Heating and hot water systems

Basically, a boiler is a device that heats water. The most common fuel for a boiler is natural gas. But diesel oil, liquid petroleum gas (LPG), electricity, biomass, heat from the air or the ground, and even logs or coal can be used as fuels.

Whatever type of boiler you currently have, if it’s more than 15 years old there’s often a good case for replacing it, as the efficiency of new boilers is dramatically better; they are very much ‘greener’; and running costs are generally substantially lower. But before you replace an old boiler with a new one, take advice about flushing out your existing system. Over time, oxygen in your heating system’s water reacts with the metals of the pipes, radiators and working parts of the boiler to produce sludge which starts to clog up your pipework and, indeed, the whole system. So be certain that all this material has been thoroughly flushed out and a rust inhibitor put into the system. Be aware, though, that caution should be exercised on older systems as power flushing can expose leaks that may not have been evident in normal operation. For safety reasons, only registered gas installers and registered electricians can be involved in the installation of gas boilers in your home. OFTEC is a trade association that sets standards in the world of oil-fired boilers. They manage a ‘competent person’ registration scheme, to help you find installers who really know what they’re doing. In ROI check the official HARP (Home-heating Appliance Register of Performance) database to compare boilers on a like for like basis. In NI/the UK, check the NCM/SAP database. In terms of the regulations, in ROI boilers must have a seasonal efficiency of at least 90 per cent. In NI all new boilers have to be condensing ones, although there are rare exceptions to this rule. Non-condensing boilers usually take their air from the room but condensing boilers are sealed units that take their air from outside through the flue. In general the regulations are gradually squeezing out fossil fuels as an energy source for newly built homes. Passive homes don’t even need central heating. For a guide on how to comply with the ROI building regulations for a new build see this Q&A. Heat pumps The modern type of ‘boiler’ is the heat pump, an alternative heating source very much becoming the standard on Irish self-build projects as the need for renewable requirements in the ROI Building Regulations squeezes out fossil fuels. A heat pump is a device that transfers heat energy from a source of heat (soil, air, water) to a destination called a heat sink, working like a fridge in reverse. A heat pump uses a small amount of external power to accomplish this transfer. The efficiency of any heat pump is maximised when the difference in temperature between the heat source and the heat sink is minimised. Thus, the most efficient systems are suited to low temperature heat distribution networks such as underfloor heating. The lowest efficiency is usually experienced when producing domestic hot water but that depends on the output temperature. The most tried and tested type of heat pump in Ireland is the groundsource horizontal collector which relies on solar energy to replenish heat in the soil. Moisture movement in the ground is important because it works as a heat transfer agent replacing the cooled water in the collector field. The area required will depend on the thermal conductivity of the ground and can be significant. Groundworks can set you back quite a bit in terms of cost unless you manage to reduce this by laying the pipes at the same time as you install your septic tank or drainage. In recent years, the air source heat pump has taken over the market with two main types, one has heat pump component indoors (split) and the other has all parts located in the one unit outdoors (‘monoblock’). Considering Ireland’s mild weather, this plug and play heat pump option has grown in popularity but efficiency starts to drop at lower air temperatures and higher water temperatures. Whilst these units make some noise, and you will have to consider neighbouring properties (as well as its location in relation to your bedroom windows, etc.) don’t locate the unit far from the house as this will reduce efficiency (heat loss in the pipe run). Vertical (borehole) systems are the most expensive but can be the only solution for some sites. If you have a stream or a lake, an easy, and highly efficient option is laying the pipes as in the horizontal groundsource system but in the water. However gaining planning and environmental consents for these may not be straightforward. A type of heat pump that’s entering the market in a big way is the exhaust air heat pump, which combines heat recovery ventilation and a heat pump – the hot air generated within the home is used to supply heat and hot water and the house is ventilated in the process. This option is especially useful for apartment upgrades or a small house (200 sqm or less) that is built to very high energy efficiency standards. All heat pump systems work on the same principles and the component parts are similar; it is the extract methods that differ. Heat pumps can be bolted on to any existing system and there are hybrids which allow the heat pump to be complemented by a fossil fuel boiler (another alternative is to install two heat pumps). Maintenance will depend on the type of heat pump but get acquainted with your system and check, once a year, that all parts are in order and that the software is up-to-date.

Condensing Boilers

These are usually fuelled by gas or oil and make more efficient use of the fuel they burn than do non-condensing boilers. The technology behind them means that they achieve better than 90 per cent efficiency by also condensing the water vapour in the exhaust gases, so recovering the latent heat of vaporisation that would otherwise be lost to the outside air. Most modern non-condensing boilers are only about 80 to 90 per cent efficient because so much heat is lost in their outflow gases. This heat is largely recovered by a condensing boiler when a low return flow temperature is maintained. Condensing boilers went through a period of being seen as less reliable than standard boilers but they are as reliable as a standard boiler, no more difficult or expensive to service, not difficult to install, and always substantially more efficient than standard boilers. Unfortunately, condensing boilers are more expensive to buy because they are more complex in design, manufacture and function.

The European ErP rating assigns efficiency ratings from A+++ to G and is displayed on a label on your boiler.

Combi Boilers A combination or ‘combi’ boiler combines a highly efficient water heater and a central heating boiler in one small unit. It takes cold water direct from the mains when you turn on a tap, passes it through a heater and supplies hot water. Though oil and electric combis are available, the vast majority are fuelled by gas or LPG. People often get confused when buying a combi boiler, thinking that if they get one that has a large heating power (lots of kW) they’ll have a better system. This isn’t the case. The quoted ‘flow rates’ for combi boilers are determined solely by the flow rate of your incoming mains water. Obviously a boiler with a higher kW rating will heat the water faster but this will not increase the flow rate to your taps. On one job I did, my first task was to put in a new (larger) incoming mains feed to the home, to enable a good flow rate of hot water from the combi. Traditional boilers Also known as ‘heat-only’ boilers, traditional boilers are best for homes that already have an existing, traditional, heating system linked to a hot-water cylinder. Such a boiler needs a cold-water storage tank in the loft to feed the hot-water cylinder and another tank that maintains the water level in the central heating system. System boilers, meanwhile, need a cylinder (hot water tank) for storing hot water but don’t need a tank in the loft. These are good for homes with more than one bath or shower room, where the demand for hot water at any one time can be high. Most of the main components are built into the boiler, making it easier and quicker to install than traditional boilers. Wood-burning stoves and furnaces It’s possible to install a wood-burning stove that doesn’t just heat your room but also your hot water. Depending on the size it can also run several radiators. Such stoves don’t rely on electricity to circulate their hot water but usually require a pump to run radiators, so are good if you have an unreliable hot water supply. If you decide to install a wood-burning boiler you may need a thermal store which can store the excess heat as hot water for taps and space heating. Furnaces that use wood pellets are becoming more popular today. The beauty of these boilers is that they can run totally automatically, feeding themselves from a hopper of wood pellets, controlled like you would an ordinary oil or gas boiler although they do require regular cleaning and ash disposal. The only real downside is that you’ll need a lot of space to store a large volume of pellets. This means, in effect, being able to accommodate three tonnes (about 80 per cent of a typical house’s annual needs) in a clean, dry place that has been approved by your local building and fire regulation officers. Electric boilers Many people don’t realise that electric boilers exist but they can be a life-saver if you are off the gas grid and have no space to store oil or other fuels. This makes them ideal for flats and small homes. Such boilers can produce both hot water and space heating but aren’t suitable for large homes because they can’t supply enough heat. Note: The scope of this article does not cover dry heating systems.