Ämne: Bosch Compress 6000 Installation

[prpr]

Hello, I hope I will not annoy you too much by writing in English, unfortunately there is no good forum in my country for discussing heat pumps, and this place has been an invaluable resource of information. I have been lurking for a long time and reading all the discussions and the advice the smart people here are giving. Mostly through a translator, I know very little basic Swedish - lived in Sweden for 3 months almost 20 years ago.

I have a 200m2 house, that has basement, 2 floors and attic (attic not included in m2). Some parts of the basement and the two floors are heated - approx. 150m2 total.
The house is very old (1959), but I have been renovating it over time and improving the insulation.
When I purchased the house it had a light oil heater connected to a new aluminum radiator system and a couple of underfloor zones.
Now I have a total of 6 underfloor zones (3 bathrooms, kitchen, foyer, one bedroom) and 9 radiators. Each underfloor zone and each radiator has it's own thermostat.

About 4 years ago, I had the light oil burner replaced with a ground source heat pump. I have a total of 400m of pipe in the ground (horizontal collector). I live by the sea, and the soil is very wet, the pipe is basically in water saturated sand, so the collector is not cooling too much.

Unfortunately the company who installed the heat pump did almost everything wrong that they could have:
* The pump was under-dimensioned. They fitted a Compress 6000 6LW. At that point I did not have all rooms in use, and afterwards I also installed recuperating ventilation system, which while 80% efficient, still is losing some heat. The result is that when the pump was installed it could work down to -5C without using the electrical heater, and after using and heating the whole house + adding the recuperation system, it could only manage down to -1C before having to turn on electrical aux.
* The hydraulics and buffer tank were connected completely wrong.
* The pump feed sensor was in the wrong place, causing the pump to not see the current temperature and almost boil the water (due to enabling additional heater), and shut off from overheat.
* One buffer tank on the bill, but another one in reality.
* They basically disappeared after and did not help me to solve any issues

Probably the only things that were done well is that they said there is no point to fit an inverter with my system, and that a an on/off with buffer tank is a better solution due to all the thermostats in the system that would make a problem with inverter without buffer tank. Also I had a lot of room in my cellar (but low ceiling), so I have heat pump, separate boiler (300L) and separate buffer tank (300L).

After reading the installation manual a little, I moved the sensor to the top of the buffer tank from the feed past the buffer tank, this stopped the pump from cutting out, as it now could see the correct temperature, even when all the thermostats were closed.
At this point I was not aware that the pump was actually using the aux heater all the time when it was colder. I left it like this for 2.5 years... However, last year the electricity price hike started, and I had quite high consumption, so I wanted to understand more what was going on.

The first thing I did was find an MBLan2 module for 50 EUR on eBay, that someone did not need. I then pulled all the data from this module to Home Assistant. Unfortunately it is very limited as to what information it is possible to get from the heat pump, from the important information it is possible to see:
* Compressor on or off
* Heat curve setpoint for heating circuit (but not the actual temperature!)
* Actual temperature of the hot water circuit.
* Outside temperature
* Additional heater activation %

Because I did not see the actual temperature of the heating circuit, I also bought a Sonoff TH10 switch with a DS18B20 temperature sensor, and put the sensor on the feed, so I could observe the feed temperature also digitally.
Around the same time I got an underfloor controller, and since I have 2 external circulation pumps (1 for radiators, 1 for floors) I connected both circulation pumps to the controller, so that the UFH pump only runs when one of the floor thermostats is open, and I added an outdoor temperature sensor and made the radiator circulator run only if it is colder than 15C outside (above 15C I don't need radiator heating, it is always >22C in the house).

Now I could actually see what is going on, and realized that what I saw was not good. Below -1C the aux heater was being used quite a lot. At -8C I already had 30% average duty cycle on the aux heater. As the aux heater is 9 kW it means that the aux heater was using more power than the entire heat pump, basically doubling my electricity consumption. In colder months 2400kWh consumption was not uncommon - I don't have a separate meter on the pump, but in summer my consumption is maybe 200-300 kWh.
The pump runtime was about 4400h per year, so this also gave a clear indication that the pump was undersized.

This all led to the fact that I decided that I need to sell this current pump while it is still more or less fresh and get a 8 kW pump instead.
I then started learning and looking at the hydraulics. Just to say - I am a normal guy, I have a electronics/software background, but I knew nothing of heating systems, and just trusted blindly before what was told to me... what I found was quite shocking.

Here is a schematic of how the tank was connected:


A such connection caused complete mixing of feed and return in the buffer tank. Because of this the HP was always working with 5-6 degrees higher temperature, than what was going into the radiators. For example if the radiators were receiving 40C then the heat pump output was 46-47C. This made the undersized heatpump problem even worse.
I then started looking at the buffer tank, and while on invoice was a 200L buffer tank with 4 ports, in reality, I had a 300L buffer tank with 2 ports and slight baffles, like this:

To prevent return and hot water from mixing, and provide the heatpump output direct to load, I had only one option to connect the heat pump, if I wanted to use this tank:


A better connection would be with three points, with the return always going through the bottom of the tank, but this requires a tank with connections on both sides, which I didn't have. With this setup care must be taken with tuning the flow parameters of the pumps to make sure that the buffer tank load is properly engaged when the HP is operating - the return of the buffer tank should mix with the return of the load, to present the lowest possible temperature to the heat pump and increase COP.

