Tankless water heating
Copper heat exchangers are preferred in these units because of their high thermal conductivity and ease of fabrication.
The main disadvantage of these systems other than their high initial costs (equipment and installation) is the required yearly maintenance.
In order to provide on-demand, continuous hot water, tankless units use heat exchangers with many small passageways consisting of parallel plates or tubes.
This increased number of passageways and small internal size create a large surface area for fast heat transfer.
Another advantage is that more than one unit may be used to supply separate heating zones or multiple bathrooms, giving greater time and temperature control.
Combination boilers are popular in Europe where market share in some countries is in excess of 70%, with a projected rise in the United Kingdom to 78% by 2020.
[citation needed] Some designs dating from before the 21st century, notably the Ideal Sprint, included as standard a flow regulator that permitted the same model to function efficiently in both high and low pressure mains water supply areas, thus accommodating wide supply pressure variations often encountered in otherwise similar urban settings such as Greater London.
While combination boilers have more moving parts and are thus widely held to be less reliable than tank systems,[4] the twin trends towards replacement of parts based on a pre-set design life and replaceable digital controls for 'traditional' systems has largely eroded this distinction.
In many situations, the initial expense of buying and installing a separate POU heater for every kitchen, laundry room, bathroom, and sink can outweigh the money saved in water and energy bills.
Regular maintenance, such as descaling, is essential for Point-of-use (POU) tankless water heaters to prevent mineral buildup, which can reduce efficiency and increase operating costs.
In recent years, higher-capacity tankless heaters have become more widely available, but their feasibility may still be limited by the infrastructure's ability to furnish energy (maximum electrical amperage or gas flow rate) fast enough to meet peak hot water demand.
In some installations, the energy lost by a tanked heater located inside a building merely helps to heat the occupied space.
During low-flow situations, the hybrid behaves like a tank-type heater by having minimum fixed fuel usage and thermostat activation.
[citation needed] Tankless water heaters can be further divided into two categories according to their heating capability: "full on/full off" versus "modulated".
[8] Under current North American conditions, the most cost-effective configuration from an operating viewpoint often is to install a central (tank-type or tankless) water heater for most of the house, and to install a POU tankless water heater at any distant faucets or bathrooms.
