Some basic HP questions for those in the know

[Gordo]

hi i'm designing my HP system in preparation for the "professionals" design. 

 

Heatpunk design comes out at 5kw heat pump @ 45 degrees for an annual heat load of just over 11,000 kwh for space heating. My historical heating works out at 12,500 kwh, with a pellet boiler and includes DHW and some supplementary heating with a small log stove. Therefore im pretty content with validity of the heatpunk design output. its a reasonably well insulated 2003 house about 180m2 floor area

 

what im intrested to know is how the heat pump modulates its output.

 

I assume the flow rate is fixed?

 

do heat pumps modulate the flow temperature up / down or is that generally set (fixed) at the controller. then it just modulates the input electric to maintain the set flow temperature until it can go no lower and shuts down for a period (cycling)

 

unassuming the flow temperature is set (fixed) at controller is it a matter of playing with controller to come on the minimum flow temperature needed.

 

how does weather compensation come into the equation. will this then adjust the flow temperature automatically.

 

probably silly questions for those in the know. lol

 

Thanks in anticipation

Edited yesterday at 00:29 by Gordo

[JohnMo]

7 hours ago, Gordo said:

assume the flow rate is fixed?

Some will modulate flow don't.

 

Basics are

Target flow temp, dT, max and min flow temp (hysterisis).

 

So compressor starts, at min flow temperature, once started tries to establish dT at minimal flow temp, as dT decreases, more flow temp is added, up to target flow temp. Once there different manufacturer do different things, but it's all about power input, to maintain dT and flow temp.

 

WC is just a variable flow target temp based on outside temp.

[ReedRichards]

9 hours ago, Gordo said:

I assume the flow rate is fixed?

 

1 hour ago, JohnMo said:

Some will modulate flow don't.

I beg your pardon, @JohnMo?  I think he means "not necessarily".

9 hours ago, Gordo said:

do heat pumps modulate the flow temperature up / down or is that generally set (fixed) at the controller. then it just modulates the input electric to maintain the set flow temperature until it can go no lower and shuts down for a period (cycling)

Heat pumps have a target flow temperature.  But it may take them a while to reach that target.  They will indeed shut down if the target is exceeded (by some pre-set margin) at their minimum electrical input.

9 hours ago, Gordo said:

unassuming the flow temperature is set (fixed) at controller is it a matter of playing with controller to come on the minimum flow temperature needed.

You would try to establish the the minimum flow temperature needed for your desired interior temperature at each particular outside temperature.

9 hours ago, Gordo said:

how does weather compensation come into the equation. will this then adjust the flow temperature automatically

Once you have done that, you use weather compensation to render it automatic so you can stop playing around.   

[JohnMo]

47 minutes ago, ReedRichards said:

beg your pardon, @JohnMo?  I think he means "not necessarily".

Just reread

Meant to write 

2 hours ago, JohnMo said:

Some will modulate flow, some don't.

And as way of example - mine does modulate the circulation pump and I cannot change that. Others will have external pump(s) or some like Grant have an internal pump that runs at a fixed speed, the speed is adjustable, but remains fixed after that.

[sharpener]

For example by default Vaillant HPs modulate the compressor, fan and pump to maintain a dT of 5C. This requires a flow rate of 170 l/hr per kW of output. It is possible to define the building circulation rate separately for both HW and CH but I don't know of anyone who has done this or why they would.

 

I recommend reading the detailed analysis by Michael Podesta of how their weather compensation works. It is complicated, and spread over this and the three succeeding articles. Not helped by some errors in the Vaillant manuals in particular the basic graph illustrating how WC works(!).

[Gordo]

thanks guys that advice was helpful.

 

so most HP have a fixed flow rate.

HP all vary flow temperature based on how far off from the set target flow temperature

weather compensation will auto adjust target flow temperature based on inside/outside temperatures based on some selected algorithm (graph)?

 

think i got it.

[ReedRichards]

6 hours ago, Gordo said:

weather compensation will auto adjust target flow temperature based on inside/outside temperatures based on some selected algorithm (graph)?

 

It's usually a straight line; nothing sophisticated.

[JohnMo]

7 hours ago, Gordo said:

 

so most HP have a fixed flow rate.

I wouldn't go that far, some do have a fixed flow rate, some don't.

 

7 hours ago, Gordo said:

HP all vary flow temperature based on how far off from the set target flow temperature

Don't confuse house temp and heat pump target temp. The heat pump has zero care about what happens in the house, it's just a heat generator. It looks after it's self. The target temp referred too is heat pump target flow temperature, the heat pump is always trying to get to that temperature, while also managing dT. It manages dT first to protect itself from damage of the refrigerator circuits.

 

7 hours ago, Gordo said:

weather compensation will auto adjust target flow temperature based on inside/outside temperatures

WC just takes reference to outside temperature only. Doesn't care about inside temperature, just simply follows a straight line curve based on outside temp. You adjust the curve so it coincides with a house temp you like.

 

You have variations of the above, like load compensation, which does take references to house temp, these work well with radiators but not with UFH.

[JamesPa]

44 minutes ago, JohnMo said:

You have variations of the above, like load compensation, which does take references to house temp

Just to add to what @johnmo says, these variations typically work in addition to weather compensation not instead of.  Some more middle-market-upwards heat pumps feature a mode called something like 'adaptive' (or similar terminology), which automatically 'tweaks' the basic WC curve based on what the heat pump learns about the house over a period of time.  This is not the same as responding directly to room temperature (like a thermostat would), it had a much longer time constant for the response.  Various third party controllers, of which the best known is Homely, do something similar (and other smarts in addition) for heat pumps that don't feature this kind of supplementary control.

 

Note though

44 minutes ago, JohnMo said:

these work well with radiators but not with UFH

 

 

Edited 3 hours ago by JamesPa