Help

Hi! I’m new here and somewhat new to PIC micros, but have a project that I believe would best be handle by a micro controller system. I have to heat a 4500 sq ft building as efficiently as possible, as the floor will be a cement slab a good heat mass I decided to go with radiant heat in the slab. The building will be divided in to three zones, each with it’s own thermostat. Each thermostat will have an on/off switch and two temperature sensors. One for the slab and one for the air, the sensor in the slab will serve as a safe guard to prevent the slab from getting to hot (<90f). If the slab sensor gets a reading above the set point it shuts the zone down (maybe turn on a fan) till it cool down. The other sensor should be self-explanatory. Each of the thermostats will send a simple signal to a central control unit that monitors all thermostats and open or closes valves and runs a two speed circulating pump or three single speed pumps (one for each zone). Each thermostat will have a 16x2 back lit LCD that will display the current temperature of each sensor on it’s own line and the set point for each at the right side of the same line. There must be a means of setting the set points for each as well. The central control unit should be able to handle five thermostats even though current design will only use three. A simple single output pin from the micro in the thermostat would go high to signal the need for heat and low when heat is not needed. It would be stepped up to a higher voltage (12V) for transmition to the central unit where it would be buffered to (5V) then in to an input pin of the central unit micro (another PIC micro). The central control micro will simply poll the input pin from the thermostats and toggle the out put pin (maybe turn on indicator LEDs) for the appropriate valves, pump(s), and monitor the heating source. With that all being said, where do I start? What sensors do I use? What would the best micro be? How and where do I start programming (I have pic basic pro)? I well be grateful for any and all help!

steelman said:

I have to heat a 4500 sq ft building as efficiently as possible, as the floor will be a cement slab a good heat mass I decided to go with radiant heat in the slab.

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I'm in the business. If this is for a customer and you have to warranty the system then I very much recommend that you just buy the controls. Unless it was for my own house I would never even consider using home-built controls (I wouldn't do it there either, except just for my own amusement). Anyway, most times everything you need comes with the boiler.

Control companies have good R&D and well tested products designed by people who know hydronic heating. I prefer Tekmar. Excellent stuff!

Due to the perversity of these things, controls, pumps or valves will always fail on the coldest night of the year, on a long weekend. You'll want to be able to buy an off the shelf product and get that damn thing heating again, pronto! You won't want to be frantically building a new board on Xmas eve because yours smoked itself for some freak reason, or having to rewire half the system to suit an off the shelf control. For that reason, if you build one, be sure yours wires up in standard ways.

If it's for your own amusement, and you're not in any hurry, and you don't mind having the thing screw up a few times while you get things right, then fly at it. Sounds like an interesting project. Just keep in mind that you may not be the guy servicing this thing later(you might sell). 99.9% of heating guys would take one look at a homebrew non-standard thing like that and it's new controls and total re-wiring time! They won't know what to do with it. Many heating guys aren't very good at electrical. MAJOR big bucks!!

What sensors do I use?

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Talk to the heating department at your local plumbing/heating wholesale. If yours are anything like my suppliers, the heating guys are experts, and will do your heat-loss calculations and a system design, and help you with choosing the proper products.

The building and the need for high efficiency sounds like a natural fit for a **broken link removed** or 200 condensing boiler (or **broken link removed**). The 100's are cheaper and still very nice. They're up to 98% efficient. In-floor radiant is perfect for making a condensing boiler work at max efficiency too.

the sensor in the slab will serve as a safe guard to prevent the slab from getting to hot (<90f). If the slab sensor gets a reading above the set point it shuts the zone down (maybe turn on a fan) till it cool down.

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What we normally do for this, rather than put sensors in the slab, is put a high-limit aquastat in the radiant supply piping, just ahead of the zone valves for the radiant, and just downstream of the mixer/diverter valve and pump (or just downstream of where the injection pump ties in if it's an injection system). Set this to cut the burner call-for-heat if supply temperature exceeds ~140F (because of other failed control or some idiot messing around with the controls). Leave the pump(s) running to allow the thing to cool down between rounds, and to keep on heating even with the malfunction. Doing it this way means that your radiant loops will never see the excessively hot water. With slab sensors, by the time you sense it it's already too late. And if nobody services it for a year or two with it riding the high limit like that, at least no damage is done.

I was gonna put in a baseboard hydronic heat system one of these days, or maybe not, with the price of propane now-a-days.

Got a Taco SR503 EXP to handle the pump relays. This unit handles 3 zones and can be expanded further if required.

The radiant slab adds an extra dimension that I don't have to worry about, so just got a couple cheapo Honeywell setback thermostats. The Taco pump controller has a 24V transformer, so the level shifting could be handled by transitors at your thermostats. With the Taco pump controller, seperate thermostat inputs control the individual pumps, so a master micro may not be required.

No expert on this stuff, but interested in what Futz has to say.

nickelflippr said:

Got a Taco SR503 EXP to handle the pump relays. This unit handles 3 zones and can be expanded further if required.

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I think Allied and Teledyne-Lars use those things (or something quite similar, possibly custom built for them). They're quite nice to wire up. I have almost no experience with them though, as I never buy Allied or Teledyne-Lars boilers. Over the years I've used tons of Weil-McLain, Slantfin and lately Burnham boilers. Only recently started selling the Viessmann high-efficiency units.

I'm no fan of Taco, but that's almost completely because of a long run of defective pumps they cranked out years ago. Cost us a fortune in callbacks. I only use quality, reliable Grundfos pumps now.

Thought the Taco controller pdf had a good illustration for the basic wiring control needed for the O.P.

futz said:

I'm no fan of Taco, but that's almost completely because of a long run of defective pumps they cranked out years ago. Cost us a fortune in callbacks. I only use quality, reliable Grundfos pumps now.

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Have to defer to your expertise on the HVAC stuff, just by chance, I had bought the Grundfos pumps .

As far as a room temp thermostat sensor, something like a DS1721 or equivalent would work nicely. Just about any 14 pin device would be able to run it and a lcd, say a 12f684.