Mod/con with small zones

hr

You said
"Either way with a 40 gallon tank and 15 gpms through the boiler loop, that tank's benefit to lower return water temps or add an artificial load are done in less than 3 minutes. It's spent at that point. It also extends the time spent at a higher fire for the boiler to establish the load."

If only one small zone is calling where does the 15 gpm flow come from, certainly not fdrom that one open zone valve or pump. So if the load is let's say 5K the boiler would not ramp to full speed to meet the demand?

I've been doing some delta t experiments with tiny load panel rads, for my next IF presentation. This Contender starts at low fire and pretty much stays with the load for 10 or so minutes with a 23 gallon buffer and a small panel load. Finally, even at the lowest fire rate this (oversized) 99K catches and passes the load and winds down.

hot rod

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Jim McKee

Mod/con with small zones

Homeowner/contacter here planning a new system for next summer (yes I'll be working with a professional installer). Unique situation: vintage house under renovation, originally had gravity radiator system, unfortunately converted to fintube baseboard about 50 years ago (when an addition was put on the house). My thought/desire is to leave the fintube in the 4 second floor bedrooms, each on it's on t-stat. This can be easily acheived with existing piping. On the first floor, I want to put in cast iron radiators that I have collected from various homes over the years. (I have Dan's book about "Every Darn Radiator" so I can size them roughly to the heat loss, or at least make them all about the same amount oversized.) For comfort, I'd also like to "microzone" the first floor radiators, maybe 4 seperate zones. I just like the idea of turning up the heat in just the family room at night, keeping my office cooler when not in use etc. I plan to put radiators in the 3 bathrooms with TRV's on a sort of modified zoning set up -- if any zone calls, hot water circulates to the bath rads unless the TRV says "hot enough already" and shuts. This is basicly my dream system for my dream house (and I've been living/freezing in another house with forced air for 8 years, but that's another story!!)

My issue/question is, with a mod/con, will the modulating save me from massive short cycling? That is, if one bedroom calls for heat, will it "know" to fire at the lowest modulated BTU's abnd only ramp up if the return water temp doesn't rise fast enough or other zones start calling? Do different manufacturers have different systems for controlling? I'm thinking that a mod/con with a really wide modulation range will be my best choice, and the NTI Trinity meets that bill, but I am not opposed to any one brand.

Thanks for any help or comments here -- I really enjoy this site and Dan's books and have learned a lot from both!!

ALH_4

short-cycling

Micro-zoning can exacerbate an already short-cycling boiler, but generally will not cause significant short cycling on its own. Part of the benefit and the difficulty of modulating condensing boilers is that they are uniquely tied to the system and therefore must function as a fully integrated part.

I would TRV the entire system and use constant circulation to. At first it sounds like you lose control of programmable functions of the digital thermostats, which is true. But once you find a temperature where a room is comfortable, a TRV has a wonderful perfectly functioning simplicity that can match the heat input to the load like nothing else.

Chronic short-cycling is, for the most part, a function of the entire structure and its widely varying heat load, which varies from 0 to design, infinitely. Most important is to size the boiler based on a detailed heat loss calculation. You are right to look closely at the minimum modulation as closely as maximum output.

McKern

Look at the controls...

Jim, it's pretty well impossible for a single boiler to modulate to all ranges. At some point, they fall below the minimum modulation and that's where the controls kick in.

My experience is limited to MCBA controllers (a common control used by many manufacturers). It has numerous settings that will allow you to very strictly control, how it fires.

For instance:

· You can set it so that when it fires, it fires for 10 minutes at a time - it then varies the time interval between firings and you won't sense that this is even going on.

· You can set a minimum time between firing cyles for up to 5 minutes.

· You can cap the fan speed for heating only so that when it does start up it never exceeds what you need for design day, like a governor - this is especially good on multi-zoned systems to limit the HP the boiler might think it needs as various zones keep coming on. If it's allowed to go to high fire when sensing the colder loads coming back as zone open up, it won't be able to run as long before it hits the ODR target return temp.

· You can also use the differential/hysterisis settings to tune how it cycles.

Until, boilers can modulate more than 10:1, understanding how the controls can be used to tune the cycling characteristics of these machines will be paramount.

You also have to remember that cycling off isn't a bad thing either. If it is only firing half the time in milder conditions you won't sense any comfort differences and you'll be saving on your electrical.

ALH_4

knowledge

Steve, you're getting pretty deep into that control. Please share with us all you know, at some point. Perhaps you have already and I missed it...

chris_86

Jim,...

