Technical FAQ: Hardware Installation

Is it possible to run wires within ducts?


Short answer: check your local building codes.

Long answer: possibly. In some places, you are allowed to use "plenum rated" wiring inside your ducts. The difference between plenum and non-plenum cable is that the latter produces a lot of smoke and toxic fumes while burning, while the former does not.

WARNING: Failure to comply with your local building codes will create problems upon selling the house, or a building inspector visit.

Is it possible to make this system wireless?

Short answer: yes.

Long answer: the cost may be prohibitive. Currently, some products are available on the market that allow the drop-in replacement (for example, PointSix wireless temperature sensors), but the cost is very high - the aforementioned sensor retails for about $80, vs. $3 or so for the 1-Wire® sensor. The price for the sensors starts to be more or less reasonable in quantities of well over a million, which, of course, is not something you'd be interested in. The only hope is that the wireless hardware is getting more commonplace and therefore cheaper, so with some luck this system will go wireless sometime soon. By the way, if you're reading this and know about cheap[er] solution, your input would be greatly appreciated.

Another aspect of it is the power delivery. The sensors will feel quite OK and probably work for a few years with no problems (especially keeping in mind that some models, like the one above, support the "standby" mode and don't have to broadcast the readings more often than once in 10-20 seconds). The servos, however, are power hungry, as well as their receivers. It is, of course, possible to pack them with the big batteries, but I'm afraid they'll have to be replaced quite often anyway.

Isn't it too complicated to use R/C servos for dampers?

As a matter of fact, no.

Despite apparent complexity, the R/C (short for Radio Controlled) servos are amazingly simple and reliable in operation. They are designed to be abused, and stand up pretty well. So, reliability is one reason to use them.

Another argument is flexibility. Standard HVAC dampers are usually open/close only, with very few exceptions (those are called modulating dampers). Open/close dampers are quite noisy mechanically (can make you jump in a quiet room), and contribute to increased wind noise - it's quite possible to hear the ductwork resonance frequencies change as those dampers open and close. Combined with increased airflow when some zones are completely shut off, this may create some serious wind noise problems. Final argument, modulating dampers can provide better temperature control than open/close ones.

With R/C servos, I can do whatever I want to the damper position. In particular, at this moment "crawling" position changes are implemented - a damper changes its position in small increments without any noise whatsoever. Transition from completely closed to completely open position completes in about 10-15 seconds, which is acceptable from airflow control standpoint.

Do these dampers return to open position on power failure?

No.

However, think about it for a second: what is going to lose power first - multikilowatt consuming HVAC unit, or a servo that can work off the 6V battery (or even USB bus)? And what do you care if the damper is open or not when the HVAC is off?

This kind of behavior is, of course, a nice safety net, however, with DZ's paranoid treatment of failure conditions it's simply an overkill.

A complete answer to this question would be: if you are planning to run DZ, it would be a good idea to provide the computer it runs on with an uninterrupted power supply anyway, so the point becomes kind of moot.

Is it possible to use dampers other than based on R/C servos?

Yes.

Any kind of dampers (as well as other peripherals) may be used with DZ, as long as the drivers exist.

Is it possible to use servos to control dampers instead of wall registers?

Yes.

The procedure is essentially the same - install the servo mount and provide a linkage from the servo horn to the damper control lever. More detailed answer is provided here.

Where do I install temperature sensors?

I got away with positioning the sensors next to the light switches - in US, they're at about 130cm (approximately 50") high. Also, it seems that positioning sensors at the height of power outlets (14", or about 35cm) works well.

Generally, you'd want them to be approximately at the height that matters to the occupants - lower in the bedrooms (and possibly family room, where people often sit), higher in the work zone of the kitchen.

Avoid installing the sensors where they would be affected by the airflow from supply ducts - this will cause short-cycling (and all problems associated with it) and uneven temperature distribution.

How close to the wall can I mount a sensor before the responsiveness and accuracy suffer?

My sensors are sticking out about 5cm (2") on twisted pair, feeling just fine. Enclosing the sensors in anything increases the temperature inertia, but there are several factors that may make this irrelevant: the PID controller I and D components may be increased to offset the increased temperature inertia of the sensor. However, a noise filter must be implemented, because 1-Wire sensors (and, I suspect, any kind of sensors) have limited precision, and D component may go nuts just because of the noise. Simple integrator may work if its time span is matched to the actual inertia of the sensor and enclosure.

Can I use existing thermostat as a sensor?

Theoretically, yes. Physical thermostat will provide the same functionality as a virtual thermostat. However, there are drawbacks:
  • You will depend on the quality of the existing thermostat. Good thermostats are expensive, inexpensive thermostats are not good. Good thermostats usually come (for the price) with the features that DZ already implements (for free), so there's no gain.
  • You will not be able to utilize advanced features of DZ that require access to the "virtual thermostat API", in particular, fine tuned balancing and zone resonance.

1-Wire sensors look ugly, can I use something nicer?

Sure, but it'll cost ya. The list will be long, so here's just one example of how a sensor may look like: Decora-Style Analog Temperature Sensor from SmartHome.

You can use any sensor as long as you have (or write) a driver for it.

There are too many choices for 1-Wire sensors, which one should I really pick?

Doesn't really matter. The answer to this question is determined by price and availability, rather than technical features. The sooner you want it, the more you will have to pay. Dallas lead times may be several weeks, if not months - last time I checked.

Should I be using XBee or XBee Pro?

Xbee Pro supports longer range, but is twice as expensive as XBee, and draws more current (read: more money spent on batteries). Since model supported by DZ (XBee ZB) support mesh networking, there's no real need to get more powerful radios. Moreover, with the deployment process (coordinator first, then routers, then sensors) you will be able to adequately estimate whether the regular XBee range is sufficient for you. All in all, if you don't like waiting, get XBee Pro for the coordinator and HVAC switches (which need to be configured as routers and constantly powered anyway), and regular XBees for sensors, where battery life is critical.
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