Next steps on setting up the heat pump - Running Weather Compensation

[MikeSharp01]

The heat pump is running, it has some teething problems but it works very effectively at warming the slab. The heat Meter (EMON Pi) is also up and running. So now I want to move on to setting up the Weather Compensation and see what happens. Looks like you choose a curve setting as best guess and then observe what goes on to fine tune. One thing I cannot see anywhere in the system we have is an internal temperature monitor the typical systems seem too open loop to me. I get the system does not use a thermostat but I don't get how it can get away without having some sort of measure of the internal temperature of the house because it doesn't know how the house is being used, what is coming in via the sun or indeed if the house is cooling or heating more than the output is providing for. Still the heating period has a few weeks to run yet and I want to make every day a school day learning how to heat the place so I guess I should just give it a go, any definitive advice as to the process I should follow.

[Nickfromwales]

16 minutes ago, MikeSharp01 said:

I get the system does not use a thermostat

Panasonic uses the wall mounted controller for internal info. How the feck will yours know what's going on indoors if you don't have one? Other than the difference between flow and return, which would be cruder than a semi-sober welsh plumber.

[JohnMo]

Some simple science 

 

Your floor even on a dull day will not be much hotter than the room, maybe 1 to 3 degs at the surface depending on outside temperature.

 

Let's assume room is 20 degrees, floor surface 23, sun comes out room increases to 23. Now as floor is 23 and room also 23 the floor no longer transfers heat to room. Sun goes away room temp drops, floor starts giving heat to room. With a thick screed (think you have this) the floor just acts as a huge buffer for energy. 

 

Thick screed doesn't act like a radiator due to its thermal capacity, think more a swimming pool, once up to heat it isn't cooling quickly. Once you allow to cool it takes an age to heat up. 

 

Setting up WC is pretty easy for UFH, start with a gradient of 0.5. so every degree outside temperature drops you increase the flow temp 0.5 degs. Start with a flow temp of around 20 or 22 at 10 degs and 28 to 30 at -5. Ideally let it run for 24 to 48 hrs. Adjust curve up down to suit. Small changes have a big change. System doesn't need to know internal temperature. You set the curve to balance heat loss and heat input. You better starting cool rather than hot.

 

WC is completely open loop.

[marshian]

51 minutes ago, MikeSharp01 said:

So now I want to move on to setting up the Weather Compensation and see what happens. Looks like you choose a curve setting as best guess and then observe what goes on to fine tune. One thing I cannot see anywhere in the system we have is an internal temperature monitor the typical systems seem too open loop to me. I get the system does not use a thermostat but I don't get how it can get away without having some sort of measure of the internal temperature of the house because it doesn't know how the house is being used, what is coming in via the sun or indeed if the house is cooling or heating more than the output is providing for. Still the heating period has a few weeks to run yet and I want to make every day a school day learning how to heat the place so I guess I should just give it a go, any definitive advice as to the process I should follow.

 

It's witchcraft - that's all you need to accept - once you accept that it's really easy to understand 😉

 

I set mine up to target a flow temp which resulted in my house being stable internally at 0 Deg C OAT (I started high and worked down - probably quicker to start low and work up but Mrs Alien was not keen on "freezing her ass off while I fecked around with the heating" (True statement)

 

My slope target flow temp at 0 deg OAT ended up at 33 Deg C which in my case was 0.6 as a slope (with no level correction)

 

Once you have the slope you can then fine tune the level

 

All the level is doing is compensating for the fact that heat loss is proportional to temp differential - higher the difference between inside and outside the faster the heat loss

 

example

 

at -2.5 OAT and 21 deg inside the delta is 23.5 Deg C   

at 10 Deg OAT and 21 deg inside the delta is 11 Deg C 

 

So heat from inside moves slower to outside when the differential is lower so proportionally you need to put less heat in to cover for the escape because the escape is at a slower rate

 

So at 10 deg OAT my target flow temp was 27 Deg C and the house over heated so I needed to drop the level to target 26 

 

When I drop the to -3.0 on the level on a slope of 0.6 it changes the target temp at 0 Deg C OAT to 32 Deg (and I want 33)

 

So I increase the slope to 0.7 and that gets me back to 33 Deg C at 0 deg C OAT

 

Told you it's bloody witchcraft

 

[SteamyTea]

WC uses partial differential equations (PDE).

These show a possible solution to a position on a complex curve.  In heat equations, complex curves can be though of as rugby ball.  If you place a rugby ball on a table, and assume it rests level, then you can easily measure the heights along the length from the table.  This will give you a curve.

Now imagine that for every 5mm you move, either left or right, towards the pointy end of the ball, that you measure the circumference.  This will reduce as you move left or right.

So you can see that you have two variables.  Positions on the X and Y axis.

Now imagine that you throw in a third dimension, Z, and you can see that you could recreate the shape of the rugby ball.

This can be written as a PDE with the form:

 

δ²u/δx² + δ²u/δy² + δ²u/δz² = 0

 

All that is really showing is that if you know two positions you can deduce the third i.e. Flow dT, OAT.  This is because of the Laws of Energy Conservation.

 

Easy really.

[MikeSharp01]

8 minutes ago, SteamyTea said:

This is because of the Laws of Energy Conservation.

I get that but it assumes a static envelope if, using your analogy, the rugby ball is shape shifting as you measure the curve would work only when the ball hits the shape it was when you measured it. In my terms if I leave the windows open when it's cold outside the heat will be removed faster than you are making it up so the temperature will drop and once you close the windows the temperature difference will linger for a period which, in a well insulated building, could last a reasonable time. Obviously the reverse also applies so if you have a big party the house will overheat and stay overheated!  Have I got that wrong?

 

 

7 hours ago, Nickfromwales said:

Panasonic uses the wall mounted controller for internal info. How the feck will yours know what's going on indoors if you don't have one?

 

We do have a number of internal temperature sensors and via the BMS connection to the heat pump we should be able to  adjust things if the Weather Compensation (WC) is not effective.

 

On the whole I suspect when I get it all going that the edge cases I set out above, windows open and parties, will not be an issue but I do think it is important to stress test all the assumptions built into the accepted truth.

[SteamyTea]

1 minute ago, MikeSharp01 said:

get that but it assumes a static envelope if, using your analogy, the rugby ball is shape shifting as you measure the curve would work only when the ball hits the shape it was when you measured it.

Yes, and it is why there are, sometimes, no solutions.

Though you can put a time element into the mix, which you, in effect, be enthalpy.

But as you are working within limits, and tight limits as well, it is probably close enough.

[JohnMo]

2 hours ago, MikeSharp01 said:

if I leave the windows open when it's cold outside the heat will be removed faster than you are making it up so the temperature will drop and once you close the windows the temperature difference will linger for a period which, in a well insulated building, could last a reasonable time

The assumption is incorrect though. Open window you let cold air in hot out, but the thermal capacity of air is very low, building fabric the thermal capacity is very high. You close the window, the air within the building is reheated by the now warmer fabric (especially the floor) pretty quickly. You floor will not loose heat for many hours/days even with heat source off.

[JohnMo]

The other acceptable running mode is pure thermostat, set a fixed flow temperature of 28, still run as a single zone. Your thick screed will demand very long times and at that temp CoP will be great.