ASHP Short Cycling

nxdmsandkaskdjaqd

This is my first season with a ASHP and wet underfloor heating system.  In the last months I have struggled to solve the short cycling of the heat pump (HP).  Initially it was thought that because the bungalow was well insulated (modern new build) and it was mild, and it was just the mild weather conditions causing the problem.

Now that the colder weather has arrived I still have the HP short cycling every 30 mins (I can see this via the system control unit, showing that the HP is off).

I have tried all I can think off, slowing and increasing the pump speed (3 speeds), increasing and reducing the flow across the underfloor manifold. But none of this has worked. The best I can achieve is a Delta of 2 or 2.5C across the flow and return.

My current strategy, is to run the underfloor pump at max speed to get the water across the heat plate/exchanger as quickly as possible and to reduce the flow to the underfloor such that the water returning to the heat plate is cooler. So my question is:  Is this the right approach?

Any other words of help would be appreciated.

BTW.  No room stats set, all flow valves open, goal is to run 24/7.  The HP is not the most common here: Hitachi Yutaki (ASHP and Indoor Unit & DHW Cylinder)

Reed_Richards

How long does the heat pump run for when it is on?  30 minute cycles are long, not short.  It would only be short cycling if your heat pump was only on for a few minutes then off for the remainder of the 30 minutes.

Exiled_Tyke

Unless I've misread your post you don't seem to have mentioned flow temperature.  Are you able to change this? If it's set by a WD curve you should be able to amend this so that for any given outside temperature the flow temperature is reduced slightly. 

nxdmsandkaskdjaqd

Reed_Richards said:

How long does the heat pump run for when it is on?  30 minute cycles are long, not short.  It would only be short cycling if your heat pump was only on for a few minutes then off for the remainder of the 30 minutes.

The HP would be off for 3 minutes and then on for 27 minutes .  It would cycle this most of the time.  So this is not short cycling?

nxdmsandkaskdjaqd

Exiled_Tyke said:

Unless I've misread your post you don't seem to have mentioned flow temperature.  Are you able to change this? If it's set by a WD curve you should be able to amend this so that for any given outside temperature the flow temperature is reduced slightly. 

Yes I can change the Temperature Compensation system and I have adjusted recently (45C down to 40C vertical scale).  My flow and return could be typically:

26C and 24C - today - mild
32C and 30C - cold

nxdmsandkaskdjaqd

If this helps, this is the flow and return temperatures for today.

Netexporter

Has it got a volumiser/buffer tank? Shouldn't need on with UFH as the system should have enough volume of water in it anyway.

nxdmsandkaskdjaqd

Netexporter said:

Has it got a volumiser/buffer tank? Shouldn't need on with UFH as the system should have enough volume of water in it anyway.

Yes it does have a 50 litre buffer tank.  i have heard that its not necessary and should be removed, but its what I have at the moment.

matelodave

As above, try reducing your flow temps by tweaking the weather compensation.

I've got an older Daikin 11kw HP and found that rather than getting hung up on flow/return difference it runs better with all the floor circuits open and the weather compensation set to run the system continuously at a low level.

I also have a three speed pump which is set to 2 (3 makes it noisy). Weather compensation slope is set to 42 at -5 v 25 at 20outside (lowest boiler temp is 25 degrees)

Most of the time it idles away at around 30 degrees unless it gets everso cold when it will increase to 37-38 very rarely does it get above 40. We do have occasions when it gets very wet and windy when I might uplift it by2-3 degrees but the controller then resets when the daily cycle repeats. The controller drops the w/c curve by a couple of degrees overnight.(the controller allows for five operation settings - on/off/temp for each day for hot water and for heating.

When the room stats were enabled (one for each room controlling the relevant floor loops) it caused the unit to short cycle as each room closed down and hot water just got returned to the unit via the bypass valve.. It now runs for longer at a lower temperature.

The only way to control the flow across the manifold is to adjust the flowrate in each of the floor loops (like a lockshield in a rad, the flow loops usually have a flow indicator so you can adjust each loop to suit the room). 
You can't easily measure the flow/return temp in each loop as the pipes are plastic and all terminate onto the manifold so all you can do is change the flow rates in the loop and wait to see what happens

However as u'f heating is very slow to respond it can take ages (days)  to gauge the effect of any adjustment. Our system has two manifolds one with five zones and nine loops and the other with three zones and 7 loops (four loops in the lounge) - I gave up.  We have a flow/return differential of around 3 degrees and I guess we could improve that a bit by tweaking each loop but TBH life is a bit too short.

When we got ours 14 years ago I spent half the first winter randomly tweaking and fiddling until I set up a spreadsheet with all the possible control variables. Adjusted one thing at a time and recorded what I did, gave it a couple of days to settle and recorded the result until I got it working to suit. I've still got the spreadsheet and it took around six iterations before I was happy. Its not been changed since. I did the same with all the prgrammable room stats and timings but it just upset the system causing even more short cycling, increased power consumption and poor comfort levels. Now all loops are open and the system is gets on with it the place is warm and comfortable all the time and it doesn't get cold overnight either.

Hot water is heated once a day to 45 degrees with a sterilisation cycle once a week. Ours also has a 6kw back-up and 3kw immersion heater, both of which are disabled.

Reed_Richards

nxdmsandkaskdjaqd said:

Reed_Richards said:

How long does the heat pump run for when it is on?  30 minute cycles are long, not short.  It would only be short cycling if your heat pump was only on for a few minutes then off for the remainder of the 30 minutes.

The HP would be off for 3 minutes and then on for 27 minutes .  It would cycle this most of the time.  So this is not short cycling?

No, that is most definitely NOT short cycling.  I agree that it's very irritating but it means that your heat pump is supplying all the heat that your system needs per 30 minutes in 27 minutes.  The most obvious thing to do to remedy this is to reduce the leaving water flow temperature (as others have suggested).

Exiled_Tyke

Agree with @Reed_Richards keep lowering the temperature until it struggles to get the room to temperature.