Residential heating systems allowing for the simultaneous production of electricity are a promising application for thermophotovoltaic (TPV) power generators. A simple cost analysis shows that today only TPV-generators making use of Si-PV-cells can be taken into consideration for residential use. We discuss the influence of various parameters onto the electricity conversion efficiency of a thermophotovoltaic central heating unit (TCHU). We find that, in such a system, the expected electricity output amounts even under optimistic conditions to only about 5% of the thermal input if Si-cells are used. Using low band gap cells increases this share to about 10%. We, therefore, conclude that cogeneration of heat and electricity with TPV is only viable if low-band-gap TPV-cells are available at reasonably low costs. However, TPV-systems using Si-cells may still be an interesting option, e.g. for self-powering heating units, i.e. in systems, where the required electricity share is only a few percents of the thermal load.