Underfloor Heating with Multiple Heat Sources and Heat Stores
There is an increasing demand from new house builders, whether from an ethical standpoint or for legislative reasons, to dramatically reduce the carbon footprint of new constructions. This means that many 'alternative' methods of heating are being considered before building begins. However, the combination of these 'alternative' methods of heating needs to be considered carefully to deliver maximum carbon reductions and cost savings. UFH's compatibility with both conventional heating solutions and renewables, which operate at lower temperatures, make it a popular choice for specifiers.
Solar thermal panels, ground source and air source heat pumps and boilers, using a variety of energy sources such as bio-fuels, gas and oil, are all being utilised in the new era of energy saving homes. Solid fuel boilers can be connected and integrated into a heat store. However, since they cannot be turned off in the same way as a conventional boiler, there must be a way of dissipating the heat from the boiler without letting the water in the solid fuel boiler boil and so any installation should allow the boiler to vent into the atmosphere.
When considering running costs, there is a hierarchy to the order in which the various heat sources should be used. For example the running cost of a series of solar thermal panels is very little, only the cost to run a circulating pump. Yet the contribution to domestic hot water can be as much as 70% of the yearly demand on a conventionally sized system. So if there is only a hot water demand, then there is no point in running any other forms of heating until the need arises. One option would be to switch to a heat pump with a boiler or electric immersion heater as the final resort.
Increasingly the panels are being 'oversized' so that they can contribute to both heating and domestic hot water demand during the shoulder months of the year (March, April, May, September, October) and even on sunny winter days. When designing a solar thermal system to provide domestic hot water and contribute towards heating, careful consideration needs to be given to the hot water storage quantity and also the safety devices incorporated into the solar design. As a result when considering controls, solar panels should be the first in line for supplying the heating. If they can't supply what is required, then other heat sources should be called upon to provide the heating.
There are many different designs of heat store but most work by having a temperature gradient through the tank, meaning there is cooler water at the bottom and hot water at the top. This is completely opposite to a conventional calorifier or hot water cylinder where temperature gradients or stratification is undesirable due to the potential build up of legionella bacteria in the cooler parts of the vessel.
This stratification within the tank aids fuel efficiency of the heating system, particularly when the heating system is designed to run off lower temperatures, such as UFH. What this means is the hot water is drawn off from the heat store at the top where the water is at its highest temperature. The hot water will normally pass through a heat exchanger, either an internal coil or external heat exchanger. The advantage of this is that the hot water does not come into contact with the water from the heat store and no domestic hot water is stored.
The solar heating is usually supplied to a coil at the bottom of the heat store. This means that again there is a physical separation between the heating solar water and the heated water within the heat store, since the solar circuit usually contains glycol to prevent the water from freezing during the winter.
Both UFH and radiators can be connected to the heat store. UFH, correctly designed, can run at temperatures as low as 35°C, dependent on heat loss from the room. This means that with the stratification in the tank the UFH connections can be near the bottom, leaving the top part of the store for hot water. This means the UFH can run off both the heat pump and the solar thermal coils. Although the temperatures required from the heat pump for the UFH are low, the UFH system should still incorporate a mixer to allow for the days when the solar heating is contributing to the hot water at a much higher temperature.
Radiator connections will need to be located further up the heat store since they require a higher temperature to run properly because they are less efficient than UFH at operating at low temperatures.
To conclude, UFH should be considered in conjunction with multiple heat sources and heat stores because its compatibility with lower temperatures means it provides a very energy efficient option.
Continental Underfloor Heating has an expert technical team on hand to answer any questions. For more information log onto www.continental-ufh.com or call 0845 108 7001.