So I ordered new pump (Compress 6000 8LW) and implemented the schematic. It looks like this in real life behind the tank, the thing in foreground is a spring loaded check valve to prevent siphoning through the HP when it is off, and there is an immersion well in the tank for the T1 sensor:


To tune the circuit, I added a lot more temperature sensors. I flashed my SonOff TH10 switch with ESPHome firmware, to read multiple sensors and daisychained 3x DS18B20 temperature sensors, so that I can monitor digitally:
* HP output
* Tank actual temperature (same as pump T1)
* Load input (to heating system)
It is possible to add many more sensors, but I did not see the point.

And then I started tuning, so that I do not get too many starts (currently 12-13 starts per day), outside temp 2C HP is at 71% duty cycle, prevent overheating as much as possible and also that I do not have any noticeable temperature fluctuation in the house. Right now it is +/- 0.1C, sometimes 0.2C, not more.
Here is an example of 2 cycles. The first one is a normal cycle, and second one is a cycle with DHW demand coming in:


I had to set the Delta T on the heating circuit of HP to 5C (from 7C), otherwise the tank takes a while to become engaged, because almost all the flow is going directly to/from the load and even though the recovery temperature is hotter, overall the input temperature of the heat pump is larger on average. The difference on the G2 circulator pump was 9% compared to 7C Delta T (61% vs 70%), I think it is worth it. Even now on turn-on the temperature recovery is very good - the input temperature jumps up directly by 4C.
This step would be not neccessary if I had a tank with 2 ports at the bottom, then I could have made the return always pass the tank without causing big turbulence and preserving the temperature stratification in the tank. My Delta T brine is between 2.6K and 3.1K right now, depending on efficiency of pump (DHW or HC and HC start vs HC end).

Final result now is that I don't have massive temperature fluctuations in the house, and my HP duty cycle at 2C outside went from 96% to 71%. Not only because the pump is more powerful, but also because it is not heating 5-6C higher all the time!

Some more general Bosch Compress 6000 (probably applicable to 5000 also) things I noticed:
* The brine pump should be adjusted to 3K Delta T. From factory it's usually set to max, but this gives Delta T of 2K and is wasting energy with most collectors, you can lower the speed by 50% and still have 3K Delta T over brine.
* The PID controllers (or rather PI) are tuned quite badly from factory, specifically:
  - PID for Additional heater should be set to P = 3 and I = 100. From factory it has very high I, this causes the pump to always do a cycle if aux heater turns on, because aux heater stays on much too long after reaching setpoint temperature, this causes it to overshoot and then turn the pump off, and you use even more aux heat, as the pump is not running for 15 mins. This goes into an infinite loop.
  - PID for G2 circulation pump for me worked better with P = 5, I = 100 and in hot water mode P = 10, I = 100 than the default settings. The pump was able to reach the target Delta T much quicker and hold it much better during changes.
* Instead of degree minutes (like on Nibe) on the T1 setpoint the Bosch Compress uses a variable hysteresis. The default is minimum 4K, max 25K, time factor 20. I never changed the time factor, but I think these settings are made for an installation without tank. If you have a tank, it is better to make it less conserative. I ended up with 3.5K min and 12K max, but 4K and 16K is also quite good. Otherwise it overheats more and lets the water cool down too much, and you start to get temperature fluctuations in the house.

If you made it through all this text - congratulations and thank you.

To the people much smarter than me - how does the pump tuning look? Right now at 2C, and generally between 0-5C is probably where I have the biggest amount of starts, this is now at ~12-14 starts per day, but in colder and also warmer weather it should be 6-7 starts or less. Do you think this is OK? You can of course reply in Swedish :)

[Carl N]

Hi!
Looks good overall, but there is one thing i see.
If you have a T-connection on the supply from the heat pump, the temperature sensor that control the compressor should be on the pipe and not in the tank.
If you balance the flow over heat pump = flow load then the flow in/out the tank will be zero.

You can also move the connetion from tank to supply the heat system to another connection on the tank and only have a T on the return flow.
This will not mix the temperature in the tank too mush, you should use these connections then.

[prpr]

Hello, thank you for reading my wall of text, and thinking also about the issues.

Hi!
Looks good overall, but there is one thing i see.
If you have a T-connection on the supply from the heat pump, the temperature sensor that control the compressor should be on the pipe and not in the tank.
If you balance the flow over heat pump = flow load then the flow in/out the tank will be zero.

