Triassic

Weather Compensation.

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The first fix is complete and over a brew the electrician says, how you dealing with weather compensation? 
Something else I don’t know the answer to!

 

I assume it’s some sort of weather / temperature sensor on the end of a cable placed on the exterior of the house?

Which side (N,S,E or W) of the house, sunny or shaded spot, ground level or eves?
Cat5 cable or something else?

 

Help me out here!

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There seem to be a load of solutions out there to this. Really to make it most effective you need to have a system that can use the house parameters, decrement delay etc, and then can get ahead of the weather by however many hours you need in all the various circumstances. You can get an API, I have been playing with this one, feed for the weather forecast in your area but just spotting the sunlight and temperature in the right places should do it. We have several south facing windows and I have put temp probe pockets in the slab 1.5m back from the windows so if it starts to get ahead of the UFH I can react to the additional heat coming in either by adjusting the circulation to this zone or take the heat to other zones. I also intend to put a sun sensor on the roof although I could get some of the data about sun from the solar array it is not 360deg. The API is very clever, you could use the hourly rain forecast to close roof lights before it starts raining - when the roof lights close (cows lay down) it is going to rain. 

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We live in a micro climate due to out location surrounded by Cumbrian fells, so the local weather is often different to what we experience. For example, everyone we talk to mentions torrential rain storm and thunder. We had one light shower!

 

Floor sensors, could you point me in the direction of the equipment I need?

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Posted (edited)
56 minutes ago, Triassic said:

Floor sensors, could you point me in the direction of the equipment I need?

Can you still get into the floor? I used some lengths of UFH tube back to the plant room and fitted one of THESE DS18B20 probes in each which are; bog standard, very cheap and easy to interface to. (I had to cross calibrate them before I fitted them - I think @Jeremy Harris describes how you do this in one of his blogs.) 

 

Edited by MikeSharp01
sense

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What heating?

 

Most weather compensation is based on an outside sensor usually on the north wall, but it must be out of direct sunlight.  Basically when it's colder outside it turns the heating up. 

 

I have never used one personally.  But a good plan if you think you might is a spare cable from your plant room to somewhere on the north wall where you might put a sensor.

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My ASHP has both the external temperature probe and compensation control built in as standard, I needed to allow for 2 cables to it. Mains power and a two core control cable.

 

The external unit is north facing so sheltered from direct sunlight for most of the year.

 

This does not compensate on weather forecast, only on temperature. Works fine.

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21 hours ago, MikeSharp01 said:

Can you still get into the floor?

I can still get into the floor, but I was thinking! Why the floor and not the ceiling? Hot air rises?

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44 minutes ago, Triassic said:

I can still get into the floor, but I was thinking! Why the floor and not the ceiling? Hot air rises?

Yep hot air rises so if you control it at the bottom it will affect the top but you probably need both and one right at the top of the house. The slab is the largest mass so controlling its temperature will use a big proportion of energy so you need to be in control of that as if things get out of hand there, heat wise, everything can go pear shaped and comfort on the floor is a key factor. You need one right at the top of the house to manage the accumulation of heat, open / close roof lights etc. Broadly the general rule that: 'if you cannot measure it you cannot control it' is in play here.  

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Good thinking! Never even considered putting one at the top of the house or controlling automatic windows to let hot air out. I suppose I could also use the feedback to control the MVHR purge option. 

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3 hours ago, Triassic said:

the MVHR purge option

Yes that is also an option - as I say if you cannot measure it you cannot control it. 

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With a modern well insulated house I cannot see the point in weather compensation.  I have seen this for big commercial buildings where it has to be x degrees at 8:00AM.  If it is brass monkeys it will switch on a bit earlier.

 

With a new house we had the heating failed and we did not even notice for a couple of days.

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It’s fitted on a few of the new gas boilers and the controllers - tends to be used to predict the start time on heating systems but isn’t entirely accurate. 
 

@TerryE has a really neat system that he designed that basically predicts load requirements but there was a lot of programming in it. 

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1 hour ago, Mr Punter said:

With a modern well insulated house I cannot see the point in weather compensation.  I have seen this for big commercial buildings where it has to be x degrees at 8:00AM.  If it is brass monkeys it will switch on a bit earlier.