If you have small zones in a well insulated house, then short cycling may not be a problem. With some very expensive models, that also have sophisticated mixing valves and variable speed pumps that go along with the firing modulation, but most of them have a minimum firing rate that when you get to small zones they have to cycle. I say may...if you have a higher heat loss with a small zone you will absolutely get any boiler, especially these mod/cons cycling on and off about every five minutes and this is not good!

Almost everyone will recommend a flash tank, or frequently called a buffer tank added in series to the boiler to prevent this. They just don't modulate, as some believe, and have written, small amounts of water. They turn on and off till they self destruct. Adding a small tank isn't a big deal and is common practice. HTP and many of the other mod/ con manufacturers have detailed piping diagrams as they have learned this from experience many years ago.

hr

you really need to put some numbers

to your question to get real exact numbers and answers.


You need to know the lowest firing rate and the load numbers of the various zones you can calculate fairly close what the cycle times will be.

Siggy has a nice program to do this in the HDS. Keep in mind those loads are at design. A mild, or less than design day will increase the cycling.

The hard part is determining what a "short cycle" is and when it should be a concern. I've installed a few MZ non modulating units and they cycle their brains out. However the manufacture clain these frequent cycles are not an issue. The on/off noise of the whole think would drive me nuts, however.

A properly installed buffer tank can really smooth out the bumps. Even a 30 gallon tnk will be a big improvement over a boiler that holds 1 gallon

hot rod

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Jim McKee

Thank you all. Maybe the buffer tank idea is the way to go. But what I'm still wondering about is exactly how these units know to modulate. In otherwords, what is the logic the controllers use (and does it vary by manufacturer).

For instance, the Trinity Ti 100 modulates fron 15,000 to 100,000 BTU's in. Not quite 10 to 1, but much more thatn the typical 3:1 with many others. If, in my situation, a bedroom with 20 feet of fin tube (3/4" feed and return) calls, does the unit "know" to hold at 15,000 BTU's, or will it start high and then slow? or what. I'm sorry if I'm being too non-technical here. I could see 15,000 BTU's running for a while before shutting down.

What I don't want is to have my multi-zone setup cause my boiler to fail due to excessive on-off cycling.

BTW, I understand (I think) the efficiency of the recommended constant circ with TRVs' Andrew. It's just that I like having rooms or sections of the house that heat up and cool down on different schedules. Like waking up to a kitchen with nice warm radiators at the same time every morning.

Jim McKee

And if I go the buffer tank route, modulation becomes less of a factor, yes? In other words, would the buffer tank reheat at full BTU output, as if reheating an indirect tank (which I also plan to have as part of this system)?

McKern

Most buffer tanks waste energy!

If the buffer is on the return boiler loop getting full flow it may actually be less efficient. The boiler is expecting a steady load. It sets its target fan speed on that - an algorithm based on how long it takes to get the return water to start to warm up. The all important fan modulates up to ignition speed and then full fire or part fire from there. Some go up to full and then modulate down once temps start to rise in the return water or adjusts on the fly and starts based on closer for odr.

Either way with a 40 gallon tank and 15 gpms through the boiler loop, that tank's benefit to lower return water temps or add an artificial load are done in less than 3 minutes. It's spent at that point. It also extends the time spent at a higher fire for the boiler to establish the load.

In the meantime the boiler isn't going to back down from that artificially higher fan speed until some time after 3 minutes. The boiler doesn't know that load is temporary. A 10°F rise for a 40 gallon tank is 3.2mbtu in far less than an hour - 4 minutes. 15 x 3.2 = 48K. So you just started your boiler off 48K too high saving you from the ravages of short-cycling. How long do these cycles last with the boiler firing that high? NOT AS LONG many times!

And every cycle you do this, the boiler is firing hotter than it needs to, wasting fuel and doing larger expansion/contractions per cycle. When that boiler tank's 40 gallons are spent, if the boiler is firing 48K too high for the sustained load and it has a small capacity HX, what happens to the water temps. Way past target, easily out of condensing range... small volume system with 48K of oversizing!

If you are going to use a buffer tank, I really feel that the effect of the cooler water has to last about the length of a full cycle. You want the cooler water from the buffer tank near the end of the cycle.

What might be best to do is have a smart controller and a very small ultra low watt VS injection pump in a separate indirect loop off of the boiler return. At the start of firing cycle it does nothing. As return temps start warming up, it mixes them down. It targets the flow to the expected duration of the firing cycle (a TRV could also work).