Here the problem happens that if some underfloor zones are open, then there is very little flow through the tank. This will cause the pump to shut off before the the tank has had a chance to warm up.
The major flow through the tank only starts to happen after the heat load has reached the desired temperature. This is because the system is configured as direct-to-load, which should be the most efficient way to connect accumulator to heat pump from COP and temperature recovery perspective, and unfortunately I can not pass the return through the tank, this would solve the problem, an then the sensor could be on the feed.

Also, the installation manual of the heat pump states:
"In case of using an accumulator tank the sensor should be placed in the upper region of the accumulator tank".
I think for the same exact reason, so you do not get a problem with short cycling the pump if the flow demand from the load is very large, because if the HP shuts off and the tank has not heated, the temperature drops extremely fast, and after 15 mins minimum timeout the HP has to turn on again. So the tank is not working correctly.

I agree with what you said, but I think it is a bit different philosophy. My philosophy is that first the house has to be warm. And then if the load temp goes a bit higher than is in the curve (because the tank has not heated yet), then it just goes... because it probably means that the heat curve is set a little too low for current conditions - maybe it is more windy than usual etc, but at least I do not get temperature fluctuation in the house, and of course once the temperature in the house is correct, then the thermostats start to close, and all the heat starts to go to the tank, and then once the tank is heated the HP turns off, as there is nowhere to put the heat anymore.

You can also move the connetion from tank to supply the heat system to another connection on the tank and only have a T on the return flow.
This will not mix the temperature in the tank too mush, you should use these connections then.

The thing is, that my tank (300L model) does not have this connection. This is why I have drawn the red cross in the picture, because it does not exist. The next larger connection is in the middle of the tank.
But even then, I think that passing the heat pump heat directly to the heat load is more beneficial, because this allows to have lower temperature curve.

How it is now, it is the lowest possible mixing happening in the tank, and my 300L tank at 2C now is sufficient to heat the house for 40 minutes after a cycle at 2C outside temp. The only thing that could be even better is if the tank had two parallel connections at the bottom, so the return from the load could enter on one side, and at same level on other side the return to the HP could be placed. But unfortunately I have only 4 connections - two in middle, 1 on top, 1 on bottom. The other connections are present at tanks of this series starting from 500L...

Here is a good article about the different ways to connect the tank:
https://www.pmengineer.com/articles/94749-john-siegenthaler-combining-heat-pumps-with-buffer-tanks

I think probably the best is the 3 pipe system for a heat pump. 2 pipe system like I have requires a lot of fine tuning of the circulator speeds to prevent the issue that the tank is not used (but the recovery is of course the fastest). 4 pipe system loses a little extra heat of the heat pump when passing through the tank, so the COP will be lower...

P.S.
Here is a bit bigger picture of tank, you can see the larger 1 1/2 connections are top, bottom and 3x in middle:

[Carl N]

Hmmmm...
Maybe you can turn the tank upside down and get better connections for the supply? 
The heated water from the heatpump vill go up in the tank, but I don't think the connection in the middle will be OK?

[prpr]

Hmmmm...
Maybe you can turn the tank upside down and get better connections for the supply? 
The heated water from the heatpump vill go up in the tank, but I don't think the connection in the middle will be OK?

Hehe, it is not possible, because the tank has the vent on the top only, and also at the bottom there are feet welded to the tank
Connection in middle for sure will not be ok, this will mix entire tank, and have again the problem like before that HP output will be 5-6C hotter than output to load. This is about 7-10% loss in COP.
I think supply is perfectly fine like it is. Here is a good video why T should be used and heat pump passed direct to load:

I do not agree with everything they say there (especially regarding stratification of the buffer tank, as I can clearly see it works very well in my case), but the COP increase from bypassing the tank on the supply side can not be denied.
In my case the problem is that the return does not pass through the tank, no issues on supply side, I would connect any tank this way. But it would be better if load return would enter at different location at the bottom (preferrably exactly opposite pump return, to reduce vertical mixing), then you get always the coldest water into the HP automatically, without having to do black magic with the circulators. This gives the best COP.

I think the system is working quite well now hydraulically. It was a pain to tune it though. Maybe I am having tiny losses because G2 is running at a bit faster speed (9%), perhaps it draws 5-10W more.

The only thing hydraulically I could change now in the tank is put the load return through the port where the draining valve is, that is just above the return to the heat pump.
But this port is only 1", not 1 1/2". And that I think can be a restriction. Now I have the 1" (Alupex 28mm) return from radiators and 1" (Alupex 28mm 1") return from UFH joining into 1 1/4" (Alupex 32mm) and then this is going into the T that is 1 1/2", and again 32mm to the pump. I don't think it will be too good idea to put both radiator and UFH return into a 1".

So I think it will stay how it is. In 7 days I had 76 starts, and now is the worst temperature possible start wise, the pump has settled into a pretty steady pattern:


I can maybe go a little lower on the curve to cut off the small spikes up when it is doing the final heating of the tank.

[prpr]

Found also the schematic from buffer tank manufacturer. The connection is exactly as I have connected it.
This is logical because with this tank any other way you create mixing. Here the heat source is a burner with mechanical mixing valves, but same idea.



So this is some confirmation that I connected it the right way.