Help me to understand GSHP performance

[Benguela]

Hello Everyone,

 

I'm new here and love trawling through the questions and answers in the GSHP part of the forum.

 

A little about me: I live in Norfolk and have a GSHP (Stiebel Eltron WPF 13 S). I've got about 700 m of brine loops in my garden and the WPF is paired with a 100 l buffer tank.

 

 

I know that one of the most important efficiency considerations is to prevent short cycling - run the machine continuously rather than stop/start. Most heat pumps are either on or off... the compressor runs at 100% or nothing. But some heat pumps are variable speed... How can I work out if my heat pump is an inverter model or a fixed output model? I googled the model number and 'variable speed' and 'inverter' but nothing obvious comes up. 

 

More important, I'm beginning to wonder if that 100l buffer is too small. If I had a bigger buffer, that would also smooth out the short cycling, not so? Any ideas what difference a larger buffer would make?

 

Thanks for any advice.

 

~Benguela

 

Edited November 14, 2021 by Benguela
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[ProDave]

If the heat pump supplier does not state in big letters that it is inverter driven, then it won't be.  I would not buy anything other than an inverter heat pump for a house.

[Benguela]

Thanks ProDave,

 

What's making me wonder is this. The performance tool for the hit pump is online at https://www.stiebel-eltron.co.uk/toolbox/waermepumpe/... and it shows a reading for Inverter Speed % (look on the right hand side in the picture below).

 

Maybe it's just a standard display thing in how Stiebel makes these graphs... or maybe it is an inverter model after all.

 

[SteamyTea]

https://www.stiebel-eltron.co.uk/en/products-solutions/renewables/heat_pump/brine-water_heatpumps/wpf_05_07_10_13_s/wpf_13_s/technical-data.product.pdf

 

From the limited info it seems it is non inverter and you just buy a different capacity model.

But have a good look in the controller submenues. Who knows what may be hidden in there.

 

[Wil]

During my research on GSHP I came across one manufacturer (Kensa) who ONLY made direct speed GSHPs and claimed that it was more efficient to run hard and stop rather than have an inverter driven unit. Possibly to do with the almost constant ground temperature for the brine? Also no inverter to die on you at some point...

 

ASHP needs an inverter all the way.

[SteamyTea]

32 minutes ago, Wil said:

During my research on GSHP I came across one manufacturer (Kensa) who ONLY made direct speed GSHPs and claimed that it was more efficient to run hard and stop rather than have an inverter driven unit.

Will that be the very small, Cornish, Kensa, that employed a mate of mine.

I would take that claim with a pinch of Cornish Sea Salt.

[Wil]

Given their sponsorship of Exeter Chiefs, they're probably in that direction. They don't look particularly small from their website/ team, but who knows. Wasn't a recommendation or otherwise...

[saveasteading]

On 14/11/2021 at 14:35, ProDave said:

I would not buy anything other than an inverter heat pump for a house.

If you can spare time to expand a little, I would be very interested.

 

1. Variable speed works better because....?

2. How is the optimum speed decided and set? Is there a +/- set on a master thermostat to avoid cycling?

3. If an inverter changes  DC to AC, and we start with AC anyway, what is happening and how does this control the fan?

 

[ProDave]

The big issue with a non inverter heat pump is the high start up current, the DNO's get worried about them, causing your lights to dip when a large direct start compressor starts.

[saveasteading]

Thanks, that matters as would be really annoying, and might trip fuses.

 

Might a slower running fan work better anyway, especially in cold weather? Just a hunch and have not troubled myself with the physics.

 

[SteamyTea]

35 minutes ago, saveasteading said:

Might a slower running fan work better anyway, especially in cold weather

You don't have a fan on a GSHP.

It is the compressor that needs the omph to get started. Think a large diesel engine starting from cold. Except higher pressures.

[saveasteading]

34 minutes ago, SteamyTea said:

You don't have a fan on a GSHP.

That would be messy, I agree.

 

4 hours ago, Wil said:

ASHP needs an inverter all the way.

 

My queries remain.

Does the inverter do anything other than allow a steady start?

Do heat pumps run at other than 100%, and how?

[dpmiller]

ASHPs modulate just like a gas boiler.

[SteamyTea]

3 hours ago, saveasteading said:

Does the inverter do anything other than allow a steady start?

Yes.

By modulating the power during a heating cycle, they can optimise the CoP.

