fan controller for larder

[KLD]

When we built our new extension we put a larder into the north-east corner of the house. It is well insulated on the inside, separating it thermally from the heated space. There are two vents to the outside, one low down, the other just under the ceiling. Until April, i.e. while the nights were reasonably cold, it was quite cool in there. Now it's almost as warm as anywhere else in the house.
I like to add 2 PC fans to the ventilation, to be able to draw cold air through when the air temperature outside is a few degrees below inside, but only until the inside is somewhere near 5°C. So, what I need is something like a basic temperature difference controller (TDC) with additional thermostat. One option is to build one (like this one, but it doesn't have the thermostatic bit) from scratch. Another possibility might be using a PIC with some peripherals?
Also, maybe I could use the existing 1-wire-based temperature logging system, and somehow control the fans (via a relay ?) from that. At the moment, the 1-wire stuff runs on a NSLU2 a.k.a. Slug. This has two USB ports and one Ethernet connection (and an internal RS232 port). Any pointers for the necessary hardware needed to setup a fan controller?
Any other possibilities you could think of?

Any advice most appreciated!
Klaus

[ericw]

Klaus,
The simplest and cheapest way would be to adapt something like the circuit here http://www.siliconchip.com.au/cms/A_30610/article.html by using the other half of the LM393 to sense between one of the temperature sensors and a fixed voltage from a pot to give the thermostat function. As the outputs of a LM393 are open collector you can just connect them together.  If the 'thermostat' side was wired so the pot went to pin 5 with the sensor on pin 6 then the fan would be prevented from turning on at  at high temperatures.
 

[KLD]

Eric, good find, and thank you for additional explanations! Looks easy and low cost enough, to quickly wire one up and try it out.
As a matter of interest, would you have any pointer for the "slug" solution?

Klaus

[Justme]

How big is the room & how much air will 2 pc fans move? 

I would think that a bathroom fan might be better even if it does use more power.

[KLD]

Justme,

the room is quite small 1.2x1.2m, but you are right, the PC fans might not be powerful enough. I'll give it a go and see. At the moment there are air single bricks there, so two 80mm would fit that opening well.

Klaus

[ericw]

Klaus

Wookey posted this script some time back which you could possibly strip down and use.
http://www.navitron.org.uk/forum/index.php/topic,8673.msg92987.html#msg92987
If you need a 1 wire device to switch the fan (via an external FET) then the clone factory can help.

[KLD]

If you need a 1 wire device to switch the fan (via an external FET) then the clone factory can help.

Wow,
that would be smart! That said, today I received a little parcel from Rapid Electronics, so will be trying the circuit you found earlier. If that fails for whatever reason, I'll be knocking on your door to get cloned   

Klaus

[KLD]

The temperature difference controller was easy to build, and works well. The output (as switched by a MOSFET) could change states it bit faster, at the moment the fan sort of is speed controlled in the switch-over area.

The bigger worry is the fan itself. As Justme predicted, even three PC fans in parallel didn't shift enough air volume, when mounted in front of a short 50x180mm duct going through the outside wall, with a grille on the outside. Off to TLC, for a 12V bathroom fan. Mounting this fan onto a 110x180mm duct and leaving it running all night (outside 10°C, inside the larder 17°C) yielded precisely nowt.

What next?
I don't really understand fans. For a water pump I'd know how to read the "flow versus pressure" graph. Are there different types of fans that can cope with "flow" resistance?

Klaus

[Iain]

Hi
Which opening is your fan on? top or bottom? I couldn't find it in your post. I would think it would be better to extract warm air from the larder, out at the top vent and allow the cool air to enter naturaly through the bottom vent. I would have thought a normal 12v/230v bathroom/caravan extractor fan would be alright. At least doing it that way around the temp gradient will help. You might have it that way around already,just a thought.
Iain

[johnrae]

Klaus,
Some pointers for consideration

a) Axial fans are useless when working against a back-pressure  -  in such cases you need a centrifugal fan

b) You need to have the fan blow "outside" air into the pantry, so producing a positive pressure within the pantry.  If you have the fan mounted as an extractor then the pantry works at negative pressure and will tend to draw warm "house" air into it, especially when the door is open.

c) However, if you take a leaf out of the oldies manual there should be no need for a fan at all.  Natural convention currents should work provided there is sufficient air port area.  Simply make decent sized  top and bottom vents, say at least 8" square, with suitable bug screens (perforated zinc).  Any slight temperature increase within the pantry should establish a convection current which will draw in fresh cool air and exhaust the warmer stale air.