Still, the tanks take up a lot of space for something that controls can totally take care of. If you add a buffer tank, try and maybe measure consumption per DD before and after. You can actually see if there's a change. I'm going to put one in, but it'll be fixed partial flow. Maybe when in years if I get a VS pump I'll control that flow more. At partial flow it stands a chance on helping a bit.

Jim McKee

Perhaps I should start without the buffer tank

And see how the unit performs in actual service. Maybe controls can solve short cycling. Or maybe in my situation, the fact that I'll have the 3 bathroom on TRV's with hot water circulating to them whenever any zone in the house calls, will add enough extra volume to let the unit operate for a while.

I think I'll also see if I can reach some one in technical service at NY Themal/Trinity to run my situation by them and see what they recommend. My other choice would probably be the TT Prestige 110, although it only modulates down to 30,000 BTU's.

McKern

Jim

I have a Prestige and also a Prestige bias...

No matter which boiler you get it will cycle. You will often only have about a 5K demand. There isn't anything on the market that'll run that kind of load through modulation.

In the meantime, the T100i requires a Taco 009 or Grundfos UP26-64 if you want to use the optional flow switch (the Prestige comes standard with one). The Prestige can drive the whole show direct with an internal UP15-58. Only 80 watts on speed 2 to drive the whole show vs 160 to 200 watts just to do the boiler loop for the Trinity. Even less for the Prestige is using P/S because it can use speed 1 (the HX is wide open).

The form factor on the Prestige is also better for many installs. Intake and exhaust both off the top, piping all on the bottom.

The T100i should be cleaned annually, the jury is still out on whether a Prestige would ever need to be cleaned.

It's hard to get much information on the Sentinal controller but it doesn't seem to be as well featured as the MCBA. The MCBA also has the advantage of being a more commonly used by a variety of mfrs. It's made by Honeywell and used by Burnham, Cleaver-Brooks, W-M... they should be easier to obtain years down the line if you need one.

The Prestige can't modulate as low but isn't going to need larger / costlier / noisier pumps and will use far less electricity. It also has an excellent control, built in low water protection and a self-cleaning / flow tolerant HX.

Still... I agree - it would be nice if they modulated lower. In Europe the same model modulates down to under 20MBTU (5.8KW). No fair...

McKern

Are we talking similar layouts h.r.?

As I said in my first sentence, my answer is based on full flow going through the boiler return piping in a P/S layout as is often mentioned as a good way to do it with a modulating boiler.

Under those conditions it's the boiler circ that determines how long the coldest water stays in the buffer tank before it gets diluted down temp-wise. It is taking out the coldest water out of the buffer at the GPM of the boiler pump and there are some boiler loops that have high GPMs.

-----

How is your buffer tanks piped? It looks like it's a LLH type of setup. It's hard for me to tell from the pic.

How much is the buffer tank cooling between firings? If it cooled down significantly, what's stopping the Contender from firing up too high initially? The only clue the boiler has as to what load it is dealing with comes through the change in ΔTs. Is your CH fan speed max capped?

jp_2

limited returns.....

if your system is trying to run long(small floor) the buffer will reach a temperature that makes it transparent to the boiler, thus start short cycling.

I'm looking at running the buffer at a higher temp than the floor needs and draining down the buffer temp over time, to be replenished by the boiler later on. being that the buffer volume is much greater than the floors. this of course requires mixing.

using the buffer with a lower temp may look good on paper but falls apart when running over time?

hr

true

the boiler to PS interface flow is constant, in yourcase. But if the seconsdary side is only flowing one zone, lets say then the actual heat load the boiler sees is that 5 k load, not the total load the boiler pump was sized for.

Mine is a work min progress and does not have a control on the tank yet. With it piped as a seperator as you noticed, it merely and extension of the boiler and the boiler aquastat is in charge of the temperature within the whole system. But that boiler fires on low speed and never ramps up if the delta t is not telling it to. As soon as I figure out how to read fan speed when it is running I can confirm what step it runs on with that low panel rad load.

When all is done that tank will have a well aquastat to maintain the tank at 120as it is an Ergomax and will become my DHW preheat, after the solar peheat, and before the plate HX that polishes the temperature to the final DHW temperature desired.

At least in my minds eye that's how I picture it working.

Have you run a Prestige with one pump yet? That to me is the strong point of that boiler. Id love to get one on the wall and test run it with delta T and delta P circs straight to the load.

hot rod

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McKern

No buffer tank here yet...

