In the past, hot water was only a small percentage of a buildings total energy demand, often around 20% for the average home. However, as the insulation levels of buildings increases, then the space-heating demand drops. Our hot water use for washing and showering has if anything increased. Over half the heat requirements can in some cases be for domestic hot water.
Low temperature emitter systems can make room heating very efficient, but with hot water, there is little oportunity to reduce the working temperatures. The fairly recent concerns about legionella health risks have lead to increased temperature requirements. It is therefore quite a challenge to achieve high energy efficiencies during hot water production when using a heat pump.
There are two main types of hot water storage cylinders.
(note heat pumps are never suitable for instantaneous heating, unlike gas boilers etc)
1. The conventional hot water cylinder. This has a heat-exchanger coil to transfer heat from the heat pump. The cylinder holds the actual water that comes out of the taps. Traditionally these were fed from open-topped header tanks in the loft. This has several drawbacks, namely: contamination of the water in the loft tank, risk of freezing in the loft, and low water pressures for showers.
The UK is slowly changing towards mains-pressure systems. These avoid the need for loft tanks, and can give better pressures for showers,and smaller supply pipes can be used. However, they do come under stricter regulations since, if not installed well, there could be safety issues
The heat-exchanger coil within the cylinder needs to be far bigger than that designed for a boiler. Ideally with only a 5°C difference between the heated water from the heat pump, and the water available to the taps. This is achievable is a big enough coil is used.
2. The 'Thermal Store' type cylinder is the traditional method used in Sweden. With this system the cylinder contains the water that circulates within in the heat pump. It is pumped direct from heat pump to cylinder. A large heat-exchanger coil inside the cylinder takes cold mains water, and heats it up instantaneously as it passes through. This system has advantages and disadvantages.
Advantages
* Legionella risk is minimal due to low water content within the heat-exchanger coil.
* The hot water can be produced at high pressure (same as cold-tap pressure)
Disadvantages
* The water temperature to tap may be flow-rate dependant.
* Storage temperatures may need to be higher than the conventional system.
The conventional cylinder is a bit easier to configure and has advantages of stratification and easier control. The thermal store will often have inferior performance unless designed well, and is more prone to dropping in temperature quicker. However, developments may improve these details.
Some heat pumps only pre-heat the water to say 45°C. The heat pump works efficiently at such temperatures, but the immersion heater that is needd to top-up to a useable temperature is very inefficient. Ideally the heat pump would go to its highest possible limit, even if the COP drops at such temperatures, (the limit varies from model to model).
Batch heating
It is very important to keep the water temperature the the heat pump is working at, as low as possible. However, most cylinders are maintained at over 50°C all the time, and usually heated to 60°C daily or weekly for Legionalla protection. This sometimes requires an direct electric immersion heater which is very inefficient.
If the cylinder is large enough, then a considerable delay can be arranged before the heating is re-triggered, by which time the bottom half of the cylinder will be cold. The heat pump can then heat the lower section from cold right up to the required temperature. The average water temperature is considerably lower, and the energy efficiency is significantly improved.
This method of control can be partly achieved with a conventional cylinder if the sensor is repositioned further up the cylinder. The total volume of available hot water will
be reduced, but this may not be a problem for much of the time.
To my knowledge, no heat pump manufacturers use this method due to the increased cost. There is not enough drive at present to improve the energy efficiency of systems. Unfortunately few people are willing to pay the extra for such refinements.
Saving energy and water in the distribution pipes
Whatever your method of producing hot water for baths, sinks and showers, you should make sure that energy and water is not wasted due to the pipework from the storage cylinder to the tap. Whenever a hot tap is first turned on, there is a time-delay before water comes through at a useable temperature. This is known as dead-leg losses.
- Firstly this is a waste of water.
- Secondly it is a waste of heat since the hot water within the pipe will slowly cool after the tap was last used and heat will be lost. Sometimes this heat is useful to the building. Nonetheless, these losses are undesirable.
One solution to save water is to use a pumped-loop around the building so that hot water is always close to the tap outlets. This may save water and it reduces waiting time, but it does not save energy. This is common in hotels, but not necessary for most domestic situations.
(note, return loop pipes are often far too big, they only need be big enough to keep the loop warm. This need only be a tiny flow rate)
Good housekeeping
In the days of cheap energy and before energy was a problem. The pipe runs from a cylinder were made nice and large, usually reducing in diameter as they got to the last tap. If your washbasin was at the end of a long run, then it will take ages for the hot water to come through.
To minimise these losses there are a few things that can be done.
- Use the smallest size pipe possible to maintain an acceptable flow rate.
- Use multiple pipe-runs from the cylinder so that each pipe is smaller (e.g. a separate pipe to the bath & separate small-bore to basins))
- Insulate pipes well. (helps most for frequently used taps).
Experiment
1. First thing in the morning (following no tap use for many hours), put a bowl in your kitchen sink and turn on the hot tap and see how long it takes before the water is a useable hot temperature. This amount of water is wasted frequently. This quantity of water is left cooling in your pipework.
2. Assess the flow rate. How quickly can the sink be filled? Is it quicker than needed? If so, you should be able to improve matters.
Repeat this experiment for all your sinks.
Solution
- Reduce the pipe diameter from your cylinder.
- make the pipe route more direct, hence shorter.
- fit a small-bore pipe direct from the cylinder to that distant wash basin.
- Insulate pipes so the heat says available in them for a bit longer.
Note,Consult an expert if you are worried about washing machines and dishwashers. My experience is that lower flows simply increase the time for things to fill, but it could be a problem.
Noise can increase when smaller pipes are used, but I have not found this to be a problem.
Pipe sizes and flow rates.
The following chart gives the internal volume of common plastic and copper pipes. Dimensions may vary slightly with different manufacturers . However, it shows the dramatic difference in internal size between copper and plastic pipes.
The following chart might be useful for assessing the size of pipe that you may be able to reduce to.
Note, this chart can only be used as a rough guide. It is interesting to note how dramatic the effects of the internal diameter is on the flow rate. The internal bore of plastic being much smaller than that of copper, this may be an advantage, or a disadvantage depending on your requirements.
Notes,
The pressure given are just a guide. The height below a loft header tank causes the pressure, these systems need the biggest pipes. Mains pressure systems (e.g. thermal store) can give the highest pressures, so pipe sizes can be smaller. However, mains pressure can vary considerably.
