@f1p just zoning the system tech makes it over sized unless a couple of bypass rads can be maintained. I will give some avice on this once I know FTC model and some other set up answers.

@patch321 how new is your home, is it 2024/2025?

Posted by: @patch321

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@f1p thanks for the post.

 

interesting, I wonder in that case its modulating down, I think the ashp is massively oversized for the heating demand for our house, I think it looses about 3-4kw but have a 11.2kw ashp. 

 

 

 

 

If this is true and you zone off a floor this system will not work at 11.2kW

@ashp-bobba Hi Bobba,

It’s an FTC 6, house was built in 2024. 

Yes, when the play button goes it switches the pumps off.

I’ve asked the plumber about the zoning, also about the pumps and the logic behind them, but I don’t hold much hope.  

@patch321 ok, so if the ASHP heating "Play symbol" switches to "II" pause and the pumps stop that is fine it is just cycling and thinks the demand is off, if wired through the UFH system this will play and pause the system for heating. 

Now it gets a bit complicated, - when the demand ends for the UFH, the UFH will stop the ASHP and the pumps will overrun for a little while, if a zone shuts down and you try to push 32l/ps through 4 UFH loops you will cause the flow to stall and the DT will overshoot cycle down very quickly on the ASHP causing it to cycle / stop, then the system waits on a compressor protection 3 min cycle timer and repeats the cycle over and over.

So hear I am trying to explain there are several reasons the ASHP can pause, UFH says stop so it pauses, ASHP flow DT overshoot so the ASHP can no longer chase the curve it says pause. 

Whole house design

If your whole house heat loss at peak (this is important, peak is just a few days/weeks a year) is 4kW or 5kW it means your system will want to run at 40% at -2, any warmer external temperature and it want to run at a lower capacity. The lowest your system can run is maybe 2.9kW. So if the external temp is say is 10 DegC your heat loss may only be 2kW, do you see the issue here?

 Now, if you close off an entire floor you no longer have a whole house design, you now have a half house design and with it a lower peak and no peak heat loss of 5kW, now your 10 degC external has gone from 2.9kW  to maybe 1.4kW if only for 1/2 a day and thats going to be very difficult to manage for an 11.2kW system, you must heat most of the house the whole time.

The above is a bit simplified to help show what zoning does and I am sure I will be forgiven by the more knowledgable colleagues on here for the made up figures. I wanted to point out max flow/kW design is a peak figure and only applies for a short time based on a whole house design, cut the house in half with extra controls and you have a 50% incorrect design at various points in the day at some points in the seasons. How we get round this is we leave 30% of emitters open loop so they are always heating needed or not to allow a minimum capacity exchange matching the minimum capacity of the unit design, this balances across the house, the best rooms for this are hallways, landing and bathrooms as these are the rooms we tend not to mind slightly overheating as we tend to prefer that or only we transit through them for a brief moment and they also often can spread some warmth to other rooms if they are central to the house like a hallway.

How we get round the over heating between rooms is we balance the circuits and emitters making sure that the flow and heat is even, then we set up a curve that just replenishes the home with the actual heat loss, so your system will unfortunately overshoot all the time if oversized as its minimum will often be more than the required replenishment V external temperature. 

The weather compensation curve programmes the inverter to change the capacity of the ASHP to offset the difference in weather temperature / heat loss, if you are already using most of this at peak (-3) then there is none left for zoning or warmer weather = short cycling.

I hope this helps and I hope I didn't ramble to much.

@ashp-bobba interesting, but currently the way I’m running the ufh it’s constantly calling for heat, it will never satisfy the room thermostats. 

so surely the heating should remain on constantly? Or until it hits the target temp from the tcc?

Posted by: @patch321

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@ashp-bobba interesting, but currently the way I’m running the ufh it’s constantly calling for heat, it will never satisfy the room thermostats. 

Great this is open loop

so surely the heating should remain on constantly? Or until it hits the target temp from the tcc?

Looking at your graph you are not dissipating the heat so it is overshooting on the DT, it is not just about the loops closing even though you are setting it to run at 35 Deg C flow you still need to use the minimum kW output, the retun flow water temp is always rising rather than steady so the system will keep back off.

Please see attached graph from 8.5kW Ecodan serving a 244m2 hose new build 2025.

see how the return flow is steady. This is dissipating it heat so it can run steady and holding the home temp at 23, they like it very warm.

can you see here it not like a set of steps every 20 mins, it runs flat for 2 or 3 hrs.

I also currently have a Vaillant that is running 808hrs for 432 compressor starts, the best systems have 1 start per running hour, this systems is only 3 months old and only known a winter run, once it has done a year I have a bet with a top Vaillant engineer that I can make it 1 start per 2hrs, I have been informed this is hard to achieve, but I have a special heat loss coils in the walls at my office as an experiment to test this by heating the walls rather than the floors.

Its about the usable energy, if you can generate exactly what you need you can see the system will run gently with fewer starts / cycles. Your emitters will be unable to dissipate the heat if the heat loss is not matched to the ASHP.

Posted by: @ashp-bobba

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once it has done a year I have a bet with a top Vaillant engineer that I can make it 1 start per 2hrs

If I understand what you are saying correctly and the unit is well sized you should win the bet.  My Arotherm, just in a normal house, is up to 1830 'on/off cycles' (as the app calls it) in 5463 hours 'running time' over the period Dec 2024-date.

@jamespa oh this is very good results on your system, yes, its just for fun and in house training but I am experimenting a few things.

1) I have an ASHP with undersized emitters by 30%

2) I have an ASHP with oversize emitters by 30%

3) I have an ASHP with perfectly sized emitters at 104% 

all 3 have the correct heat loss sized ASHP for the building. My bet is the over sized emitters win, better than the perfect sized. its just a test for fun.

My theory is oversized emitters at each room help the gain, volume and over run with less seasonal effect, so kind of flys in the face of zero disruption, it worth noting I don't expect them to be better by much and probably would not be worth the capX for larger stuff but if you own a company with ASHP kicking around everywhere you may as well use them for testing, training and fun.

So far it looks like the under sized effected the COP by 0.7, the over size increased it by 0.2 and the normal sized seems inline. None of the tests running are for official recording just meeting room discussions and allows our engineers to think about the dynamics of heating, nothing like making them think.