EP0943877A2

EP0943877A2 - A domestic water heater

Info

Publication number: EP0943877A2
Application number: EP99650026A
Authority: EP
Inventors: Stephen William John Grant
Original assignee: Alley Enterprises Ltd
Current assignee: Alley Enterprises Ltd
Priority date: 1998-03-18
Filing date: 1999-03-18
Publication date: 1999-09-22
Also published as: IE990215A1; IES990216A2; EP0943877A3; IES81007B2

Abstract

A domestic water heater (1) for use with a domestic water boiler (3) is provided. The heater (1) has usually more than one electrical heating element and preferably three which increase the total heating capacity by forty percent or so. The heater (1) comes into operation when the draw off from the boiler (3) exceeds a flow rate of 8 litres per minute or so and the temperature of the water falls below an acceptable output temperature. It is particularly useful for Combi boilers which are demand draw off combination central heating and domestic hot water supply boilers with relatively little hot water storage capacity.

Description

The present invention relates to a domestic water heater and in particular to an electrically operated domestic water heater.
Domestic hot water supplies are of three general types. The first is a dedicated hot water supply boiler with or without limited storage, which usually depends on its heating capacity and storage, if available, to provide sufficient hot water to satisfy the maximum demand that is likely to be placed on it, namely the maximum draw-off of water required at a predetermined temperature. The major problem with this type of unit is the cost in providing a separate heating unit simply for water.
The second type of water supply usually incorporates a cistern which is either heated directly by electricity or by indirect heating coils from a central heating system. The problem with the provision of a cistem is that a large quantity of hot water has to be stored at a relatively high temperature and thus there is considerable heat loss and hence cost to the householder.
The final type of domestic hot water supply unit in use is the so-called "Combi" boiler which is a demand draw-off combination central heating and domestic water boiler. These Combi boilers have a relatively small supply of hot water and rely on the boiler itself providing sufficient heat to heat the water as it is drawn-off from the boiler.
The problem with many of these Combi boilers fitted in domestic houses is that when being fitted the size of boiler chosen is chosen on the basis of average demand, namely the demand to heat the radiators in the house, together with the likely requirement of domestic hot water over an extended period such as a day. Thus, many a Combi boiler which is perfectly adequate to provide the domestic hot water and heating requirements of a house on the basis of an average demand is not able to supply sufficient domestic hot water at the required temperature when demand increases, such as, for example, when filling a bath.
There are a large number of Combi boilers fitted in domestic houses, with a rating of the order of 80,000 Btus (23.4 KW) which, unfortunately while sufficient for heating the house, is not sufficient to provide the necessary hot water. Generally speaking they will be able to provide 8 to 10 litres per minute of hot water at a temperature of 40°C or so. This is the usual temperature required for domestic hot water. When the domestic hot water exceeds for example 45°C it becomes too hot to leave one's hand in, but not sufficiently hot as to burn.
Combi boilers are often rated in Ireland and the United Kingdom as 80,000 Btu boilers which have an output of 23.4 KW. Such a Combi boiler, hereinafter referred to as conventional Combi boiler as the majority have this output rate, will on a flow rate of 9 litres/minute impart a 35°C temperature rise to the incoming water.
When a bath, shower or the like is required, the draw off rate usually exceeds 8 litres/minute. At 8 litres/minute, the conventional Combi boiler will impart a 39.4°C temperature rise to the water. Thus, with very cold ambient conditions, a Combi boiler will, at draw off rates up to 8 litres/minute, provide hot water at a temperature of 49.4°C and in warmer conditions such as between 15°C and up to 20°C, the temperature of the output of the Combi boiler can be up to 70°C. However, if the flow rate increases to 10 litres/minute, then the heat rise drops to 31.5°C and at 12 litres/minute to 26.25°C. Thus, if, for example, this higher draw off rate is required, then, in very cold conditions, the Combi boiler would only be able to produce water at approximately 36°C which would be unacceptable. Thus, the flow rate is of considerable importance.
At the same time these Combi boilers are a very efficient way of providing hot water, since the hot water is only provided on demand and there are no heat losses and consequent costs associated with the storage of hot water until it is required.
The present invention is directed towards overcoming this problem and in particular towards overcoming the problem of Combi type boiler arrangements, where the rating of the boiler is not sufficient to provide adequate draw-off of water over a relatively long period of time such as 20 minutes or so at the desired temperature.

