Ecodan short cycling problem?

[owh2021]

Hi all,

 

We are having some problems with a new Ecodan install and with the heating now on it's clear that it's not performing as expected.

I want to check I'm on the right tracks before going to the installer and getting another TADTS type response.

 

8.5kw Ecodan serving two UFH manifolds:

GF - set in flow screed approx 70sqm, 4 zones

FF - grooved overlay panels (12mm pipe), 70sqm, 3 zones.

 

When there is a call for heat everything fires up as expected. However if only a couple of zones are calling for heat it will run for just a few minutes before switching off. Not just the compressor cycling on/off but also the heating pump (I would have thought with short cycling that the pump would continue to run and just cycle the compressor on/off depending on return temps?).

It then sits dormant for a good 15 minutes before repeating. It often won't run long enough for the UFH return loop to even become warm.

 

I can see from the thermistor readings that the return temp matches the flow within a few minutes. I expect the problem is exacerbated by the small-ish 12mm pipes on the first floor as the flow through these must be tiny compared to the flow from the heat pump.

 

I think we might have a minimum volume problem and/or a flow rate problem. Can anyone confirm if that sounds right and what the correct solution would be?

 

Many thanks, Olly

 

 

 

[SteamyTea]

Welcome

I am no expert, but take an interest in all things ASHP.

Sounds to me that there may not be much volume in the pipework, so the heat pump quickly heats fluid in there, compares flow and return temperatures, sees little difference and shuts down.

So our initial diagnostic seems correct.

 

How large is the buffer tank, if there is one?

I am assuming it heats the DHW no problem.

Edited October 31, 2021 by SteamyTea

[owh2021]

Thanks. There is definitely no buffer tank. The DH all runs without fault.

 

I'm a bit confused between buffer tanks with 2 ports, seemingly acting as a volumiser, or 4 ports where presumably a stat is fitted to the tank which controls the call for heat.

I assume I need one of the above. Any idea how buffer tanks are sized? We have quite a limited space to work with since everything is fitted in a cupboard off a wardrobe area.

[SteamyTea]

6 minutes ago, owh2021 said:

Any idea how buffer tanks are sized

Find out the minimum volume the ASHP can operate with, then double it.

You can work out an optimal volume from the power output of the ASHP, al the temperature differences, run times etc.

Or just double the minimum volume.

 

I think there is little difference in performance between a 2 or 4 port buffer/volumiser.  Just down to easy of plumbing in for the given design.

[Stones]

4 hours ago, owh2021 said:

Hi all,

 

We are having some problems with a new Ecodan install and with the heating now on it's clear that it's not performing as expected.

I want to check I'm on the right tracks before going to the installer and getting another TADTS type response.

 

8.5kw Ecodan serving two UFH manifolds:

GF - set in flow screed approx 70sqm, 4 zones

FF - grooved overlay panels (12mm pipe), 70sqm, 3 zones.

 

When there is a call for heat everything fires up as expected. However if only a couple of zones are calling for heat it will run for just a few minutes before switching off. Not just the compressor cycling on/off but also the heating pump (I would have thought with short cycling that the pump would continue to run and just cycle the compressor on/off depending on return temps?).

It then sits dormant for a good 15 minutes before repeating. It often won't run long enough for the UFH return loop to even become warm.

 

I can see from the thermistor readings that the return temp matches the flow within a few minutes. I expect the problem is exacerbated by the small-ish 12mm pipes on the first floor as the flow through these must be tiny compared to the flow from the heat pump.

 

I think we might have a minimum volume problem and/or a flow rate problem. Can anyone confirm if that sounds right and what the correct solution would be?

 

Many thanks, Olly

 

 

 

 

I think you have correctly identified the problem, not enough volume when only a couple of zones calling for heat.

 

2 options really to increase the volume:

 

Run the house a single or dual zone, i.e. set the thermostats so that when the heating is on, all the loops in the zone are open. The obvious for dual being GF and FF. If dual doesn't work, try as a single zone.

 

Install a buffer tank in the primary circuit to allow heating as per your current zone set up.

 

We have an 8.3 kW Ecodan and preplumb cylinder like you, 155sqm of UFH which we operate as a single zone, temp set by master controller. The house simply settles at the target temp we have selected. For the past four years we have returned seasonal COP of 3.5 for heating, and do not have any short cycling issues.

 

Heating as a single zone may seem alien or wasteful, but I've found its the optimal and most cost effective way of running things.

[owh2021]

Many thanks both.

Stones, I think I understand what you are saying re. running as a single zone. The problem with that for the way we live is that we tend to have the bedrooms quite cold and the bathrooms/living rooms warm. That makes running the first floor as a single zone particularly difficult. I could also see some difficulty balancing things whereby we would have some rooms overheating whilst the bathroom is still trying to play catch up - as I'm convinced there isn't enough ufh pipework in there.

 

How exactly would it work installing a buffer? As simple as 2 ports - return-in and return-out on the primary return? That would solve the volume issue but would it do anything to improve flow rate with only one or two small circuits calling for heat. 

Presumably given enough time with low demand, the buffer temp would also match the flow temp, then the unit goes back into cycle mode, and the UFH loops can't benefit from all the hot water in the buffer because for some reason the heating pump also turns off with the cycling. OR perhaps the pump will continue to run, supplying heat to the ufh because there will no longer be such a flow restriction...

