Multistage heat from a thermostat's point of view

giantsean

Hi All... hope you can pardon the strange questions here, but I'm really trying to figure out my next move. I have a hydrocoil forced air install which is just about to face it's first full winter. I have an AS branded Trane TAM7 with a Navien boiler. My installers are Rheem guys and happily took my money to put in the AS without really understanding how to set it up, but that's a whole other story.

Now I have it tweaked to my satisfaction, I am looking to upgrade from the cheap stats which they provided (as well as a zone controller which was made for heat pump.. I don't have a heat pump. Another whole other story). I noticed the TAM7 supports multistage heating via terminals W1-W3. Jumping them makes fan blow fast. NOT jumpering them makes fan blow so slowly that the house will never get warm. I know there are some thermostats (Nest comes to mind) that have separate W1-W3 terminals. I also know that in heat pump setups these are often configured for different things, like AUX/EM or various stages of electric supplemental heat within the A/H.

My question is, is it worth getting stats which support W2/W3 for a hydro air handler? W1 does almost nothing by itself. I have not tested W2 extensively but W3 does the job by sheer force. What I am not sure about is by what logic the thermostats will try stepping up to a higher heat level. Is it based on temp over time? Also, when a thermostat calls for more stages, does it only energize that particular stage or does it do so sequentially? For example during a 3rd stage call are all "W's" energized, or only W3?

Again, it seems to work ok but I'd love to take advantage of the full capabilities of the system. Thanks for any advice!

Zman

Making the building windier does not make it heat faster. In fact from a comfort point of view, it actually makes it worse (think wind chill)
If I am understanding your setup correctly, you want to run the fan at the lowest speed that gives you a balanced distribution of air. If you want to stage the heat beyond that, it needs to be done at the heat source. In your case varying the water temp using outdoor rest at the boiler would be a good idea.

ratio

Multi stage heating just means that you have several levels of heat - "some heat" (W1), "more heat" (W2), & occasionally "lots of heat" (W3). With regards to the air handler, some of them ramp the fan up higher to move the extra heat out faster, but without different stages of heat source that won't heat the house faster. If the air handler had electric heat strips installed those could be wired to W2 (wouldn't want then as W1 unless electricity was free in your area!)

Regarding the stat, I believe all modern electronic stats use both distance-from-setpoint & time-since-previous-stage-energized to determine when to bring on the next stage. Some of them expose those numbers in the programming, some don't. My personal preference are the Honeywells, because they Just Work.

Rich_49

Static pressure of the duct system should not be more than .5" ever . Fan speeds can and will kill you and start many threads about how terrible a forced air system can is . ODR will be your most valuable tool here and a boost function would not hurt either . Keep the fan speed as low as you can , pressurizing the house more will only increase the heat lost to outdoors .

njtommy

If you really want to do it. Install an aqua stat on your supply water piping at the air handler. Once the temp of your water increases with the out door reset you can have the fan speed up, but like stated above it maybe and bad idea, but it would trial and error on how the house feels and to your comfort level. I plain on trying this at my own house this winter to see if it makes any difference or effects my fuel usages.

Rich_49

A blower door test and figuring out the ACH natural would allow one to get the fan right or close to where it needs to be for a particular home . This would benefit performance in both heating and cooling seasons . Once that is done and you have ODR and fine tune it as you live there no other devices are necessary

giantsean

It's actually funny because these guys seemed to know so little that the original settings on the TAM did not take an internal hydro coil into effect at all. The coil has a control board which runs the circulators with DIP switches to tell it what it is (model and coil size). It was set for triple off which is the setup for an external add on hydro coil (vs an slide in). THAT was reallyi cranking. When I dialed it in correctly BUT did not jumper W2/W3 it absolutely did not heat the house, and this was during the spring when temps went down to 45 at worst. The only other options I can see besides varying the supply temp (more on that below) is experimenting whether W2 or W3 does the job best. It's a bit frustrating that this is supposedly the "modern" heating solution but it seems like immature technology. Maybe I'm just expecting too much.

On another note... I always kinda figured that more wind = more delta T and more heat passed to the ducts/house. The BTU output matrix supplied to me by Trane indicate higher BTU's as CFM/GPM/temps increase in general.

Re: the ODR. The Navien is working off the ODR sensor however there is some tuneability via the "K-Factor" which creates a more or less aggressive curve. Right now at K6 (the hottest but least efficient) it supplies around 150. I have not been able to see it work in bitter cold yet (and it's been a mild fall so far) so it's possible that it will put out higher temps. What concerns me is that there is nothing telling the thermostats what the boiler is doing and nothing telling the boiler what the stats need. It makes hot water based on how cold it is outside and what it gets back via return temps, and I guess it figures 150 is enough.

Rich_49

What thermostat is in the house ?

giantsean

This little gem right here...
http://yourhome.honeywell.com/en/products/thermostat/digital-non-programmable-thermostat-rth111b1024

I absolutely want to upgrade to something better, and that's actually one reason I'm asking these questions... to ensure if I get a stat that supports multi-stage it will work the way I need it to.