Posted by: @chastity_outward053simplelogi

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Okay good to hear about fan coil cooling with water temp above  dew point - I had toyed with being below but thought I may save myself potential condensation issues, but if it is not worth the time I can go back to no slab cooling, and just fan coils at low temp with vapour sealing on all cold pipework.

Currently, I send water above the dew point (17°C) into the slabs. The effect is... subtle. In fact I can't really tell whether it's doing anything. Yeeeah it is that effective lmao.

On the other hand, my second hand 3kW mobile AC unit has no trouble bringing the living room down to 19°C with an outdoors temperature of 33°C...

However, at night, I turn the mobile AC unit off, because it is noisy as hell. Then, normally, when I turn it off, room temperature rises quickly because it cools the air but it doesn't cool the masonry much. However, when I cooled the slab earlier in the day, even after turning off the AC, temperature remains at civilized levels until the next morning.

Therefore, cooling the slab is not very useful on its own, but it is a good complement to cooling the air with fan coils.

I have not installed the fan coils yet, they're sitting in a pile while I finish that damn ceiling. I am sure the vapor insulation will be a challenge, we'll see. But I'm going to do it, because I absolutely want quiet AC.

Posted by: @chastity_outward053simplelogi

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Please can you expand on why PICVs are useless? My intent was to use a 0-10v actuator on them as well as the FCUs ESC fan, controlled by a Zennio KNX product (MiniBox 0-10V x2) that modulates both - and has a dedicated module that is designed for modulating both.
In my head the PICV is just a slightly more fancy individual zone valve, that caps it at a set max flow rate, but allows me to modulate below that? So I could still open it up (via KNX) and pass water through without turning the fan on etc to avoid minimum flow rate issues.

Looking at Caleffi 145 Flowmatic PICV datasheet I see it needs a minimum pressure drop of 30kPa in order to open and do its job. 30kPa is 3 mH2O or 0.3 bar which is already more than half of what the circulator in your heat pump can provide, so... problem. It would be more rational to use the circulator's pressure budget to push water through pipes and emitters rather than waste it on a valve. These valves are designed for big buildings with powerful circulators. They don't make sense in a residential setting.

If you want to modulate flow through your fan coil it is simpler to use something like a radiator valve with an actuator on top. I tried it and fitted a Siemens SUE 21P on a radiator TRV in place of the knob. It works absolutely fine. Control is by two wires (open/close), if you power the motor for longer it will open and close more. These also fit on UFH manifolds but they're too wide, and the manifold valve they fit on is only good for on/off so it won't modulate your UFH loops. Honeywell also makes valves for this but I couldn't find them, apparently only available in the US and Asia. I also tried Honeywell M6410C-7410C actuator on a radiator TRV: the adapter broke on the first use so nope. You can also use a small motorized ball valve, reduced bore 1/2", that should work fine, and it doubles as a zone valve. They're simple, cheap and easy to use, so that's what I'm going to use. None of these options have the minimum pressure drop requirement from the PICV so they are a much better fit for a single circulator heat pump system.

So in order to keep it simple, since I was going to put ball valves on the FCUs to isolate them in case of maintenance, I simply went with motorized ball valves instead. I might use them as flow control, but most likely not.

Posted by: @chastity_outward053simplelogi

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I was under the impression that FCUs can require quite a high flow rate? Will the ASHP pump be able to cope with them at full tilt if required?

FCUs are a bit counter intuitive: in cooling mode, if you increase the fan speed, the output temperature will increase, that's because its maximum power is determined by the water (flow * deltaT). So increasing the fan speed will blow more air, but it will be less cool. If you want more airflow AND cooler output air then you need more water flow. In cooling mode, your 5kW heat pump should have enough power to max out 2-3 fan coil units, you plan to install more so if they're all on, the heat pump won't be able to cool the water enough to max them all out. This isn't about flow, it's about power. This isn't necessarily a problem: if they are balanced and one doesn't hog all the flow it will work fine.  All that matters is that the FCUs and heat pump have enough power to suck the heat that gets into the house. But do you really need this many?

FCUs only have a flow requirement in so far as to get the cooling power you need the flow, but if the flow is not high enough then you just get reduced power. The issue is more the other way around: you have to make sure the heat pump gets enough flow so it runs at low deltaT and doesn't trigger an error. Low deltaT is important, because water freezes at 0°C, so you can't get below 5°C to avoid cold spots in the heat exchanger... but you also want the water to be as cold as possible so the FCUs work efficiently. So if you work with 7°C/10°C flow/return (deltaT 3°C) the FCU heat exchanger will be 1-2°C cooler than at 7°C/13°C (deltaT 5°C), so it will boost performance a bit.

I'm about to run the risk of oversimplifying and asking a dumb question. However.....

This is a new build we're talking about, and one where insulation and airflow are capable of being kept to PassivHaus standards. MVHR has already been mentioned as a desirable inclusion, and that means ducting. Cooling is getting a lot of discussion, and heating capacity required is expected to be low. Given all this, is there any reason why an air to air system isn't being considered? The only thing it wouldn't be able to do is DHW and there are several options for that besides an A2W heat pump.

Seems to me that going from the start with A2A would simplify the design significantly as well as removing the need for radiators entirely and having the summer cooling inherently included. Am I missing something and talking spherical objects?

@bobflux Perfect - thank you. Okay the PICVs I were looking at were 16kPa, but even with that 7 of those robs me of too much, and I achieve the same goal with actuated ball valves. I had been looking at he belimo ones, so I may have another look.

@majordennisbloodnok Now this is a completely new train of thought for me - and no there is no reason other than I have very (even less) working knowledge of A2A. Lets say I got a ducted unit, would that sit alongside MVHR? Or can you combine the two? Is that advisable, or a fallacy? 

Would give me the question about DHW - which, and I haven't broached this here, I have had some interesting thought cycles about. I like the idea of POU instantaneous boilers, I know they only achieve a COP of 1. But, if I have a large bank of batteries (3 phase) and a suitably large inverter, it saves me me all standing losses. Significant pipework, a secondary recirculating loop, the pump, valves, a tank etc. So it may be a case that actually it is worth it in the long run.
I did toy around with very low tank temps, and then DHE27's to ensure it maintained a nice temp (because they modulate the power, costing very little) and gave instant hot water.
I had binned off the idea after doing a fair amount of research, as I came to conclusion with the A2W heat pump it worked out easier to have just a larger UVC and a secondary loop to get instant hot water.
But with just an A2A, I could do one POU boiler (something like the DHE27) for each bathroom (so bath/shower/sink), a smaller unit for the kitchen and another one for the garage. I reckon the cost is probably similar to tank+ancillaries and pipework. Without even including the recirculating loop.
Plus saved cost on UFH etc.

The batteries and invertor required were part of my plan anyway - so that is neither here nor there in terms of cost. 

What about comfort factor? Anyone with thoughts/opinions on A2A heating vs UFH for example?