diff --git a/src/Blocks/continuous.jl b/src/Blocks/continuous.jl
index 81a78aa0a..de76b19cd 100644
--- a/src/Blocks/continuous.jl
+++ b/src/Blocks/continuous.jl
@@ -20,7 +20,7 @@ Initial value of integrator state ``x`` can be set with `x`
 @mtkmodel Integrator begin
     @extend u, y = siso = SISO()
     @variables begin
-        x(t) = 0.0, [description = "State of Integrator"]
+        x(t), [guess = 0.0, description = "State of Integrator"]
     end
     @parameters begin
         k = 1, [description = "Gain"]
@@ -65,7 +65,7 @@ Initial value of the state ``x`` can be set with `x`.
 @mtkmodel Derivative begin
     @extend u, y = siso = SISO()
     @variables begin
-        x(t) = 0.0, [description = "Derivative-filter state"]
+        x(t), [guess = 0.0, description = "Derivative-filter state"]
     end
     @parameters begin
         T = T, [description = "Time constant"]
@@ -122,7 +122,7 @@ See also [`SecondOrder`](@ref)
         lowpass = true
     end
     @variables begin
-        x(t) = 0.0, [description = "State of FirstOrder filter"]
+        x(t), [guess = 0.0, description = "State of FirstOrder filter"]
     end
     @parameters begin
         T = T, [description = "Time constant"]
@@ -545,7 +545,7 @@ linearized around the operating point `x₀, u₀`, we have `y0, u0 = h(x₀, u
     end
     @named input = RealInput(nin = nu)
     @named output = RealOutput(nout = ny)
-    @variables x(t)[1:nx]=x [
+    @variables x(t)[1:nx]=x [ # FIXME: should it be guess = x, not default? 
         description = "State variables of StateSpace system $name"
     ]
     # pars = @parameters A=A B=B C=C D=D # This is buggy
@@ -618,7 +618,7 @@ See also [`StateSpace`](@ref) which handles MIMO systems, as well as [ControlSys
     @parameters a_end = ifelse(a[end] > 100 * symbolic_eps(sqrt(a' * a)), a[end], 1.0)
 
     pars = [collect(b); a; collect(bb); d; a_end]
-    @variables begin
+    @variables begin # FIXME: should it be guesses, not default? 
         x(t)[1:nx] = zeros(nx),
         [description = "State of transfer function on controller canonical form"]
         x_scaled(t)[1:nx] = collect(x) * a_end, [description = "Scaled vector x"]
diff --git a/src/Blocks/utils.jl b/src/Blocks/utils.jl
index 10ad0261f..f6d72cdec 100644
--- a/src/Blocks/utils.jl
+++ b/src/Blocks/utils.jl
@@ -115,36 +115,47 @@ Connector with an array of output signals of type Real.
 """ RealOutputArray
 
 """
-    SISO(;name, u_start = 0.0, y_start = 0.0)
+    SISO(;name, u_guess = 0.0, y_guess = 0.0)
 
 Single input single output (SISO) continuous system block.
 
 # Parameters:
 
-  - `u_start`: Initial value for the input
-  - `y_start`: Initial value for the output
+  - `u_guess`: Initial value for the input
+  - `y_guess`: Initial value for the output
 """
-@mtkmodel SISO begin
-    @parameters begin
-        u_start = 0.0
-        y_start = 0.0
+@component function SISO(; name, u_start = nothing, y_start = nothing, u_guess = 0.0, y_guess = 0.0)
+    if u_start !== nothing
+        Base.depwarn(
+            "The keyword argument `u_start` is deprecated. Use `u_guess` instead.", :u_start)
+        u_guess = u_start
     end
-    @variables begin
-        u(t) = u_start, [description = "Input of SISO system"]
-        y(t) = y_start, [description = "Output of SISO system"]
+    if y_start !== nothing
+        Base.depwarn(
+            "The keyword argument `y_start` is deprecated. Use `u_guess` instead.", :y_start)
+        y_guess = y_start
     end
-    @components begin
-        input = RealInput(guess = u_start)
-        output = RealOutput(guess = y_start)
+    pars = @parameters begin
+        u_guess = u_guess
+        y_guess = y_guess
     end
-    @equations begin
+    vars = @variables begin
+        u(t), [guess = u_guess, description = "Input of SISO system"]
+        y(t), [guess = y_guess, description = "Output of SISO system"]
+    end
+    
+    @named input = RealInput(guess = u_guess)
+    @named output = RealOutput(guess = y_guess)
+
+    eqs = [
         u ~ input.u
         y ~ output.u
-    end
+    ]
+    return ODESystem(eqs, t, vars, pars; name = name, systems = [input, output])
 end
 
 """
-    MIMO(; name, nin = 1, nout = 1, u_start = zeros(nin), y_start = zeros(nout))
+    MIMO(; name, nin = 1, nout = 1, u_guess = zeros(nin), y_guess = zeros(nout))
 
 Base class for a multiple input multiple output (MIMO) continuous system block.
 
