Passive Slab UFH Cooling Control Strategy

[Dan F]

Given we have UFH embedded in a passive slab with a huge mass, standard thermostatic control is not a feasible control approach given the large lag.  A correctly configured weather compensation approach would be suitable (and this is what I use in the winter), but the Vaillant controller AFAIK doesn't use weather compensation for cooling.  Given Vaillant doesn't support external "call for cooling" to date I've simply been manually controlling cooling in an attempt to keep a decent temperature in the summer.  The issue with this approach is that slab temperature creeps up without you noticing and then it can take quite a while to get it back down.

 

Now that I finally got my hands on an eBus adapter ( https://adapter.ebusd.eu/v5/) I can now think about putting in place a control strategy that will hopefully (once tuned) look after itself all summer!

 

Keeping the slab temperature constant isn't enough, as depending on the outside temperature if it's a cool week the slab may need to be at 21.5C, but if it's a very hot week the slab may need to be at 20C to provide the necessary cooling power to cool the house.

 

What I think I need to do is implement my own external weather compensation using the forecast average 24-hour exterior temperature in order to determine the necessary target slab temperature and then switch ASHP cooling externally.

 

I don't know how accurate the PHPP data is and maybe this is a bit too theoretical, but it seems a good place to start.  I can then tune this over time.  Using the PHPP data I get that the cooling power required, based on exterior temperature is:

required cooling power = (outdoorAvg24hrTemp-16.8C) * 270W

 

Given 120m2 UFH and 10W/m2/K UFH cooling power, I then get:

slab target temp = (outdoorAvg24hrTemp-16.8C) * 270W  / 1200W

 

With an average 24-hour external temperature of 25C, this would give a target slab temperature of 20.2C for example.  Of course, for an internal target temperature of 21C (instead of 22C) the constant would need to change, or be enhanced to include an additional parameter.

 

Any thoughts anyone?  I remember @TerryE has some kind of similar algorithm working well for him, but not read much about how others do this.

 

Edited September 5, 2023 by Dan F

[JohnMo]

I just flow mine with a target flow temp of 11 from around 0930 until 5. Basically using solar when it's there. I only put it on, when it's likely to be sunny after about 3pm, if it's not sunny I don't need it. Anything lower than 11 deg, I get condensation on the pipes etc. so don't go there. Not sure you need to be scientific nor do any weather compensation.

[Dan F]

2 hours ago, JohnMo said:

I just flow mine with a target flow temp of 11 from around 0930 until 5. Basically using solar when it's there. I only put it on, when it's likely to be sunny after about 3pm, if it's not sunny I don't need it. Anything lower than 11 deg, I get condensation on the pipes etc. so don't go there. Not sure you need to be scientific nor do any weather compensation.

 

Right, thats roughly the same approach I've been taking, but flow temps more around the 17C mark.  If the house is getting warm and/or "heat wave" appears in headlines, then I turn it on for a number of hours while the sun is up and PV is generating.

 

When you say "put it on" is this manual?  In the winter, do you manually turn the heating on/off based on the weather?  My view is that if heating can be 'set and forget' (using a heat curve), then it should be possible to do the same thing for cooling.

 

Edited September 5, 2023 by Dan F

[JohnMo]

I have a cooling thermostat, but it's set to 21.5 deg.  But this morning the house is about 21, I know the suns going to out all day, so I will manually override so the cooling comes on earlier than it otherwise would do.

 

I found over the last couple of months when it's been wet the house was most days hovering around 21, even though it was raining most the day, the cooling coming on would have been a waste a time. So have resorted to overriding the thermostat.

 

I am just doing batch cooling instead of heating, so I use PV instead of paying for it.  

[Dan F]

2 hours ago, JohnMo said:

I have a cooling thermostat, but it's set to 21.5 deg.  But this morning the house is about 21, I know the suns going to out all day, so I will manually override so the cooling comes on earlier than it otherwise would do.

