Howdy,

I can't seem to figure this out. If a mod/con boiler comes with an outdoor reset control, how do you compensate for the passive heat gain/loss? The boiler must accept a thermostat, does it interact with the boiler in a special way?

AMAC

The thermostat makes the call for heat to the zone (typically controlling a pump or zone valve) while the outdoor reset determines the output temperature of the boiler. The zone controller "let's the boiler know" if and when there's a call for heat.

The passive gains may keep a zone thermostat from calling for heat during sunny daylight hours, but it's still the outdoor reset that determines the water temp, and the curve needs to be tweaked/programmed to be able to deliver sufficient heat from the radiation during periods of zero passive gain. To get the absolute highest efficiency out of it programming the curve is an iterative process of tweaks + observations, but the starting point can be set pretty close using room-by-room heat loss calculations and the radiation specs.

Well, I need to revisit my book and review this section again. My understanding was that in an ideal situation, the reset line dictated the supply temperature to balance the emitters vs. heat loss. Because things aren't perfect (passive gain / infiltration variables) the curve could be off from desired. These variables raise the need for reset line shifting positive or negative. My question is how does the reset line shift, I hope it's a secondary closed loop such as with a TT. I guess/assume the reset controller will read the TT and if it's off by some set differential it will make adjustments on the fly? I certainly hope it's on the fly and i don't need to adjust the reset line every time it's sunny!

OK, I just found the section. It says many modern controllers can be equipped with indoor temp sensor. Also,, Systems without interior temp sensors should use a TT wired to override the output of the reset controller if internal gains push interior temperatures above the desired indoor setpoint. I guess in my boiler research i never read a spec stating the controller accepts a TT.

outdoor reset does not adjust to indoor conditions and no one bothers with overrides unless you're using an external control with indoor feedback capabilities or, in the case of a few boilers, a single indoor sensor. I hate the single indoor sensors though... where should it go? the east facing breakfast nook, south facing great room, west facing study? that's why we like multiple point indoor feedback like tekmarnet 4 and 2 systems.

but basic outdoor rest is pretty good. during high internal gain periods the zone just shuts off. no biggie. Unless you're doing a constant circ system with no external control or something.

Outdoor ambient temperature makes for a fairly crude but effective model for heat load, yet the addition of an indoor sensor isn't necessarily a huge improvement. It makes for a more complex & dynamic mode but it doesn't necessarily improve efficiency or comfort in any significant way.

A room with much solar gain is going to be heated almost WHOLLY by that passive gain when the sun is out, so the fact that the now-tiny heat load in that room could now get by with 80F water for a few hours rather than 110F water based on the outdoor reset curve, even if you dialed it in PERFECTLY it would likely make less than a 1% difference in the annual fuel use. I s'pose there will be an application somewhere that it makes some sense, and if it auto-adjusts the curve it may reduce the effort & thought required for getting a reset curve dialed in.

Rooms with significant solar gain need their own thermostat. You can make things harder than you want to.

Posted By BadgerBoilerMN on 20 Feb 2012 06:20 PM
Rooms with significant solar gain need their own thermostat. You can make things harder than you want to.

This makes sense. Thanks.

actually, indoor sensors can make a big difference. in a zone with direct solar gain, you can't adjust the water temp because the zone is off. but the adjacent zones can pick up heat gain from that zone, or heat gains can come from other, more marginal sources like people, lights, electronics, cooking, etc.

Further, indoor feedback allows a system to react to how far off of target it is. so it can speed response as well. Now you can have setback as well as reset water temps.

and finally, it can adjust UPWARD if the load is higher than expected for some reason.

it certainly provides a comfort and efficiency improvement path. but for most of those benefits you'd need more than a single indoor sensor to be truly effective.

Indoor sensing tweaks to the water temp may result in better response times, sure, but whether it's going to buy you even 1% in fuel savings will depend a lot on the particulars of where the outdoor reset curves are starting from, and 5% savings would seem out of the question unless maybe your design-condition water temp requirements are well north of 140F, and average temp requirements on the barely-condensing edge of the curve.

Idling along at min-modulation with return water temps below ~125F it takes a ~20F drop in return water temp to squeeze another 1% in efficiency out of it, and at 50% fire it would need to drop ~10F, at least according to this graph: http://www.mnshi.umn.edu/images/boilergraph-1.jpg Unless you're operating at the knee of the condensing curve I wouldn't expect the efficiency gains from indoor sensing to be very dramatic at all.

If you got just indoor feedback with no other features, and you're using a boiler (not heat pumps, which are significantly more sensitive to water temp changes) then I would agree with your assessment of savings. However MOST systems that include indoor feedback mechanisms bundle other features which help a lot like post purge, zone sync, etc. When you add up all the additional features, it can be a significant savings, and indoor feedback is a potential contributor. It's important enough that I consider indoor feedback standard issue on water to water heat pumps, as well. Unfortunately the altherma can't "let go of the reins" enough to benefit... ah well.

If you consider that almost all heating systems are at least somewhat oversized though, and thus the reset curves are high, then you've got a long list of caveats on your statement too. Are you at 25% or 50% output more often? how far below a calculated reset curve will you typically run due to internal heat gains, etc?

larger savings are possible if it allows a real night time setback due to faster response recovery as well. with it's own long list of caveats, of course. it doesn't make sense for all systems... definitely. but for convenience, comfort and reduced callbacks it should be a pretty compelling package on an awful lot of systems. You get constant circ with the 'lead zone' determined in real time.... that's consistent comfort, right there.

I'll defer to your experience on that one, but it's by no means a no-brainer that you'd get an easily measurable efficiency boost out of it. (But not having to come back & tweak the curves for a client is incentive enough to sell the concept to a heating pro! :-) )

In my design work, I focus on comfort first. The graph you site is interesting but theoretical. Actual combustion and heat transfer may differ. With control strategies including full time flow, room temperature balancing problems can be mitigated or eliminated. Unless the we are talking about glass walls of course.