My Prestige was piped so that it can run either direct or P/S.

When valved to run direct, I run the internal 15-58 at speed 2. When valved for P/S I run the heating loop on a second 15-58 at speed 2, and the boiler loop at speed 1.

It's a real crap shoot to say which way works better. I get the feeling that I get a slightly wider ΔT running P/S at colder temps. It's hard to tell because the North American MCBA controllers count by 2s.

I followed the direct vs P/S debate, but I think with monoflo piping all bets are off because you need so much flow on the heating loop that it is hard to widen the ΔT on the heating loop. Is it possible in that case to just widen it on the boiler loop? I don't know...

The monoflo mains can almost be looked at as another P/S system. The monoflo mains run much warmer than the branches. Between that and the higher flows, it can't be seen as an ideal way to make these puppies condense their best!

The only thing I know for sure is that it costs around $7/mo in electrical costs to run it direct and $10/mo to run it P/S. One switch and 3 valves switches me from direct to P/S. The slowest part switching modes is opening the boiler door to switch speeds on the internal circ.

chris_86

On small zones a buffer tank is manditory, there realy isn't any debate. The voices of experience are trying to tell you you can debate till someone is totally confused but the first manufacturer learned the hard way...that being HTP... that regardless of modulation, setback, or retoric untill you throw up...you need a buffer tank, period!

Why, because some small zones in 3/8 pex use about 6 gallons of water. No heater can modulate that low. And if they do I suspect the effecincy: burner: hx size: ratios will be scewed, this is also the reason WM is looking at variable volume stratigies for the future. This will make sizing a boiler a thing of the past...you heard it here first folks...

Uni R_3

What makes you so sure?

Jim (he started this post) is considering a Trinity or a Prestige. I can't speak for the Trinity - I haven't seen any in depth information on their controls. I can speak for the Prestige's controls.

When a Prestige falls below minimum modulation it fires for 10 minutes at a time. That's how they work. How does a buffer help that if it is firing for 10 minutes at minimum modulation? If that small zone can't absorb a total of 5 mbtu coming over a 10 minute period without a huge overshoot, then how are you going to pipe and control that buffer tank and keep all the extra heat away? Is a 10 minute firing cycle too short?

Jim McKee

Uni-R: What has your experience

Been with your Prestige? What is your smallest zone and what kind of cycle times are you seeing? I think I understand your advice to be to skip the buffer tank, but I am confused on which way to go here.

Perhaps my idea of zoning each bedroom is not practical, even with a mod con that modulates down to 15K (Trinity) or 30K (Prestige). I understand the Prestige has some nice advantages in it's heat exchnger design.

I am in Chicago, there should be some good professionals in such a large Northern city, but I only find one guy in the far suburbs through this site. And whenever the topic comes up, the opinions on when and how to use buffer tanks really vary here.

Any final thoughts/advice?

Uni R_3

So far so good...

My experience so far has been limited to using the Prestige on a single monoflo zone using fintube. I've spent considerable time trying to fully understand the controls because I'm not done yet. I want radiant floors in the kitchen and baths and possibly a way to boost the temps in the back of the house in the springtime.

My boiler only modulates down to 40K and T-T is looking into this. My home's calculated heatloss is 56K. I now know that my actual heatloss is 40K or less. So my modulating boiler rarely ever modulates. But unless you are monitoring the machine, you'd never know what's going on. I have monitored it and that is why I know its cycling characteristics. The machine was installed mid-December and started up in extremely mild weather but it worked flawlessly. It times its 10 minute firings to match the house load with perfect comfort.

The only thing, I'd change if I were doing this all over from the start would be to rip out the monoflo. It really isn't well suited to life with MC boiler - if only they were as restrictive as many people here say they are. Anyway, they have very high flow which results in a low ΔT.

Ideally, I'd try and design for a single temperature like I am doing but I'd just pipe the boiler right into a manifold and only use the internal circ. Total electrical consumption would be 100 watts firing, 85 watts when it's not firing but there is demand but it has met target water temps and 14 watts if there is no demand and it goes idle.

If you can modulate lower instead of cycling but need an extra 160 or 200 watt pump to do so (as in the case of the Trinity with a 26-64 or 009 for the boiler circ) are you saving on energy costs? I don't know.

The piping connections for a buffer tank are already in place on my system. I'll probably still put the tank in, but the controls already 'modulate' the cycling so I'm a bit worried about what I'll actually gain in the process other than having another 40 gallons of water that will surpass the capacity of my 30# expansion tank.

Keep it simple...