Statements of Invention

According to the invention there is provided a domestic water heater comprising a water cylinder housing an electrical heating element and including a water inlet port and a water outlet port, characterised in that the heater includes a throughput flow rate measuring device, a water inlet thermometer and control means for causing the electrical heating element to operate when the flow rate exceeds a critical flow rate and the temperature falls below a preset minimum temperature.
The advantage of this unit is that when fitted in a domestic hot water supply pipe it will only provide additional heating firstly when there is a throughput through the heater exceeding a preset amount and secondly when the temperature of the water flowing through the heater is below a predetermined level.
Ideally a domestic water heater according to the invention forms part of a demand draw-off combination domestic heating and domestic water boiler namely a Combi boiler.
It will be appreciated that the unit can be incorporated into any Combi boiler casing and controls and further it is envisaged that the water heater according to the invention may be fitted in a remote position from the boiler and the controls fitted in position for ease of access, such as, for example, near a bathroom.
Ideally, there are a plurality of heating elements which are progressively activated as the temperature drops below the preset minimum temperature by a preset critical temperature increment. It is envisaged that the control means includes means for varying the setting of the critical flow rate and/or the critical temperature.
Ideally, the preset minimum temperature is between 42° and 56°C and preferably it is in the order of 45°C.
Ideally, the preset critical temperature increments are between 4°C and 6°C and ideally are approximately 5°C. The critical flow rate is generally of the order of 6 to 10 litres/minute and in most domestic situations ideally 8 litres/minute.

Detailed Description of the Invention

The invention will be more clearly understood from the following description of an embodiment thereof given by way of example only with reference to the accompanying drawings in which:-
Fig. 1 is a diagrammatic representation of a domestic heater according to the present invention,
Fig. 2 is a diagrammatic representation of the heater according to the invention in use with a Combi boiler,
Fig. 3 is a perspective view of an alternative construction of heater according to the invention, and
Fig. 4 is a plan view showing the heater of Fig. 3 with part of the cover removed.
Referring to Fig. 1, there is illustrated a domestic water heater indicated generally by the reference numeral 1 mounted intermediate a hot water supply pipe 2, fed from a main draw-off Combi boiler 3 fed by a water mains feed pipe 3(a). See Fig. 2 which shows a domestic heating system. The water supply pipe 2 will supply taps, etc. at 4. The water heater 1 comprises a housing 5 for water having a water inlet 6 incorporating a throughput flow rate measuring device, namely a flow sensor 7. Mounted within the housing 5 are two sets of electric heating coils 8 and 9 connected by wires 10 (shown by interrupted lines in the drawing) to a control unit 15 which forms a control means for the water heater 1. The control unit 15 is connected by further wires 10 to the flow sensor 7 and also incorporates a water inlet thermometer in the form of a control thermistor 16 mounted in the housing 5.
The control unit 15 includes a temperature controller operated by a dial 17 in conventional manner. A further temperature sensing device 18 is provided and will usually be set to operate for example at 90°C to shut-off operation. This acts as a fail safe.
It will be appreciated that the illustrated water heater is a diagrammatic or schematic representation of the boiler and for example the flow sensor and all the units would be incorporated in the one housing.
In use, the domestic water heater 1 may be used in various ways. For example, on the flow sensor 7 sensing a flow rate greater than a certain amount, the control unit 15 can be activated so that when the control thermistor 16 senses a temperature in the housing 5 below a predetermined temperature, one of the heating coils, for example the heating coil 8, is activated to bring the temperature in the housing 5 to the desired level. If the heating coil 8 is unable to achieve this, then the second heating coil 9 is brought into operation. The advantage of this is that when, for example, fitted to a Combi boiler, if the draw-off is at an acceptable level the unit is not operated. Alternatively, the water heater 1 may be operated immediately there is any draw-off of water through the water heater. However, it will only supply heat to the water when the temperature of the water falls below the predetermined level.
A test was carried out using two 4KW heating elements in the water heater 1. It was found that there was an increase for a Combi-boiler having a normal output of 8 to 10 litres per minute of useful water to as high as 15 litres per minute. Since a conventional Combi boiler has an output of approximately 23KW, the provision of these two heating elements effectively increases the output to 31KW and thus boosts the output. However, many Combi boilers have higher outputs such as 90,000 Btu or 26.3 KW.
Table 1 shows the various tests carried on a 90,000 Btu Combi boiler where the supply to the boiler was at 8°C. In the first column it will be seen that after 12 minutes or so the water being produced by the boiler was effectively below an acceptable level and was largely useless for domestic use and approximately 200 litres would have been provided to a bath. Then on the 16th minute the 8KW of heating coil was operated and the water immediately increased to 47°C and further hot water was provided for 21 minutes. Then it was switched off and as can be seen, the water from the boiler was again below an acceptable level. It should be noted that the affect in each case of the water heater according to the invention on the water was instantaneous.
In the second column it will be seen that after 5 minutes one of the two heating coils was switched on and this gave acceptable water and would have continued to give acceptable water for 20 minutes or so.

