reset on top of reset

archibald tuttle

have a system with two 140 btu munchkins where i'm replacing one that has leaking heat exchanger with a 150 btu noble (a knight would be more appropriate, I don't use combis ever, don't like the stack temps from making hot water, but i got a deal on a scratch and dent noble so i've got some parts i ain't using (well that's true on the body as well but thats for the dating sight :-).

now I never really knew what the munchkin algorithm was and it's opaque and i don't believe addressable but these worked perfectly as primary main loop, twin secondary boilers. I'm using a PC702 (the green version of some Tekmar controller) for outdoor reset and staging control. when it decides the main loop needs some heat input it calls one boiler (switches lead, i forget every 48 hours or every week or something like that). it is a dumb TT call and the munchkin comes on turns down and i assume is looking at inlet and outlet temp and if it doesn't see enough of a differential after some time period , 30 seconds or a minute or something the munchkin turns up. rinse and repeat until it is running at full (i believe 5 stages on the munchkin).

meanwhile the PC702 waits 5 minutes to see progress on heating the loop and if it doesn't see sufficient progress (defined by its own algorithm which maybe learns from system experience in order to prevent short cycles, not sure what differential or change it's looking for is) it calls the second boiler.

so, according to theory of opps in manual and some discussion with lochinvar the noble compares inlet temp to either outlet temp or system tyemp if you install remote supply temp sensor. And it has its own outdoor sensor and wants to run its own reset curve whereas the munchkin had no such think and just had some kind of load or differential based algorithm that just worked perfectly for this kind of setup.

it seems the only way i'm going to approximate that is to set up a reset curve on the noble to create targets. i'm assuming what i would want to is set modestly higher design tempts than i have on the PC702 but not drastically higher so i still might experience the benefit of condensing (building has fan cools and maybe 50% or more of the time I can keep returns in the 130 to 135 range or lower.) Of course what I would like is for the noble to take the low return temperature and raise it slowly to keep that condensing and have an outlet target where you get mixing with the system diluting it to the target temp.

that's my novel about what i'm thinking about this novel setup. if i like the noble, it being the first lochinvar i've dealt with, eventually i would pair it with something control compatible in their line. maybe a knight although i don't know if they make the knight in a floor model and part of the reason i got this noble is it is the floor model so it is good replacement-albeit a bit taller- for a munchkin floor installed. once i do that i don't really see but imagine they may have some kind of twin boiler link with a master slave staging arrangement that might be native to the lochinvars. i'm going to nashville for the STAFDA show this weekend and going to stop at the lochinvar offices in nearby lebanon so i will bring a bright light and interrogate whatever design engineer wanders into my clutches. in the meantime, i'm with the PC702 for staging and taking any advice from those with experience of theory and perhaps of the lochinvar itself in a mixed manufacturer setup of this sort.

best, brian

mattmia2

I think you can also set minimum and maximum firing rates which might help with the ramp up.

I assume you already have a pair of specific boilers because a boiler with a 10:1 turndown ratio can probably cover your load in one boiler.

mattmia2

Some controls can modulate on 0-10v and some BMS controls can output 0-10v, that is another option. Read the manual to see what the 0-10v is modulating too, the swt or the firing rate.

hot_rod

I think the Knights have a more robust control with staging options and Conex for remote monitoring and adjusting.

Find one of the control gurus when you are in Lebanon. Maybe schedule an appointment ahead so you are sure to connect

They have a control simulator at the Lochinvar website, in the “tool kit” maybe you can play around with that?

Gregg Gibbs at Shamrock Sales in Denver is very knowledgable with those Knights also. Or maybe he can hook you up with the experts in Tennessee

archibald tuttle

@mattmia2

in old fashioned conservative sizing I need these two boilers to cover the building in order to make up if all units were on high demand, as in coming back from a setback in traditional thermostat arrangement although in practice I have been able to hold my own with a single boiler on some pretty cold nights utilizing constant circulation and outdoor reset in the fan coils. it's a brick building and once you get the place warm just kicking 140,000 into it keeps up although the PC 702 often calls both boilers in deep winter. But the really ideal part of the staged boiler setup is, if one goes down, the other can more or less keep up. That has given me 30 years of no no-heat calls in the building. The original plant was 2 250,000 btu heatmaker hotwatermakers which I bought because they could take a higher blowoff but they did not modulate and I was a neophyte as to stack temp and return temp for condensing. Bad idea, they spec'd ABS flue, but that didn't last two years. I went to CPVC and downfired them to 175.

(ironically, the fan coils were made to run all the time with speed adjust for comfort and a temperature controlled zone valve with the sensor in the air at the intake filter. These old honeywell eliptical ball zone valves gave up years ago and the piping is so tight and there are no unions or provisions for replacement so I cut them out and just circ constantly which actually proved to be a much much better approach. And the fans are manually controlled for comfort and are often left off by tenants during shoulder season. wish they had instead installed thermostats in each room away from the units that I could use to control the fans. maybe when remote, wifi units get cheap i'll do that. the tenants can just access the fan speed switch.)

Point being, that I don't really need 10 stages of turndown that i'm getting with newer boilers. the 5 stages of the old munchkins were plenty. Almost a two stage gas valve kind of setup would be fine but obviously atmospheric boilers couldn't handle the return temps. instead of trying to limit the min/max fire, for now i just set the reset curve on the noble 10 degrees above the curve on the PC 702 to get started, and then i'll talk to the guys in lebanon speaking of which, thanks for the Lochinvar name drop @hot_rod I'll call Gregg on Monday and get a name and set a meet for tuesday. I got a knight in the scratch and dent sale but its wall mount and its a 110 so I went with the noble. hope i can joust with the control enough to make a good operating motif . . .

hot_rod

You are on a good path, keep going.