Derating a ModCon?

jad3675

SlantFin app shows my house (Cape Cod in SW Ohio) at 52K btu. Two zone radiant, 3/8 copper tube in the ceiling. ODR setup, with a design point set at 12F. The boiler ranges from 80F @ 68F outside to 140F @ 12F outside. The boiler is a TT Instinct 110 derated on CH to 50%, which should be slightly more than needed output.

Here's where it gets interesting...

Last night overnight temps were around 19F. Both zones (main floor and upstairs) called for heat. Boiler set for a 129.2F target temp via ODR. Boiler fired from 3am to 5:30am at full derated power and it was never able to satisfy target temp - it appeared to peter out at 118F. Once the upstairs thermostat was satisfied @ 5:30am and that zone closed, the boiler was able to meet the set temp and the firing rate fell to 30%. At 6am, the main floor thermostat was satisfied (it climbed from 68.5F to 69F between 5:30am and 6am).

I think I have the derate set too low, right? Is it worth the time and effort to find the sweet spot? Should I just set it to 100% and forget about it? Is it better to have the boiler meet the target temp quickly and then modulate down to maintain the temp? Or have it just at the edge so the boiler can barely meet the target temp?

John

PC7060

Assume you are referring to the ODR temperature curve? 

If the ODR system curve is increased it is like to satisfy thermostat and shut off.  Lower curve with gradual recovery is better since it keeps the boiler running longer periods. 

JohnNY

Why would you bother?

jad3675

PC7060 said:

Assume you are referring to the ODR temperature curve? 

If the ODR system curve is increased it is like to satisfy thermostat and shut off.  Lower curve with gradual recovery is better since it keeps the boiler running longer periods. 

No, not the temp curve. I'm pretty comfortable with the current ODR curve I'm using. I was asking about lowering the max fire rate of the boiler - trying to get it close to running for long periods of time while keeping the house comfortable.

JohnNY said:

Why would you bother?

Well, there are buttons to be pushed and numbers to be looked at.

But I do feel this is a fool's errand that really won't pay off.

John

Ironman

It could also be that your ODR needs adjusting.

I’d try increasing the maximum firing rate. It may overshoot the target temp on its first cycle, but the logic is pretty good and it will find its sweet spot quickly enough.

jad3675

@PC7060 - ah, I see what you're getting out with ODR.

Hm. I'm not really married to the idea of the ODR having a max of 140. I have two more curves available to me - 120F and 98F. It's surprisingly difficult to find any information though on what temp a radiant ceiling should - heck, people barely agree on what radiant floors should run at.

Daveinscranton

I spent a lot of time understanding this model of mod/con.  My suggestion is to keep the max fire, or de rating, as low as possible, as long as you are warm and comfortable.  

At the low end of the heating curve, for me, max firing control (de rate) matters a lot more than the ODR.  ODR will tell the boiler the target temp of the boiler.  The lower your flue temperature, the better.  In my case, supply water temperature never hits the target.  House is satisfied before the target is hit.

The factory default settings will keep you warm. They probably are not the most energy efficient settings though.  Probably works the best for the most.  Meaning warm house, not having to experiment.  Leads to short cycling though.

I added an old 120v dial clock to the house circulator.  Set it to 12 o’clock to keep track of hours/minutes for dialing in.  And a counter for cycles.  The “flame” output on the Triangle Tube Instinct is handy but adds several minutes of “run time” if you hook a timing device there because of the post purge.

I haven’t used the mod bus output.  A bridge to far for me.  The clock and counter works well.  The Triangle Tube Instinct counts cycles in units of 10.  It counts hours in units of 1 hour but not fractions of hours.  I needed greater resolution on cycles and time.  The screen graphics were helpful but not granular enough for me.

My application is very low temperature radiant pex in slab floor.  It made a difference in my application.  Actually a big difference.  Condensing mode 100% of the time.

A thank you to The Wall.  You helped me get to the bottom of this boiler in this application.

best wishes.