@@ -152,15 +163,26 @@ Base class for a multiple input multiple output (MIMO) continuous system block.
 
   - `nin`: Input dimension
   - `nout`: Output dimension
-  - `u_start`: Initial value for the input
+  - `u_guess`: Initial value for the input
   - `y_start`: Initial value for the output
 """
-@component function MIMO(; name, nin = 1, nout = 1, u_start = zeros(nin),
-        y_start = zeros(nout))
-    @named input = RealInput(nin = nin, guess = u_start)
-    @named output = RealOutput(nout = nout, guess = y_start)
-    @variables(u(t)[1:nin]=u_start, [description = "Input of MIMO system $name"],
-        y(t)[1:nout]=y_start, [description = "Output of MIMO system $name"],)
+@component function MIMO(; name, nin = 1, nout = 1, u_start = nothing, y_start = nothing, u_guess = nin > 1 ? zeros(nin) : 0.0, y_guess = nout > 1 ? zeros(nout) : 0.0)
+    if u_start !== nothing
+        Base.depwarn(
+            "The keyword argument `u_start` is deprecated. Use `u_guess` instead.", :u_start)
+        u_guess = u_start
+    end
+    if y_start !== nothing
+        Base.depwarn(
+            "The keyword argument `y_start` is deprecated. Use `y_guess` instead.", :y_start)
+        y_guess = y_start
+    end
+    @named input = RealInputArray(nin = nin, guess = u_guess)
+    @named output = RealOutputArray(nout = nout, guess = y_guess)
+    @variables begin
+        u(t)[1:nin], [guess = u_guess, description = "Input of MIMO system $name"]
+        y(t)[1:nout], [guess = y_guess, description = "Output of MIMO system $name"]
+    end
     eqs = [
         [u[i] ~ input.u[i] for i in 1:nin]...,
         [y[i] ~ output.u[i] for i in 1:nout]...
diff --git a/src/Thermal/HeatTransfer/ideal_components.jl b/src/Thermal/HeatTransfer/ideal_components.jl
index fc6858bbf..70dfe446c 100644
--- a/src/Thermal/HeatTransfer/ideal_components.jl
+++ b/src/Thermal/HeatTransfer/ideal_components.jl
@@ -24,8 +24,8 @@ Lumped thermal element storing heat
         C, [description = "Heat capacity of element"]
     end
     @variables begin
-        T(t) = 273.15 + 20
-        der_T(t)
+        T(t), [guess = 273.15 + 20]
+        der_T(t), [guess = 0.0]
     end
 
     @equations begin
diff --git a/src/Thermal/utils.jl b/src/Thermal/utils.jl
index e2a991494..fe50d4c7d 100644
--- a/src/Thermal/utils.jl
+++ b/src/Thermal/utils.jl
@@ -1,6 +1,6 @@
 @connector HeatPort begin
     T(t) = 273.15 + 20.0
-    Q_flow(t), [connect = Flow]
+    Q_flow(t) = 0.0, [connect = Flow]
 end
 Base.@doc """
     HeatPort(; name, T = 273.15 + 20.0, Q_flow = 0.0)
@@ -35,8 +35,8 @@ flow rate through the element from `port_a` to `port_b`, `Q_flow`.
         port_b = HeatPort()
     end
     @variables begin
-        dT(t)
-        Q_flow(t)
+        dT(t) = 0.0
+        Q_flow(t) = 0.0
     end
     @equations begin
         dT ~ port_a.T - port_b.T
@@ -69,8 +69,8 @@ flow rate through the element from `solid` to `fluid`, `Q_flow`.
         fluid = HeatPort()
     end
     @variables begin
-        dT(t)
-        Q_flow(t)
+        dT(t) = 0.0
+        Q_flow(t) = 0.0
     end
     @equations begin
         dT ~ solid.T - fluid.T