Balancing hot water and heating requirements with a heat pump system

There is a design challenge that modern properties face whilst balancing a heat pump system requirement for both heating and hot water. Space heating loads are reducing as building insulation levels increase, which often means you need quite a ‘small’ heat pump for a relatively large home.  Selecting the heat pump to suit the space heating requirements maximises overall efficiency whilst keeping capital costs to a minimum. 


So far so good. 


However, it does mean that domestic hot water tanks can take longer to heat, resulting in longer recovery times than a comparative system with a larger heat pump. This leads to the risk that draining the hot water cylinder would result in the heat pump diverting its heating capacity to the hot water cylinder for a longer period. The consequence of this could be a drop in the heating system temperature, and potentially, the backup immersion heaters being used for heating during the colder months of the year.


 All of this is against a background of consumers generally increasing their demand for hot water. 


Moreover, there are occasions when households have a demand for hot water that exceed the expected design parameters of the system. A houseful of guests for a family party might be one such situation. These are exceptional times, but the times when it is going to be the most important to householders that their heat pump system can cope. Yet to oversize your heat pump system to cater for these special occasions reduces efficiency, leading to increased running costs and CO2 emissions.  Oversizing a heat pump for these few days or weeks of the year results in the heat pump spending less time working at its most efficient during periods of normal demand. Additionally, you will be building into the system the greater heat losses (and thus energy costs) of a larger hot water tank, which may also increase the plant room space required for the solution as well as initial capital costs.


To summarise, installers and customers are worried about the heat pumps potential to provide adequate hot water when using an appropriately heat pump. So, how do we satisfy the customers’ demands for sporadic increased hot water demands, whilst ensuring we provide the most cost effective and efficient solution?


I would like to propose two possible solutions for this dilemma. One or both can be built into the system to manage periods of high demand without compromising the overall running efficiency.  This will result in the consumer receiving the most cost effective and efficient system, whilst providing everyone with the peace of mind that the solution will always provide ample hot water.


1. Speed up recovery times by settting your heat pump to switch to booster heater if hot water is taking too long to heat. All STIEBEL ELTRON heat pump are supplied with a backup/booster heating element.  In periods of high demand this can be used in conjunction with the heat pump.  Although this does not increase the volume of water that you can draw off, it does mean that the temperature in the hot water tank recovers very quickly.  This can be set up automatically, for example, a timer can be installed to switch on the booster element if the heat pump is taking too long to heat the tank. The advantage of this solution is there is no additional hardware to purchase, as we are using the components supplied with the heat pump. The downside to this solution is it will require some level of operation from the customer to control when the booster comes into operation, even if this is just controlling the timer. For those customers who don’t need to regularly speed up the heating of their hot water cylinders, or who don’t want to manually control their heaters, we have a fully automated solution too.



2. Extend the available volume of water with an instant water heater that accepts pre-heated water. These water heaters can utilise the pre-heated water from the heat pump when available, whilst maintaining the required temperature when the stored hot water is depleted. Our new range of instantaneous hot water heaters (our DCE-S Plus and DCE-X Premium models) can accept pre-heated hot water.  This is all thanks to our patented bare wire technology which also prevents the build-up of limescale inside the unit, extending its life and ensuring continued powerful performance. The hot water outlet from the heat pump cylinder can be fed through the DCE so that the most efficient source of hot water is always used first.  When consumers use hot water and the heat pump cylinder starts to run cold, the DCE tops it up so that there is always a supply of hot water. 


The DCE has enough power for only one shower and basin per unit. Thus, the best solution is to install the DCE near a shower, so at least one shower will always be available.  This has the added benefit of supplying the shower with hot water the instant you turn on the shower, no matter how long the pipe run from the cylinder, saving water and energy whilst increasing comfort. When hot water from the heat pump’s hot water tank reaches the DCE it will switch off until the water temperature drops below the minimum setting. The benefit of this solution is a guaranteed supply of hot water at a temperature fixed by the consumer. This offers total peace of mind that the client will always have plentiful hot water.


Find out more about our full compact instantaneous water heater range here or Contact us to discuss your project