Hitachi Yutak and system controller

[tsh]

So I have my heat pump installed now, and the house is warm, using roughly the same amount of electricity as storage heaters were the month before. The controller seems to have some quirks. (version 9 of the firmware)

The installed default for DHW was for 50C water, and a flow of 60C to achieve this. However, 60C is the thermal trip point for the heat pump which resulted in a run-time of about 1 minute before tripping out. Easily resolved by reducing the flow temp a little.

More problematic is the control of when the house heating is on. Weather compensation seems to apply on top of the target room setpoint, and I needed to wind that back too to avoid overheat. Not too bad.

Current problem is that the circulation pump is running even when the room temperature is above the room setpoint. The circulating water is above the heat pump setpoint, and I seem to be pumping heat out of the system.
Troom=20
Sroom=17
Tflow=34
Treturn=35
Sflow=28
Text=3

This flow setpoint of 28 is below the 'heat' setpoint that I set (about 35, which may be a little low for this weather, but we'll see...)

I have reduced the frost setting, so it's not that. Surely this is wrong - is there anyone who can shed some light on what the controller thinks it is doing here?

[JonG]

Could you give us a bit more detail about the pipework layout, whether it's 2 port or 3 port to hot water, whether there is a buffer, type of heat emitters and so on please?

[tsh]

I'm not sure the layout makes much difference to this question, however:
3 port valve (heat/DHW)
Emitters are large rads, and I'm trying to work out how low I can go with flow temp. 46 is high enough, and the lowest modulation output provides more heat than the house needs to maintain a steady temperature. All but 2 of the rads have TRVs, so I can imagine the load is quite variable.
The DHW tank has a solar coil which is plumbed as a bypass. I'm not sure I like this, but I have been experimenting with opening the bypass to persuade it to act as a buffer - the absence of 'pump off' when the house is warm doesn't seem to be affected by this though.

[dhaslam]

It is  probable that the circulation pump stays running  while the system is on.  If you have a buffer tank  the idea usually is to  set the water circulation return temperature to one that it cannot  reach, or at least  for a long time.  The system running time is  limited by the timer settings.   This keeps the compressor running  constantly for a one or two periods each day .     You could  use a similar  system  for  heating radiators direct i.e. set a high temperature that it cannot reach  and limit the running time  by time settings.    It  does mean  adjusting the running time in line with expected weather conditions.    Although heat pumps come with  room temperature controls  it doesn't match  the need  to run constantly.   

[JonG]

Is it also possible that if the solar coil is being used as a bypass the flow is cool coming off the heat pump, but accumulating heat from the cylinder due to thermodynamics and therefore showing a higher temp via the sensor on the return.

Long running via the bypass could decay the cylinder temp if this is the case.

How is the bypass set up, is it an auto bypass or a gate valve, whichever it is you have to be sure you can get design flow rate through out with the resistance of the bypass and the coil, but avoid dumping too much through it if its resistance is less than that of the house.

A buffer would be better or if this is not feasible open circuit the rads by removing the trvs.

Jon

[tsh]

I have the two rads in the room with the system thermostat set with the TRV on max. The bypass is on an automatic (pressure) valve, and currently it is taking a fraction of the flow (my guess is less than 50% when half the TRVs in the house are closed).

I can close the bypass more easily enough to remove this as a variable - I'd been hoping to get a feeling for the impact on cycling/run time but it seems that I have a more fundamental problem or two:

1) circulation pump running too long, indicated by the controller demanding heat despite the room being up to temp.

2) 'Stoppage by Thermo-OFF (oF)', which I think is just an overshoot of the setpoint. This is exacerbated by the controller seeming to ramp down the setpoint to meet the room target, and still calling for heat when in fact the room is hot.

I am slowly starting to reverse engineer the behaviour of the controller, but it is kind of counter intuitive at times. Dropping the setpoint for heating on the controller from 43 to 39 yesterday morning resulted in the house not getting heated much at all yesterday! It should be easier over the weekend...

[titan]

A couple of general points, cylinders for heat pumps require much larger coils usually in excess of 3m2surface area , not many makers do them, without this the low volume of water in your dhw heating loop will soon reach the heat pump trip temp. The Ecodans require  "s" plan rather than the "y" plan you have installed.  Also most weather compensation circuits have the circulating pump running continually and have no other controls ie trvs or multiple room stats they just match the  desired internal house temp to a programmed curve controlling flow temperature to the external temperature.

