connections
B	A	H	L	P	Secondary	Primary
Height	Diameter
mm	mm	mm	mm	mm
1600	300	1250	100	150	1" female BSP	1" female BSP
900	350	700	100	150
900	400	700	100	150
1050	400	800	100	150
675	450	450	100	150
750	450	550	100	150
825	450	625	100	150
900	450	700	100	150
1050	450	800	100	150	1 1/4" female BSP	1 1/4" female BSP
1200	450	950	100	150
1500	450	1200	100	150
1200	500	950	150	200	1 1/2" female BSP	1 1/2" female BSP
1500	500	1200	150	200
1200	600	950	150	200	2" female BSP	2" female BSP
1500	600	1200	150	200
1800	600	1350	150	200

Combination Cylinders

Indirect

Usually supplied with 2 No. 1" male BSP boiler connections and ¾" female BSP hot draw off, ½" female drain connection and 2 ¼" female BSP immersion heater boss.

Direct

Usually supplied 2 No. 1" female BSP boiler connections, ¾" female hot draw off, ½" female BSP drain connection and 2 ¼" female BSP immersion heater boss. These boss sizes can be changed to meet customers specific requirements.

Alternative and additional connections

Whilst the British Standard specifies connection of 1" BSP and upwards it is possible to specify alternate connection e.g. ¾" BSP or compression fittings. Equally it is possible to specify additional connections such as shower tappings, secondary returns, drain connections and to vary the position of tappings.

Factory applied insulation

Cylinders and combination tanks can be supplied either plain (with a metallic paint finish) or with factory supplied CFC free foam insulation to conform with British Standard heat loss requirements.

Aluminium protector rods

Also known as sacrificial anodes these are provided to resist copper corrosion. They are not effective in all water areas and, indeed, in some areas their fitting can prove detrimental to the cylinder. An alternative is to fit a thicker bottom to the cylinder and this option is reflected in the draft new British Standard specification. Local plumbers will be able to advise whether sacrificial anodes are the preferred option in your area or not.

Improved specifications

The performance and durability of these British Standards are, of course, capable of improvement and various options are available to you the customer. Predominant amongst the preferred option are:

Quick recovery - Increasing the heating surface of indirect units, these "Quick Recovery" cylinders give improved reheat times, a must in homes where hot water is at a premium.
Heavier base - The introduction of a heavier base, either to grade 1 or grade 2 specification, offers improved resistance to corrosion. The new draft BSI specification effectively supports this improvement in that grade 3 cylinders will in future have this heavier duty base.
Extra insulation - Additional factory applied insulation will improve the thermal retention properties of the cylinder.

Direct water heating and cylinders

Indirect water heating and cylinders

Single feed indirect cylinders

Proposed specification for British Standard cylinders

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Choosing a standard hot water cylinder