Off[General::"spell1"];

Tin = 25;
TextNom = -12;
kW = 1000. ;
PzNom = 10  kW;
reskDum = Solve[
            PzNom == 
              kDum*(Tin - 
                    TextNom)][[1]];
c2 = 4186;
resPom = Solve[{PzNom == 
                  kTopeni*(((T1nom + T2nom)/2) - Tin), 
                m2*c2*(T1nom - T2nom) == PzNom} /. {T1nom -> 
                  55, T2nom -> 
                  50}][[1]];
dosStav = Union[reskDum, resPom];

c1 = 1000;
m1 = 0.5;
rceCerpadlo = {Qin == m1*c1*(Tv - Tout), 
              Qout == m2*c2*(Tk - T2), Pel + Qin == Qout, 
              Qout == ϵ*Pel} /. ϵ -> 
              0.5 (( Tk + 273)/(Tk - Tv)) /. 
          dosStav;
rceKotel = {(T1 - Tk)*m2*c2 == 
              Pk, (T1 - T2)*m2*c2 == 
              kTopeni*(((T1 + T2)/2) - Tin), 
            kTopeni*(((T1 + T2)/2) - Tin) == kDum*(Tin - Texter)} /. 
          Union[{Pk -> 0}, dosStav];
Δ1 = Tzem - Texter;
Δ2 = Tzem - Tv;
kS = 150;
Tzem = 2;
rceZemniVymenik = {Qvymenik == m1*c1*(Tv - Texter), 
          Qvymenik == 
            kS*((Δ1 + Δ2)/2)};

ClearAll[rceDohr];
rceDohr[Texter_] = 
      Union[rceKotel, rceCerpadlo, 
        rceZemniVymenik]

res[
          Teplotavenku_] := Solve[
            rceDohr[Teplotavenku]][[1]];
prikon[Teplotavenku_] := 
        Pel /. res[Teplotavenku];
Plot[prikon[tt]/1000, {tt, -12, 13}, 
      PlotStyle -> {Thickness[0.008]}, 
      AxesLabel -> {"Tvenku[°C]", "P[kW]"}]

?If

rceMe[tepl_] := 
      If[tepl < 
          Tzem, {a == 5  b}, {a == 
            6  b}]
rceMe[-5]

?Piecewise