Using Outdoor Reset as 2 stage

Flash_2

I would like to set up the AQ475A outdoor reset controller as a 2 stage system with the T8624D thermostat. The first stage would call for lower temperature (140-190*) water, as regulated by the AQ475, and the second stage would call for max temperature (180*) hot water. See diagram below.

Setting things up this way would permit the AQ475 to be set to a lower design temperature, thus using lower temperature water and keeping the circulator running more constantly. This is because the T8624 will automatically engage the second stage (ie full temperature hot water)when it comes out of overnight setback, as well as when the thermostat is manually increased in the evening when people get home, giving a reasonable recovery time. I expect the second stage may also engage if the front door is left open a bit, but that it might take a while.

I do have indirect fired DHW, but it does not need priority. So, I would like to use the AQ475 DHW demand terminals to sense the call for the second stage, and wire the DHW aquastat in parallel with the AQ475 boiler output. I think the AQ475 should be fine with this, as the last page of the instructions states "Note: Factory supplied boiler controls can also energize the boiler independently from the AQ475A." The thermostat may require a resistor in parallel with the HW inputs if they do not present the thermostat enough of a load.

Doesn't a 2 stage outdoor reset scheme make sense? I haven't seen any discussion about it.

Also, does anyone see a problem with the control wiring I describe?

Thanks!

Plumbob

The basic idea of using the 2nd stage of the thermostat to bypass reset and deliver 180F water has been discussed here before, and there is no technical problem with it. I am not familiar with the Honeywell reset controller so I can't comment on the details of the circuit.

Note that any efficiency gains caused by setback would be negated by the efficiency loss due to higher temperatures during recovery. And during recovery you are back to "bang-bang" heating (heating with a burst of too-hot water), causing expansion/contraction noise and room temperature overshoot. It might be better to forget about setback.

BillW@honeywell

I don't see any...

problem with the wiring as shown, but I must agree that you are kind of defeating the purpose of a reset system, as Chuckles described. The AQ475 is the least sophisticated of our reset controllers, basically, it just varies the boiler run time to meet the demand. Why not take a look at the AQ675, that works with a V5013 series 3-way mixing valve and an ML6XXX direct-coupled actuator. The piping is a bit more complex, but not too difficult, and the slighly higher initial cost should quickly be recouped by energy savings. You could use a setback thermostat if there is no one home, but as long as you building enevlope is well insulated, and your windows are tight, the reset should be all you need.

Flash_2

But...

>>>The basic idea of using the 2nd stage of the thermostat to bypass reset and deliver 180F water has been discussed here before

Do you know how I can find the thread?

>>>Note also that any efficiency gains caused by setback would be more than wiped out by the efficiency loss due to noncondensing temperatures during recovery.

The oil boiler is noncondensing anyway, but are the efficiency gains still wiped out?
The reset controller says to allow at least 2 hours for recovery. Wouldn't it be better
to set the usual water temperature even lower, such that perhaps 20 hours would be needed
for recovery, and give an extra boost during recovery to make it 20 minutes?

>>>And during recovery you are back to "bang-bang" heating (heating with a burst of too-hot water), causing expansion/contraction noise and room temperature overshoot. It might be better to forget about setback.

I'm hoping the "proportional + integating" software anticipator in the thermostat would
control the room temperature overshoot. The expansion/contraction noise shouldn't be
a problem.

Does the efficiency gain from outdoor reset dwarf that of setback? I would think both together would be better than either separately.

>>>Why not take a look at the AQ675...

The AQ475 is easier to apply in my retrofit. How would the AQ675 help, by reducing radiator temperature fluctuation when in stage 1?

BillW - thanks for examining my nonstandard wiring plan.

Plumbob

Plumbob

> I'm hoping the "proportional +

> integating" software anticipator in the

> thermostat would control the room temperature

> overshoot.


The thermostat is either on or off, it is not proportional. Nor does it have any integration (which you DO NOT need) or differentiation (anticipation). Honeywell likes to claim that manually setting the cycle rate is the same as anticipation. This is cr*p.

>

> Does the efficiency

> gain from outdoor reset dwarf that of setback? I

> would think both together would be better than

> either separately.


The gain due to setback is quite small, in spite of thermostat manufacturers' claims to the contrary. But the gain due to reset is also small if you don't condense. So it is not worth splitting hairs about small quantities. What is certain is that you will not save enough in the lifetime of the boiler to pay for the 2-stage thermostat (those are expensive, significantly more than two 1-stage thermostats, which makes no sense).

hydronicsmike

Flash...

You can search for older posts using the search tool bar in the top right hand corner, but as Bill stated (and I in the other post you are refering to) it will work, but is not ideal or recommended.

If you go against Bills recommendation on not doing it this way, then at least make sure that the thermostat you will be using uses PI staging logic rather than step-staging. A PI thermostat would allow stage 2 only to come on if it feels that stage 1 is already operating at 100% of its output. Where the non PI thermostat would bring on stage 2 if the temp drops below 2°F below target (and in many cases prematurely, in my opinion) and then you are definitely eliminating or overriding the Outdoor Reset too often and then you might as well not put in a Reset Control at all. If I remember correctly from the other post you are refering to, I think the second stage contact on a thermostat was only used to recover quicker from setback, where during normal operation the second stage was never required.

Those are just my thoughts, for what it is worth.


Regards,


Mike

Brian Macdonald_4

old posts

The posts you are looking for are from October 18 and November 25, 2004. I was one of the authors.

I have my non-condensing boiler wired as you describe. This was done because my outdoor reset units curve is non adjustable and did not match the needs of the house well enough. If the curve was set high enouch to heat the house in cold weather and recover from setback, it did not "reset" much at all in warmer weather. I think a reset unit with adjustable slope curve (or better yet a multi point curve) would be the best choice by far. Mine was sold to me and installed and out of warrenty before I knew any better.

Having said that, the system as currently wired actually works pretty darn well. Operates on reset almost all of the time, except when the front door is left open to load in groceries or when coming out of setback. By the way, it does not overshoot or have expansion noises. When coming out of setback the second stage shuts off one degree shy of setpoint (this is adjustable on my t-stat, a White Rodgers)and goes back on reset (first stage heat)and the house temp "coasts" up the last degree to the actual setpoint, or near it. Usually the t-stat never stops calling for heat, just switches from no reset(second stage) to reset (first stage) and keeps on truckin'.

I have been tracking gas use in CCF per degree day for the last three seasons and can measure no difference in efficiency between running the t-stat on setback vs. constant temp. Admittedly I have the worst possible system for using setback - gravity conversion with standing cast iron. However, I find it more comfortable (as do many) to have the house a little cooler at night.

To all who post here: I can't tell you how much I enjoy this forum. Almost makes me consider a second career - in hydronics!

Brian Macdonald