In the third column it will be seen where both heating elements were switched on after 8 minutes and acceptable hot water was provided for 21 minutes when the heater was switched off.

COMBI BOOSTER-TEST RESULTS
WATER IN AT 8°C
TIME	TEMPERATURE AT FLOW RATE OF -
MIN.	15 LITRES/MIN.	15 LITRES/MIN.	15 LITRES/MIN.
1	50.0	52.0	52.0
2	49.2	49.6	49.8
3	48.7	49.0	49.0
4	47.1	48.0	48.0
5	45.3	51.0 (4KW ON)	47.0
6	44.5	50.0	45.0
7	43.3	49.0	44.0
8	42.0	47.7	52.0 (8KW ON)
9	41.0	47.1	51.2
10	40.7	46.2	51.0
11	40.0	45.8	49.8
12	39.7	45.0	49.6
13	39.4	44.5	48.9
14	38.7	44.2	48.6
15	38.6	43.8	48.0
16	47.0(8KW ON)	43.8	47.8
17	47.0	43.5	47.3
18	46.6		47.3
19	46.6		47.0
20	46.5		47.0
21	46.5		47.0
22	40.0(OFF 8 KW)		39.0 (8KW OFF)
23	37.6(FROM BOILER)

Table 2 shows a further test on a Combi 80,000 Btu boiler in which both heating elements of the domestic water heater were switched on for the full 30 minutes and 14 litres per minute where drawn-off. After 12 minutes the water temperature from the boiler had dropped to 42.5°C, while output was at 50.8°C and it will be seen that after 30 minutes the water boiler temperature had dropped to 38°C. During all this time, the draw-off water was above the acceptable level and at all times approaching what is above what is considered to be very hot water, namely water in excess of 45°C.

Thus, it can be seen that with a slightly larger Combi boiler, 8 KW of additional heating capacity is very effective.

COMBI BOOSTER - TEST RESULTS
TIME	HEATER ON FULL FOR 30 MIN.
MIN.	14 LITRES/MIN.
1	53.0 (MAINS WATER 9.4°
2	47.0
3	47.0
4	47.0
5	47.0
6	47.2
7	47.1
8	47.5
9	48.6
10	49.8
11	50.3
12	50.8 (FROM BOILER 42.5)
13	50.5
14	51.0
15	50.0
16	49.5
17	49.0
18	48.8
19	48.6
20	48.2
21	48.0
22	47.6
23	47.2
24	47.0
25	47.2
26	47.0
27	47.0
28	46.7
29	46.6
30	46.3 (FROM BOILER 38°C - MAINS WATER 8.6°C)