jack

[KLD]

Jack

The "oldies manual" idea was exactly what I planned and built. The openings are a little smaller than what you recommend, the top vent is one 9" air brick, the bottom a double. There are terra cotta sleeves trough the cavity, the clear openings of those are stated above. These openings clearly are not large enough to provide for passive ventilation.  So I hoped a little help from a fan would be enough, but sadly not so. Try and spot the day during the last week when the fan was on over night 

The larder is well sealed from the rest of the house, with double depth insulated door leave and proper door seals all round. The two internal walls have extra insulation (100mm Rockwool between studs, plus 30mm Kingspan overlay). To the outside (North East corner of house) is the standard insulated cavity wall (didn't quite dare leaving the insulation out in that wall section).

Maybe the temperature gradients aren't good enough yet, or the cool nights aren't long enough. Last winter we had 5 or 6°C in there most of the time.

Klaus

[KLD]

Ah, difficult spotting without the graph. So, here it is:

[johnrae]

Your larder temperature is clearly not following the ambient air temperature so therefore it not being influenced by it, rather it seems to be picking up heat from the house interior.  This would suggest that the outside wall insulation is effectively acting as a barrier so preventing the pantry and the ambient from forming a coupled system.  In days of old the larder usually had  thick solid walls so the wall acted as a heat sink (barrier) during the warmth of the day and dumped the gathered heat to the cool ambient during the night (a passive heat pump ?)  They also had a "cold slab" installed either of thick marble or concrete.  By the way is the floor solid concrete or wooden.  If the latter you might want to consider pouring some concrete.

You have several choices :

1) Break out the external wall insulation and make the walls solid (with vapour barrier of course)

2) Put in a decent sized fan and a control system

3) Put in a heat sink (radiator) and run cold water through it and again some control

Options 2 and 3 need power.  Option 1 probably goes against the grain but is totally passive - and will make a great fridge - or freezer - in the winter.

I take it the pantry has no windows - if it has then you need to eliminate them.  Cold and dark are mutually inclusive terms for a pantry.

jack

[KLD]

Jack,

The floor is screed over insulation over concrete slab. It has a 60mm Kingspan layer separating it from the rest of the internal floor.
When we started to plan the extension (of which the larder is a small part), one of our neighbours offered us second hand 70mm thick slate slabs he'd salvaged out of somebody else's larder. Our larder was then sort of purpose build around those slabs.    It might actually be that the thermal mass in the larder is so high that it needs a lot more air changes with the cooler night air to bring the temperature down. An indication for that is that when we bring the weekly lot of shopping in, there is a brief peak in the recorded air temp, but half an hour later it's all back to the same values as before.

Hm, that makes me think. Let's see just how slow the larder's response is with respect to changing outside temperatures: here is a plot of the larder temps (as before), but the outside air temperatures are averages over the last 1, 2, 3, or 4 days. Well, looks like the thermal time constant between temperature changes outside and inside is about three days.  So, how many air changes do I need over night to shorten that time constant?

Klaus

[johnrae]

Hi Klaus,

Pantry temperature seems to be tracking the 3 to 4 day ambient average reasonably well but with a 5 degree offset, which must be coming from the building structure.  I feel you may have insulated your larder too well and it cannot therefore lose heat to the ambient via its external walls (and floor).  I take it the roof is also insulated to prevent heat intake via that)

As far as air changes goes it's a pure guess but you might want to look at around 2 air changes per hour.  However with a simple thermostat set at your 5 degrees the fan would be running all day and night in summer months and only cut out during cold winter nights.

I think you'd be better with a differential control looking at inlet and outlet air plus a low temperature cutout on exit air.  That way the fan runs whilst there is a differential  (larder warmer than outside) but once the larderr temperature equals the outer air it stops running.   However the fan doesn't run if the pantry gets down to 5 degrees.  Also in winter you could run the fan less since the heat extraction rate from cold winter air will be greater then from warm summer air.

Inlet sensor would need to be outside the pantry air inlet and the exhaust sensor would be inside the pantry air outlet.

Sounds like a good arduino application.

The post shopping peak could simply be you filling the room with warm house air. 

Keep the slate slab, apart from being one of the larder basics for thermal mass (cool mass) - on a hot summer night you can always take the food off it, strip off and lie there in wonderful bliss.

jack