Water Storage Temperatures and Their Efficiency Impact:

A guide for Improving Energy Efficiency in Irish Homes

In this article, we will explore the advantages of storing water at lower temperatures and how it can positively impact advantages of storing water at lower temperatures and how it can positively impact the efficiency of your heating systems. We will delve into physics behind heat loss and discuss the benefits of low-temperature water for various heating systems. Additionally, we will provide insights into determining the ideal water storage capacity for your household.

Firstly, while a boiler can quickly heat a cylinder to the desired temperature, we have discovered that using a heat pump is more cost-effective option, especially utilising Night-Rate electricity. Heat pumps operate more efficiently at 35°C compared to 55°C. This raises an important question: Why not store water at 35°C instead of higher temperatures like 50°C or 60°C?

Advantages of Lower Storage Temperatures

Storing water at lower temperatures offers several efficiency benefits, particularly for heat pump systems. It is important to note that the efficiency of heat pumps decreases as the required water temperature rises. By maintaining lower storage temperatures, you can optimise the performance of your heat pump.

One often overlooked advantage is the reduction of heat loss. According to the fundamental principles of physics, energy always moves from the hottest point in a system to the coldest. The rate of heat loss is directly proportional to the temperature difference between these points. Consequently, the hotter your water storage cylinder, the more heat energy it loses.

In older homes, poorly insulated hot water cylinders acted as inefficient heaters in the hot press. Even in newer homes with better insulation, heat loss from standing water can have a significant impact, especially if the hot water remains unused for extended periods. Operating at 35°C can also help minimise losses within distribution pipework.

Utilising Low-Temperature Water

Greentherm Underfloor Heating Systems are specifically designed to operate efficiently at 35°C. The heat output of a system is directly proportional to the surface area of the heat emitter and its temperature. A larger surface area combines with higher temperatures results in increased heat transfer.

Conventional radiator systems require higher temperatures to meet the heat load of most rooms. However, low-temperature water distributed over a large floor area can efficiently and rapidly heat a room to the desired temperature. Other technologies, such as low-temperature radiators or fan-coil radiators, enable the use of low-temperature heating system water. It’s important to consider specific design considerations when implementing these technologies.

Determining Water Storage Requirements

The required water storage capacity depends on the temperature of the water used. Higher temperatures necessitate smaller hot water storage cylinders. Typically, a strategy involving the blending of cold water is employed to reduce high-temperature water to a comfortable and safe level. This is commonly achieved through the use of a thermostatic mixing valve, such as those found in showers.

For example, a 10-minute shower with a consumption rate of 18 liters per minute at a comfortable set temperature of 38°C requires a specific amount of hot water. The table below illustrated a comparison of the hot water requirements for a heat pump and a boiler, using the maximum storage temperatures. (60°C for the boiler and 50°C for the heat pump without use of immersion heater)

Boiler Heat Pump
Temperature (°C) 60 50
Hot Flow (L/min) 10 12.6
Per Shower (L) 100 126
Cost of Energy (€) 0.80 0.40

To determine the total water consumption, multiply the required hot water per shower by the number of showers taken. If teenagers are added to the household, the required amount per shower may more than double. Additionally, incorporating Zypho waste-water recovery units can reduce hot water requirements by up to a third.

For a four-person household with a boiler system, a minimum of 400 liters of hot water storage is needed to prevent running out of hot water. With a heat pump system, at least 500 liters is required. If a solar coil is added to the storage tank, an additional 100 liters of capacity must be considered.

Considering Your Heating System

The design of your heating system also impacts the required water storage capacity. In modern heating designs, the hot water cylinder serves as a thermal store of heat energy, similar to a battery storing electricity. It provides both your hot water and the heat for your central heating system. The necessary water storage depends on your domestic hot water requirements, the heat load on your central heating system, the type of heating (underfloor, radiators, or a combination), and the heat sources installed)

At Greentherm, we perform precise calculations to design and specify heating systems that meet your hot water needs while efficiently providing heat to your home, all while keeping fuel bills low.

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