This is because heating is not linear.  When you have a large temperature difference between a hot body and a cold surroundings, you have high losses, when the temperature difference is small, you have lower losses.  Therefore, when heating initially starts, you can put in less energy for a similar temperature rise.  As temperature parity gets closer, you have to put in more energy (mathematically, when the surroundings is an infinite heat sink, you can never reach parity, unless you have infinite energy.  This is why 'warp drives' don't exist, but the infinite probability drive might).

By having the control to modulate during the heating cycle, frosting can be reduced i.e. work the ASHP less hard or take less heat out the ground, for a bit longer.  Then increase the flow temperature from the HP as you reach maximum temperature, but for short amount of time.  Frosting is a combination of time, flow rates, temperature differences, and in the case of ASHP, relative and absolute humidity.

It is all about finding the sweet spot as all the above varies.

Worth remembering the the overall temperature differences that a HP works are are similar to a thermal boiler, just lower down the scale.

Why we should really use the kelvin scale and not the pesky, arbitrary celsius one.

Or create a new temperature scale that has zero at the coldest that the refrigerant gas gets to.  Then you will see temperature rises of 90 STs (going to name this scale after me, have to be careful though as we have an St, for stanton number, St = h/pvcp, where h is coefficient of heat transfer, p is density, v is velocity, and cp is specific heat capacity at constant pressure, it is used in geothermal energy, well it should be, but I think it gets forgotten about).

[Wil]

Using Steamy Teas as a measurement of a unit of temperature seems like something that will destroy the minds of our US inhabitants. Cup or mug? With milk or without? Does sugar effect the relativity?

[SteamyTea]

56 minutes ago, Wil said:

Using Steamy Teas as a measurement of a unit of temperature seems like something that will destroy the minds of our US inhabitants. Cup or mug? With milk or without? Does sugar effect the relativity?

Mug.

Milk.

Sugar is best in a pudding.

 

The real secret is that if you try to extract more than half the energy avialable, then systems start to get inefficient. This us not the same as having a system that is double the required size.

This is really what sets the sizing if a system.

[IanMcP]

On 18/11/2021 at 15:56, ProDave said:

The big issue with a non inverter heat pump is the high start up current, the DNO's get worried about them, causing your lights to dip when a large direct start compressor starts.

I’m in the process of choosing air or ground (plenty of field to dig). I know the start up draw is an issue as I have single phase and limited amps. Does an inverter address this ? 
 

got a zillion other questions but starting here ??‍♂️
 

[saveasteading]

4 hours ago, IanMcP said:

I’m in the process of choosing air or ground

Ground source as in borehole depends completely on the ground composition. Do you know what is there?

[IanMcP]

7 hours ago, saveasteading said:

Ground source as in borehole depends completely on the ground composition. Do you know what is there?

i don’t need to go borehole but not aware of ground composition yet. Just comparing performance and costs as standard. GS seems better - more regular , quieter ,  stable ? 

[joe90]

1 hour ago, IanMcP said:

i don’t need to go borehole but not aware of ground composition yet. Just comparing performance and costs as standard. GS seems better - more regular , quieter ,  stable ? 

I have plenty of ground but the general opinion here was GSHP needs a lot of brine, is noisier, more expensive, a lot of work!

[SteamyTea]

28 minutes ago, joe90 said:

is noisier

That is just bad installation really.

Not as if you would put an oil furnace in the living room.

There is no real reason they have to be inside, just lazy plumbers really.  Easy enough to build a small plant room outside, most units are pretty small.

[joe90]

56 minutes ago, SteamyTea said:

That is just bad installation really.

Not as if you would put an oil furnace in the living room.

There is no real reason they have to be inside, just lazy plumbers really.  Easy enough to build a small plant room outside, most units are pretty small.

From my reading the “mechanics “ were an indoor unit rather than ASHP outdoor units and others have said they were a tad noisy .

[Nickfromwales]

Be interesting to hear what the installed cost is for a typical GSHP with a borehole, fitted and finished….

[SteamyTea]

4 minutes ago, joe90 said:

From my reading the “mechanics “ were an indoor unit rather than ASHP outdoor units and others have said they were a tad noisy .

They are just the same as a split ASHP really, except the bit with the fan is replaces with a pipe.

But, just like a split ASHP, the compressor and pumps can be in an outbuilding.

 

These are the sizes of the two Kensa pumps

H X W X L (mm)

515 (H) X 480 (W) X 360 (D)

585 (H) X 610 (W) X 595 (D)

[SteamyTea]

15 minutes ago, Nickfromwales said:

with a borehole,

That bit depends where you are, it is geology dependant.

 

GSHPs should really be called water source heat pumps.