Referring now to Figs. 3 and 4, there is illustrated an altemative construction of domestic water heater according to the present invention, indicated generally by the reference numeral 20 for use with a conventional Combi boiler, namely one having an output of 80,000 Btu's or 23.4 KW. This heater 20 comprises a housing 21 having a display panel 22 and on its exterior a trip switch 23. Water inlet and outlet pipes 24 and 25 are also illustrated.
Referring to Fig. 4, not all of the water heater 20 is illustrated, however, there is illustrated a boiler housing 30 containing three 3 KW heaters (not shown). The water inlet pipe 24 feeds a throughput flow rate measuring device 31 having a flow switch 32. Suitable thermometers are provided as before. There is provided a control unit 35 the only feature worth noting is that it contains three sets of relays 36, 37 and 38 respectively, each of which combines a pair of relays for the live and neutral of each of the supplies to each of the 3 KW heaters in the boiler housing 30. The control unit 35 includes thermistors, etc. as before. A limit stat 40 of conventional construction is connected to a valve 41 which incorporates the trip switch 23. The display panel 22, as illustrated in more clearly in Fig. 4, there are provided indicator lights to show when the limit stat 40 has operated, the mains is on and either one, two or three of the 3KW heaters is in operation.
In operation, the limit stat 40 will operate when the output temperature in the outlet pipe 25 exceeds a predesired temperature such as, for example, 80°C. In this particular embodiment of the invention, the motor heater 20 is generally set to operate when the flow rate exceeds 8 litres/minute and the heaters are set to come into operation such that when the temperature reduces to below 54.7°C, the first 3 KW of additional heat are applied; when the temperature falls to 50.5°C, the second heater comes into operation such that there is now a boost of 6 KW; and finally, when the temperature drops to 45.5°C, all three heaters, namely a total of 9 KW of additional heat, are provided to the boiler.
For example, at a flow rate of 8 litres/minute, the use of one of the three heaters, namely, the application of a further 3 KW to the conventional Combi boiler will cause a 5°C rise in the temperature of the water. Similarly, at 10 litres/minute and with all three heaters operating, namely with a total of 9 KW, the temperature rise will be 12.1°C due to the use of the water heater 20 and similarly a flow rate of 12 litres/minute and the additional heating capacity of 9 KW, there is a temperature rise of 10.1°C. Thus, a conventional Combi boiler operating at 12 litres/minute with the addition of the water heater according to the present invention, the total temperature rise imparted to the incoming water will be in excess of 36°C and thus under the most adverse ambient conditions in which the mains water feed to the Combi boiler is of the order of 10°C, the output water will still, at a flow rate of 12 litres/minute be a very acceptable 46°C or so. Thus, for a conventional Combi boiler, the use of the water heater according to the present invention in combination therewith will overcome the problems inherent in Combi boilers that have been experienced in use heretofore.
It will be appreciated that many combinations of use may be provided in that the water heater according to the invention may be incorporated into the Combi boiler casings and controls, it may be fitted at the output of the Combi boiler. It may be fitted at water input to the Combi boiler, but this generally would not be the most efficient way of placing it, or, altematively it may be placed totally remote from the Combi boiler in another part of the house adjacent for example bath taps, showers and domestic hot water supplies generally.
In this specification, the terms "comprise", "comprises" and "comprising" are used interchangeably with the terms "include", "includes" and "including", and are to be afforded the widest possible interpretation and vice versa.
The invention is not limited to the embodiments hereinbefore described which may be varied in both construction and detail within the scope of the claims.

Claims

A domestic water heater of the type comprising a water cylinder housing an electrical heating element and including a water inlet port and a water outlet port, characterised in that the heater (1) includes a throughput flow rate measuring device (7), a water inlet thermometer (16) and control means (15) for causing the electrical heating element (8, 9) to operate when the flow rate exceeds a critical flow rate and the temperature falls below a preset minimum temperature.
A domestic water heater as claimed in claim 1 in which there are a plurality of heating elements (8, 9), which are progressively activated as the temperature drops below the preset minimum temperature by a preset critical temperature increment.
A domestic water heater as claimed in claim 1 or 2 in which the control means (15) includes means for varying the setting of the critical flow rate.
A domestic water heater as claimed in any preceding claim in which the control means (15) includes means(17) for varying the setting of the critical temperature.
A domestic water heater as claimed in claim any preceding claim in which the preset minimum temperature is between 42°C and 56°C.
A domestic water heater as claimed in any preceding claim in which the preset minimum temperature is approximately 45°C.
A domestic water heater as claimed in claim 2 and in any of claims 3 to 6 in which the preset critical temperature increments are between 4°C and 6°C.
A domestic water heater as claimed in claim 2 and any of claims 3 to 7 in which the preset critical temperature increment is approximately 5°C.
A domestic water heater as claimed in any preceding claim in which the critical flow rate is between 6 and 10 litres/minute.
A domestic water heater as claimed in any preceding claim in which the critical flow rate is approximately 8 litres/minute.
A domestic water heater as claimed in any preceding claim in which there are three electrical heating elements.
A domestic water heater as claimed in any preceding claim in which the heating element provides approximately 3 KW/hour.
A domestic hot water heating system comprising a main hot water boiler having an outlet pipe feeding the domestic water heater as claimed in any preceding claim.
A domestic hot water heating system as claimed in claim 13 in which the main hot water boiler is a demand draw-off combination boiler.