 

With a new house we had the heating failed and we did not even notice for a couple of days.

+1. I am sure both @TerryEand @Jeremy Harrisboth said this. KISS

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Posted (edited)

My heating system calculation is pretty simple in essence.  It is driven by three measures:

  • The delta between the house target temperature and the outside temperature averaged over the day
  • The day-averaged actual temperature delta'ed against the target temperature.

These two are calculated each midnight.  For the actual temperature, I have a DS18B20 buried inside an internal wall (actually the one my control system back onto so it was easy to drill a hole into the plaster and drop one down into the studding).  IMO, doing this is a lot better than measuring air temperature as it gives a better estimate of the overall temperature of the internal fabric.  For the outside temperature, I actually use an API that the Met office publishes and query the outside forecast for the coming day.  We don't have any funny microclimate issues and this is good enough for what I need.  Another option that I have considered is that our outsidel meter box is flush-mounted in our external stone skin; I have thought about dropping a DS18B20 down the wall void behind this between the outer skin and the inner lagged Larson strut timber frame of the house, as this will smooth out any spot surface solar heating noise.

 

So each night at midnight I use these two terms in a simple linear model (i) to calculate the amount of heat in kWh that I need for the next day, (ii) to uplift or drop this to adjust if the house is too warm or too cold.  I set the two constants in this calculation initially from my Jeremy-like heat calcs for the house, but tweaked them after a year by a fit to the actual house data.  So what this daily calc does is to work out that I need to put, say, 12 kWh heat into the slab tomorrow to keep the temperature of the house on target.  Given that I still haven't bothered to install an ASHP but use  a 3kW heater into the slab, this equates to 4 hrs heating is needed so I heat the slab from 3-7a.m. (to use E7 low rate electricity). 

 

The last measure is the actual house temperature.  If I need any more than 21 kWh, then I add this first 21 kWh midnight to 7a.m.;  I only add the extra heat drip-feed N mins per hour if the house temperature drops below the target temperature. 

 

Because all (or sometimes most) of the heat is added overnight, there is a residual <1°C daily ripple on the actual temperature, but we find that we don't really notice this.   The scheme works really well and the day-to-day average temperature varies maybe ¼°C.  The one thing that does kick the temperature off is having visitors to stay because these extra warm bodies aren't factored into the heat calcs.

 

This system runs for about 6 months a year, but cuts out for the summer hump where we don't need any heating.  Note that we have smallish cottage-style windows so we don't have solar-gain management issues.

 

As to slab temperature, we run our pump 6 mins every hour when the heating isn't on, just to redistribute any local solar gain across the entire slab.  I take the average temperature of the flow returns coming out of the slab at the end of this 6 min period and this is a very accurate measure of the average slab temperature.  No other probes needed.  We already have a ~300m long probe.

Edited by TerryE
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On 19/06/2020 at 07:24, Triassic said:

We live in a micro climate due to out location surrounded by Cumbrian fells, so the local weather is often different to what we experience. For example, everyone we talk to mentions torrential rain storm and thunder. We had one light shower!

+1 - except that when the wind's from the SW we always get far more rain here due to one coastal fell chucking up clouds.

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Posted (edited)
11 hours ago, TerryE said:

I have a DS18B20 buried inside an internal wall

That's interesting.  Depending on the wall orientation to the sun, it is really measuring energy change (of the building fabric), rather than temperature.

It is really energy levels that need controlling with either additional inputs to the house i.e. heating, or subtract energy i.e. extra ventilation or active cooling.

 

With a few weeks of data collection it should be possible to collect enough data to work out the characteristics of the house to make a system that is a bit more predictive., even if it is just a simple lookup table, rather than a formula (but the formula is pretty simple, Newton worked it out a few centuries ago).

Edited by SteamyTea

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No need to calculated cepstrums and try to calculated estimate impulse responses or the like.  🤣 The thermal inertias in a passive-class house like mine as so high that a simple liner fit to historic data is enough to estimate the basic constants.  The calcs are run once a night and decide "tomorrow we need 3½ hrs heat into slab" or whatever.  Magic happens, and we never really think about it.  I used to plot all sorts of plots and analyses, but I never bother these days.  The house just sits at the temperature we like. 

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