 

I found over the last couple of months when it's been wet the house was most days hovering around 21, even though it was raining most the day, the cooling coming on would have been a waste a time. So have resorted to overriding the thermostat.

 

I am just doing batch cooling instead of heating, so I use PV instead of paying for it.  

 

That's all great if you are happy with manual control.  What I wanted to do with this post though, was discuss an approach to automating cooling.  Initially, the basic approach, but then the plan would be to add electricity tariff/pv into the mix as well.

[Conor]

I just manually turn it on at the room stat with the ASHP controller set to cooling mode and a flow temp of 13c. You'll only need to use the cooling once in a while during heatwaves. You certainly wont need to treat it like winter heating. The effect is rather subtle, you wont overly cool the house and you only really need to run it during sunny hours. We ran ours 24x7 during the June heatwave for a few days and it never felt too cool. If you need it more cooling than this, I'd be looking at other strategies - shading, air cooling etc.

[JohnMo]

Should be easy enough to automate and include a cooling curve. Based on outside temp. But you need to make sure the cooling need is outside temperature dependent. Ours isn't.

 

So a simple automated outside temperature cooling curve just wouldn't work for us. It can be 12 or 32 outside, if the sun isn't shining it's not an issue, but if the sun is shining the living room can overheat.

 

But with "home assistant" and solar irradiation forecasting, I could automate a relay to kick the cooling on, only on the days required. But what happens when I move to heating, will the relay cause issues to the heating system functionality, as it a shared control system? Possibly

 

Another variable, if it's not sunny and outside temp is below 22 deg, you just open a window or two if the house is to warm.

 

Lots of variables with cooling and what if and buts, that are not there with heating.

[joth]

Sorry not much to add other than, I use per room zone controls for cooling upstairs - each room has an FCU, fed from a 100L buffer tank.

When I charge the buffer with 10° water I also circulate it through the UFH downstairs (so long as any downstairs zone, or the house as a whole, as cooling demand). This is mostly to increase the demand on the ASHP to reduce short cycling, but also has a marginal cooling effect downstairs.

The only thing I do off the forecast is flag the risk of overheating (if today high will be over 27 or tomorrow over 28). And if this is true, various additional measures are taken Inc reducing the house target temp by 1° during cheap rate window. 

Not very scientific, but to be honest I'm reluctant to base too much on any forecast data as it has too many failure modes. 

 

Fwiw on another project I integrated visual crossing weather API directly into Loxone miniserver, very easy to use free API  👍

Can share some config it you'll find it useful 

Edited September 6, 2023 by joth

[Dan F]

@JohnMo We have external shading on most windows, so while solar gain will still play a part in cooling demand, it's primarily outdoor temperature that impacts cooling demand.  That said, I could tune the calculation to include solar radiation if required.

 

@joth It's easier to use thermostatic control when there is a reasonable reaction time, but this approach doesn't work with slab.  I was just planning on doing some work to get the forecast daily average temperature into Loxone so if you have a template file (or can export one) for Visual Crossing that will likely be very helpful.   Couldn't get Weather4Lox to work for some reason, but the Loxone weather service attributes only have a limited set of data points anyway

[joth]

17 hours ago, Dan F said:

 

@joth It's easier to use thermostatic control when there is a reasonable reaction time, but this approach doesn't work with slab.  I was just planning on doing some work to get the forecast daily average temperature into Loxone so if you have a template file (or can export one) for Visual Crossing that will likely be very helpful.   Couldn't get Weather4Lox to work for some reason, but the Loxone weather service attributes only have a limited set of data points anyway

Yeah I used my rudimentary "overheating risk" approach with slab only in summer 2021 and was better than nothing, but thermostatic controls on FCUs per room is much better approach for us now.