 

Clearly I'm confusing myself, but I can at least approach the installer now and see what they suggest.

[Stones]

@owh2021

 

You have three circulation pumps on the cylinder, one for the DHW plate heat exchanger, one for the primary circuit between ASHP and cylinder, and one for the heating circuit. You could try turning up the speed of the heating circulation pump (as pictured the one lowest on the left).

 

I appreciate you say the way you live. Why not try an approach of all zones other than bedrooms always open. Might just be enough.

 

I take it you are using the Mitsubishi master controller and set your target temp on that?

[Stones]

You have nothing to lose trying a few different configurations before going back to installer and being charged for the privilege.  Assuming you have thermostats for every room, crank them up so they are always calling for heat, but leave the bedrooms off.

[owh2021]

I think that pump is already at its max - currently speed 3/3 proportional pressure curve.

 

I'll have a go playing around with some zones always open as you say. Bathrooms and bedrooms are on one manifold and the living rooms on another. What seems to happen is the living areas reach temp long before one of the bathrooms does, so the actuators close, leaving only the bathroom running and the cycling begins.

 

The master controller is just setting the flow temp. The rooms temps are by individual Heatmiser stats in each room.

[Stones]

Are your room thermostats linked into the controller at all or simply operating the respective actuators on the manifold?

[Temp]

We effectively have a 4 port buffer (actually a TS) so I can describe that..

 

The boiler and UFH sides of the TS are totally independent. They only share a fused spur. 

 

The boiler (in our case oil fired) and its pump is controlled by a thermostat on the TS. If the TS is cold it fires the boiler which runs flat out until the TS is hot. Ours set up relies on the hysteresis of one mid height thermostat but its probably better to have two stats one nearer the top and one nearer the bottom of the tank to control the boiler/ASHP.  Ours seems to work fine though.

 

On the output side.. There is a pump on the output side of the TS that circulates water from the TS to the UFH manifolds and back to the TS. This is controlled by the room stats. Our UFH manifolds have a control box that performs a logical OR on the room stats to control the manifold loop pump and this also generates a Boiler Enable (BE).  Its called BE because without a TS it would normally control the boiler. In our case these 2 BE signals are OR'd together to control the pump on the output of the store only.

 

So the UFH treats the TS as if it was a boiler. It calls for heat as and when the room stats demand.  The boiler only fires up when the TS becomes depleted. 

 

Our TS also supplies DHW and powers a towel rail in much the same way as the UFH. eg If someone has a shower the TS might become depleted and trigger the boiler but a bit of hand washing probably wouldn't.

 

Difference between 2 and 4 port..

 

There is little difference between a 2 and a 4 port in the way they work and are controlled. With a 2 port the hot goes in and out of the same ports on the buffer where as on a 4 port it flows "across" buffer tank. (I think there might be an additional one way valve on the 2 port to make sure water is drawn from the buffer not the ASHP when the ASHP is off?) 

 

The 2 port has some advantages which make it better if you just need a buffer...  1) The flow rate into the buffer is lower as some goes direct to the UFH. This is claimed to reduce turbulence and provide better stratification. 2). The UFH gets access to the hottest water direct from the source some of the time. Perhaps more important for ASHP than an oil boiler? 3) The tank is slightly cheaper to make.

 

4 Port

2 Port

 

Source: https://blog.heatspring.com/2-pipe-versus-4-pipe-buffer-tank-configurations/

 

Edited October 31, 2021 by Temp

[owh2021]

Thanks Temp, really helpful. Looks like I had totally misunderstood how a 2-port buffer works. I think that looks like the best solution if we have the space for it. Time to see whether the installers agree if a buffer should have been designed in.

 

 

[owh2021]

14 hours ago, Stones said:

Are your room thermostats linked into the controller at all or simply operating the respective actuators on the manifold?

 

The thermostats are linked to a Heatmiser control panel which controls the actuators and manifold pump, and also sends call for heat direct to the FTC6 controller.

[SteamyTea]

3 minutes ago, owh2021 said:

Time to see whether the installers agree if a buffer should have been designed in.

My view is that a buffer would always be designed in with a heat pump system.

 

[Stones]

6 hours ago, owh2021 said:

 

The thermostats are linked to a Heatmiser control panel which controls the actuators and manifold pump, and also sends call for heat direct to the FTC6 controller.

 

So in effect, your heat pump could be asked to come on by only one thermostat sending a call for heat, let's say the bathroom.  The volume of water that the ASHP has to heat in those circumstances will be low, as the other zones will be closed off.  It will heat up to your target flow temp and due to the low volume, will quickly reach parity or near between flow and return and shut down. 

 

One thing you could try is decoupling the heatmiser from the FTC6 controller, and let the heat pump run on the target temp function.  Heating would come on / off according to the temp as measured at the FTC6, with individual zones opening and closing depending on what you have the thermostats set at.  Hopefully after settling down you would find enough zones were open at any one time to prevent short cycling.  As set up you need more volume in the form of a buffer of some sort I think.

 

You should have the ability to set change the economy setting on the circulation pump so it stays on for longer after the heat pump has switched off.  I have the FTC5 controller, so it may be different, but you will find that parameter under Aux settings in the service menu.