Daveinscranton

You can put a 27k or 33k resistor in series with a 50k linear taper potentiometer instead of the ODR for experimenting.  Serves 2 purposes.  You can limit supply temp max this way.  Very quick.  And you can develop a graph quickly from the display of supply temp target vs outdoor temp.  The display does not change instantly, but it will respond to a change every 30 seconds or so.  The ODR is quite linear.  How they actually made the ODR device, I don’t know.  Resistance goes up with decreasing temperature.  Very slick.

You can put the potentiometer set up on a SPDT switch with the ODR if it has any ongoing value to you beyond experimenting.  It is also possible to switch in a big fixed resistance with a relay.  I plan to do that for a bit more heat in a basement shop on a copper loop that runs at 120 degrees.  (Way hot by my standards).

There may be ways in the menu to accomplish all of this.  In my application, it works really well. I doubt Triangle Tube would endorse the idea.  To their credit, they were amused and withheld judgment.

Daveinscranton

BTW.  The boiler appears to assign different values to the actual resistance of the ODR device depending on the curve selected of your 3 options.   It is internal to the computer as far as I can tell.  Not any big deal but it was a head scratcher for a bit, with my potentiometer experiments.

The engineering in this model of boiler is really slick.  And it is forgiving.  And the tech support is superb.  With a sense of humor too.

hot_rod

jad3675 said:

SlantFin app shows my house (Cape Cod in SW Ohio) at 52K btu. Two zone radiant, 3/8 copper tube in the ceiling. ODR setup, with a design point set at 12F. The boiler ranges from 80F @ 68F outside to 140F @ 12F outside. The boiler is a TT Instinct 110 derated on CH to 50%, which should be slightly more than needed output.

Here's where it gets interesting...

Last night overnight temps were around 19F. Both zones (main floor and upstairs) called for heat. Boiler set for a 129.2F target temp via ODR. Boiler fired from 3am to 5:30am at full derated power and it was never able to satisfy target temp - it appeared to peter out at 118F. Once the upstairs thermostat was satisfied @ 5:30am and that zone closed, the boiler was able to meet the set temp and the firing rate fell to 30%. At 6am, the main floor thermostat was satisfied (it climbed from 68.5F to 69F between 5:30am and 6am).

I think I have the derate set too low, right? Is it worth the time and effort to find the sweet spot? Should I just set it to 100% and forget about it? Is it better to have the boiler meet the target temp quickly and then modulate down to maintain the temp? Or have it just at the edge so the boiler can barely meet the target temp?

John

Some mod cons have a boost function, if the boiler is unable to keep up, or catch up in a certain amount of time the control allows it to ramp up.

Daveinscranton

There are 2 separate heat circuits available.  CH1 and CH2.  They can have separate curves assigned.  I believe that it defaults to the higher temperature curve when both call. Could use that combined with max fire rate to dial it in for your application.

Lot of flexibility built into this boiler.  You will get it humming just fine.  

jad3675

hot_rod said:

Some mod cons have a boost function, if the boiler is unable to keep up, or catch up in a certain amount of time the control allows it to ramp up.

Yeah, the Instinct doesn't appear to have that function, at least that I can find.

John

Zman

I don't see any advantage to derating the boiler. It sounds like you have the outdoor reset set correctly, you just need a little more boiler when everything is calling. I would turn it up to 100% and see how it goes.

jad3675

Zman said:

I don't see any advantage to derating the boiler. It sounds like you have the outdoor reset set correctly, you just need a little more boiler when everything is calling. I would turn it up to 100% and see how it goes.

I've got it turned back up to 100% - I'll watch it this week. I was trying to prevent excessive cycling and eek out some efficiency.
I noticed that if both zones are calling and the upstairs gets satisfied first, the swt will exceed (spike) the set point by 5 degrees or so. This causes the boiler to shut down for a few minutes and then start back up. I noticed this generally happened when the firing rate was > 80%.

I'm probably chasing my tail on all this, really. I should be happy that a) house is warm and b) boiler makes water hot.

John

jinbtown

jad3675 said:

Zman said:

I don't see any advantage to derating the boiler. It sounds like you have the outdoor reset set correctly, you just need a little more boiler when everything is calling. I would turn it up to 100% and see how it goes.