 

Very basic Visual Crossing template here: 

https://drive.google.com/file/d/1WL7LQopIfSp9aBHYtfYuHW-ti-nlTnnC/view?usp=drive_link

 

Notes

1. You need to put the API key in the place marked PUTKEYHERE in the URL. (see docs)
2. You need to put your postcode in place of the POSTCODE placeholder
3. I'm using JSON output as (bizarrely) that's much easier to scrape in Loxone than CSV.
4. For some reason I never figured out, the date range URL params only work on CSV, JSON output seems to ignore them
5. I'm only parsing current values right now, but adding forecast values is pretty straight forward if you copy the pattern from the params I've added

 

The above is mostly used in another house with no weather station. On my house I primarily use Weatherflow Tempest weather station via the Weather4Lox gateway. This mostly works (occasional outage excepted) but the temperature sensor on it seems to have totally pooched and gives bonkers readings. (My other two sources are the MVHR and the ASHP, both via Home Assistant integrations, both of which recently broke too for various reasons, gotta love Home Assistant)

 

IIRC The Weather4Lox plugin is tricky as it hijacks the Miniserver DNS query to redirect the weather request to its own implementation, so you need to setup the miniserver with that custom DNS server address and/or change your DHCP server to send every DNS query via it (no thank you). The Tempest has a neat(ish) feature that it automatically broadcasts all weather updates via UDP which is really easy to grab in loxone (no loxberry needed), but with the downside it won't populate the weather forecast tab in the loxone phone app (only their official weather service or the Weather4Lox spoof implementations support that)

 

 

Edited September 7, 2023 by joth

[Dan F]

1 hour ago, joth said:

I'm only parsing current values right now, but adding forecast values is pretty straight forward if you copy the pattern from the params I've added

Thanks!  Any idea how to match the second instance of "temp:", jump to the second object in the "days" array, or use an argument (tomorrow's date) to jump to the relevant section? JQL would be a bit easier!

 

1 hour ago, joth said:

IRC The Weather4Lox plugin is tricky as it hijacks the Miniserver DNS query to redirect the weather request to its own implementation, so you need to setup the miniserver with that custom DNS server address and/or change your DHCP server to send every DNS query via it (no thank you).

I set this up (don't have a weather station), but something is not quite working as no data in Loxone. I need to diagnose.

 

1 hour ago, joth said:

thermostatic controls on FCUs per room is much better approach for us now.

Of course.  I'm sure you mentioned this somewhere else already, but:

- Are you now only using FCUs for cooling, or combined with UFH cooling too? Did you install FCU on the ground floor too in the end?  

- If you are using UFH cooling, do you have a strategy for controlling this?

- What actuators are you using for per-room FCU's do they each take 1-10v input or something?

Edited September 7, 2023 by Dan F

[joth]

1 hour ago, Dan F said:

Of course.  I'm sure you mentioned this somewhere else already, but:

- Are you now only using FCUs for cooling, or combined with UFH cooling too? Did you install FCU on the ground floor too in the end?  

- If you are using UFH cooling, do you have a strategy for controlling this?

- What actuators are you using for per-room FCU's do they each take 1-10v input or something?

 

FCUs only in upstairs room, UFH only downstairs. 

For cooling the FCUs run when the respective rooms call for cool. As mentioned up thread there's a buffer tank on the FCU circuit and when the buffer needs charging I also run cool water (slightly higher set point temp) through the UFH too just to increase the flow rate to avoid short cycling. In essence using the slab as an additional buffer. 

The manifold closes off any loops for any GF rooms that are already below target temp. So using thermostat control to avoid overshoot. In practice GF just sits at a pleasant temp regardless, it's only FF that really needs active cooling. I expect a lot of cool air tumbles down the vaulted hallway and into the GF open plan area. I don't have ducted return plenums upstairs, so each room's FCU circulation goes via the landing and shared spaces to eventually get into the service riser back up to the loft where the FCUs are, so helps mix up the air and give secondary cooling to the whole building.

 

The FCUs have multiple windings so it's 3 or 5 channels of relays per fcu to control speed. 

(Nit: one FCU is a massive thing salvaged from a restaurant refit, which feeds two rooms with electronic baffles on the air side to direct which room(s) receive air. So the speed I run that one at is a function of the sum or demand from those room)

 

Edited September 7, 2023 by joth