[JonG]

I am not familiar with the Hitachi per se so bear with me.

Its not unusual for circ pumps to run continuously regardless of whether the compressor is recruited or to pulse in order for the thermistors to interpret the system and respond as required. Sometimes though the pump operation can be user set and can be aligned with the on off signal from a room stat.

The influence from a room sensor can often be switched on or off to suit but if on usually offsets the selected curve up or down depending on the information from the room stat, so the curve will change from what your settings are. It might be an idea to turn this off to begin with if you can while you get used to the system.

Given that it looks as though the set points have been exceeded is the compressor running or are you just seeing interrogated data on the controller?

ABV's are very difficult to set up without flow meters in the pipework to see what they are doing. I would certainly try closing it off to get an understanding of what is happening with the flow and return temps.

I am not sure how the solar coil will work as a buffer, because if the cylinder is at 50 and max flow temp on heating is 35 heat is always going to come out of the cylinder and back to the heat pump via the return switching the heat pump off.

[tsh]

Thanks for the input so far...

The system does appear to be designed for Y-plan, but it does have both room and external temperature compensation (with parameters to adjust each curve). I might turn off the room one (since my system is mainly going to be over-powered and cycling to some extent whatever I do).

I'll report back on the viability of the solar coil as a buffer eventually. What I know so far is that with the solar coil un-used, the lower part of the tank stays below 30 even if the DWH sensor is adjacent to the lower HP coil tap and the system is doing it's best to heat it up.

This is a heat pump specific tank from Gledhill (180l), and it heats the tank up from 30-50 in maybe 20 min. I do need to check there are no airlocks though on the heating side...

Generally, I'm checking the setpoint and output on the pump itself, and assuming that the controller is not using the flow/return sensors it has for anything other than a remote readout (although they do match the pump readings with a short time lag - pipe runs are short between controller and the pump, just through the wall and over a door)

[JonG]

Just be aware that the HP coil on the Gledhill 180 is only 1.36m2 which depending on the size of your HP may not be sufficient to achieve correct flows and delta t's. You might want to consider incorporating the solar coil into the DHW circuit too to increase the surface area available, you will then get the lower part hot too.

The solar coil is 0.68m2 so even adding this in you will still be shy of the 3m2 that is usually specified by most HP manufacturers.

 

[tsh]

The primary issue seems to be that the strainer fitted in the return to the heat pump (22mm Y) is still getting blocked every day or so. Now I know what to look for, it is fairly obvious if the flow/return are more than 10 degrees different (which seems to imply the flow is on the low side even when it seems to be clean)

[titan]

The primary issue seems to be that the strainer fitted in the return to the heat pump (22mm Y) is still getting blocked every day or so.

I would say this is a major issue if you have that much crud in your system. You say you had storage heaters before so is this a completely new wet system, did the installer flush the system when commissioning it.

[tsh]

Well, I didn't see him flush it. It is completely new, so I will ask him to confirm...
After cleaning both the strainer and the magnetic filter, the flow/return difference is down to 5 degrees - so i'll be keeping an eye on this from now, and see if the crud is mostly gone!

[tsh]

Well, my system has been working better since the start of the year, and I've got as far as starting to log some data...
This is logged as air temp directly on top of one of the radiators.



Outside temperature was 1 degree, whole-house consumption for this day was 31kWh.
Here, I had the flow setpoint at 38 degrees (and I think the controller will back-off the flow temperature when the room stat is calling for 17 degrees)

[tsh]

Thanks to the magic of ebay and heat meters, I've got instantaneous COP measurements now.
Flow rate before cleaning strainer (after leaving it alone for 1 month) 0.42m3/h. After cleaning 0.5m3/Hr

External Temp 3 degrees
Flow 43.0 degrees
Electric input 2.75kW
Heat flow 5.52 kW

COP=2.0 (accuracy unknown)

Kind of disappointing, but the flow temp is higher than I need (maybe the controller will back off a bit once it realises the pump is back on line - room stat is reading 23, set for 21 at the moment!)