I've got it turned back up to 100% - I'll watch it this week. I was trying to prevent excessive cycling and eek out some efficiency.
I noticed that if both zones are calling and the upstairs gets satisfied first, the swt will exceed (spike) the set point by 5 degrees or so. This causes the boiler to shut down for a few minutes and then start back up. I noticed this generally happened when the firing rate was > 80%.

I'm probably chasing my tail on all this, really. I should be happy that a) house is warm and b) boiler makes water hot.

John

I have my Lochinvar derated from 150k to 25% (37.5k which is approx my load at design temp)
The vast majority of the time the thermostat acts like high limit switch and the ODR curve being dialed in keeps the boiler from ever shutting off. 38 degrees outside, or 0 degrees outside, I never run less than 18 hours a day and am usually calling for heat approx 22-24 hours a day, exactly how I want it.

My problem with keeping the max fire rate above 25% is that in scenarios where temps fall rapidly like day to night transition, heat will kick on (lots of south facing so on a 35 degree day with southern sun I may not be calling for a couple hours in the afternoon) and my slow circulation rate with Alpha pump will mean that the boiler will overfire the volume of water in the heat exchanger, rapidly raising the temperature, since it raises too rapidly it'll shut off on a safety (even though it's raising from say 100 degrees to 140 degrees with a set point at 130. The return water hasn't had a chance to cool things down yet. By lowering the max fire rate it fires at 45% and immediately drops to 25% until it the return water makes a complete loops and the supply temps start to rise. I don't get nuisance fire, lockout, re-fires this way. Just one added benefit for me, and there are no downsides to maxing the fire rate. If I were you I'd just bump it 5-10% so that your ODR curve is what's limiting it vs the max fire rate limiting.

My Lochinvar is a Noble 150k with 10:1 turndown so I can turn down all the way to 14k output BTU. Most of the winter I've been cruising between 10 and 15%

pic attached of my run times for the last month

hot_rod

My design day load is 18,000 in my shop. I have a Lochinvar Knight 50 derate to 30,000. The issue I had turning it all the way down was recovery. I’m trying for a 5 degrees setback. I also use the ramp delay to help with short cycling, that too limits recovery from setback 

So you need to juggle all these setting options, ODR, derate, ramp delay, setback, and boost. 

Plus the always present flywheel effect of concrete slab systems. See what combination fits your wants and needs, make it your ideal comfort system

Daveinscranton

What I am sorting out now is the shoulder parts of the day. Mid morning and evening.  Next experiment is to add a timer such that the minimum burn time is 15 minutes or so.  As it is, it flickers a bit at the shoulders of the day. Maybe a couple of burn times lasting 6-8 minutes.  

This almost certainly is folly.  That said, it has been entertaining.  The goal has been long burns and no sacrifice of comfort.  May add to the life of the unit.

The default settings kept the house comfortable right out of the box.

best wishes 

hot_rod

Water under the bridge at this point, but the higher end brands or models have all these functions built in. Add the Conexus  to the Lochinvar and you can tweak all day long from your phone😉

jad3675

hot_rod said:

Water under the bridge at this point, but the higher end brands or models have all these functions built in. Add the Conexus  to the Lochinvar and you can tweak all day long from your phone😉

TT has TSync now, which is similar. It comes standard with the floor standing models. I'm not sure if it can retrofitted to the wall hung boilers though.

...but, the instincts have a modbus interface which is pretty easy to interact with. I monitor and control mine via a web interface I made. No phone app, though.

flat_twin

A few years ago when choosing a new boiler, the installer favored the Eco 110 over the Eco 70 modcon (no heat loss calc performed by them, just going by the previous boiler rating), I was curious if 70k btu would have been enough to handle our heating and DHW. So after the outdoor reset was dialed in and working well through a couple winter seasons, I derated the boiler down to 70k btu and it's had no problem keeping up. Of course the minimum fire is the same as it was before (21k vs 14k btu) so not exactly mimicking an Eco 70 but still proving the point that the smaller boiler would have worked.

Comparing part numbers from the Eco 70 and Eco 110, the only difference is the gas valve and one setting option in the initial setup menu.