These are different from vertical systems in that either there are no vertical pipelines at all, or at most just a few. In systems with horizontal piping, metallo-polymer pipes (metal-plastic or reinforced polypropylene) are used as they allow installation inside walls or floors (see Figures 21 and 22).
For balancing one-piped systems, special fittings are used for installation of radiators and these fittings are equipped with bypasses which are installed in them. Balancing of two-piped systems is done by installing thermoregulators on the radiators. Of course all these systems can be designed not only with «dead end» motion of warmth carrier, but also with «continuous flow» motion.
Systems with horizontal piping have problems with air removal. Modern metallo-polymer pipes most often are installed under the floor or behind the wall. Besides that, these pipes have high temperature expansion. Therefore on the pipelines, it is necessary to install U-, L- or ring-shaped compensators for the length. Inclination of the pipelines which allow air to be deleted by counter-flow as was designed in the case of gravity systems is not applicable here. Therefore these schemes can be made only in pumped systems and the air is removed with the help of automatic air removal devices or Mayevsky valves installed on the radiators or including an air separator into the scheme.
All the schemes of gravity or pumped circulation of warmth carrier which have been looked at above are three-way systems of connecting pipelines. Nowadays, not only three-way systems are used, but also «collector» schemes, which can be managed more efficiently.