Short cycling munchie.

Jamie_5

"You don't have to raise the water temp in the buffer tank above the zone temps"

That is particularly significant if you're using outdoor reset. If the buffer is piped as an extra zone and you are resetting the water temp in conformity with the outdoor temperature, what temperature would you keep the buffer zone at? If you keep it at the maximum you will need, standby losses increase and you need to add controls to mix the temperature down when you ask the buffer to supply the demand. If you want to keep it at the reset water temp, you will need some sort of self-adjusting setpoint control.

In contrast, if piped in series at the return, the buffer tank adds a load to the boiler no matter what zone is calling, gives the Munchkin cooler return water to aid in condensing, has its temperature reset automatically, only has as much standby loss as necessary, and requires no additional controls.

JimmyJam

Short cycling Munchie

I got called to go check on a boiler that is short cycling yesterday and found the following:

140m, (3) microzones (mudroom,mstr bath floor and a 12 x 12 solarium), (1) zone is a 20' high "sport court". (2) radiant zones in the basement.

I have not gotten the heat loss completed yet but am estimating the heat loss at -14 design to be in the 75k neighborhood.

Of course the most important zone to the homeowner is the Master bath floor and she notices alot of boiler action when this zone is calling for heat.

This system is zoned with zone valves w/end switches and a 0011 circulator and excludes a differential bypass valve. It does not have a Vision control.

My thought is to install a "buffer" tank to either maintain a setpoint temperature or to add water capacity to the boiler with a Vision control.

I appreciate any help on this I can get!!

71Gibby

hr

If the load

really is 75 K or less I'd replace it with an 80K.

A buffer is an option HTP makes a nice insulated 20 gallon with 4 large connections.

If you have Siggy's HDS progrsm you could run the buffer tsnk calcs and see how it would change the run time with various zones calling.

hot rod

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[Deleted User]

Short of replacing the boiler with the right one....

You're on the right track. Even a V1 is not going to keep it from short cycling when a micro load is on line.

THe added mass from the bufferin tank will extend on and off times, but it will still short cycle comparitively. Just longer on and off cycles.

Mod Cons are somewhat forgiving if somewhat appropriately sized, but when they get stuck into a 2:1 over sizing condition, there is STILL just so much modulating that they can do under part load conditions.

Good luck.

ME

Jamie_5

do both

I have a T50 Munchkin installed by Ken Secor and company. (They were wonderful contractors.) We have radiant ceilings installed by us, rather micro-zoned and designed for only a 10 degree F drop across loops. The first season of use, it seemed to short-cycle. After consultation with Scott Secor, Palmer Heating installed a 30 gallon electric water heater as an inexpensive buffer tank and added the Vision 1 control. Wonderful change. Longer cycle times, at low fire in these relatively warm shoulder season days, almost no expansion noise, and faster recovery of the indirect tank.

Constantin

Sounds like a couple of issues

Besides seconding the suggestions by hot rod and Mark, I would suggest using HDS to see what circulators are really needed. A 011 without a bypass sounds like trouble and may be causing your HO to hear flow noises.

I don't know what the least expensive option is, though since you have to change the near-boiler piping anyway, it may be less expensive to retrofit the Vision 1 controller and the buffer tank. I'd lay the options before the client and explain cost vs. benefit. Let them make the call.

Perhaps the buffer tank can be used a oversized low-loss header? That way you could continue to use it even if the heating plant is right-sized in the future.

JimmyJam

Piping

Thanks for the replies.
I have attached a piping diagragm for you to look at.

71Gibby

PS. I have ordered the HDS program.

Jamie_5

piping

Your diagram appears to use the buffer tank as suggested by Constantin, i.e. as a low loss header. I am not sure that will work any better than simply piping the tank in series in the primary (radiation and expansion tank) loop, since the Munchkin's control logic is determined by the supply and return sensors on the boiler outlet and inlet. If the boiler circulator and the radiation pump are to run simultaneously, as required to let the Munchkin sense the primary loop supply and return temps, I don't see any real advantage to the low-loss header approach, since it would require four tappings rather than two.

Even if you could mount the Muchkin's temperature sensors remotely on the primary loop, I don't see the benefit of the low-loss header approach over just piping the boiler as a secondary loop to the the primary radiation loop.

If it's physically easier to pipe it as you diagrammed, I don't see any problem with doing so and you can simply ignore what I said above.

John Ketterman

If there are microzones, there will be short-cycling. An HO will generally notice the circs going on and off, and this cycle will get longer with Vision 1 and outdoor reset, but the underlying boiler on/off cycles will still be the same (short).

Personally I am not a fan of buffer tanks in this situation; would first consider a smaller boiler, and second, consolidating the zones and adding TRVs. Ths will reduce short-cycling, although not eliminate it.

Of course using a single circ for two or more zones is not always possible, e.g. when the head is very different, but even then, it is simple to wire the circs so they run at the same time.

Microzones in general are a bad solution to the problem of precise temperature control in individual rooms; TRVs are better. Of course the stat should be in the coldest room.

jp_2

tie small zones together

why don't you just tie the small zones together?
instead of 3 zones under 3000 btu you have 1 zone at 6800btu's?

quickiest solution and easiest to test run.

edit: added idea

option 2.) control so heat will come on only by "pairing zones" in other words it will take 2 zone calls for heat, so you are always running boiler with 2 zones running.

JimmyJam

Thanks

> why don't you just tie the small zones

> together? instead of 3 zones under 3000 btu you

> have 1 zone at 6800btu's?

>

> quickiest solution

> and easiest to test run.

>

> edit: added

> idea

>

> option 2.) control so heat will come on

> only by "pairing zones" in other words it will

> take 2 zone calls for heat, so you are always

> running boiler with 2 zones running.

JimmyJam

Thanks

Thanks for the replies, I am going to complete the actual heat loss after getting the original plans from the contractor.

Going to spend the day with Dan today. Should be entertaining as always!!!

71Gibby

Wayco Wayne_2

Sounds like

good advice all around. My first Munchkin install done years ago I made the mistake of over sizing it. (What the heck. It's modulating, right? :O !!) It gave me lots of trouble until I did it right with a heat load calc. Another "tuition" job. I put in the right size and have had no trouble since. The old over sized Munchie is now heating my house. I would go right size and buffer tank if the customer will pay for it. WW

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Constantin

Good Point

However, I thought the whole point of using a buffer tank was to allow the boiler to take time off and rest while the thermal battery is being depleted.

One way to achieve that is making the buffer tank the only "zone" that the munchkin deals with, i.e. the Munchkin only comes on whenever the "buffer tank zone" is calling for heat. Meanwhile, a simple Taco relay box or whatever can take care of the actual zone circulators.

A downside to a buffer tank approach is that the return water temps to the Munchkin may in fact be higher coming out of the tank than coming back from the emitters (depends on the system).

Wayco Wayne_2

Connie

I sometimes use the buffer tank in series with the return water line instead of in parallel P/S style like you just described. That way it adds an artificial load to whatever zone is heating at the time and you dont have to raise the water temp in the buffer tank, above the zone temps, therefore keeping the condensing temps. Did you follow that? I'm not sure I did. The P/S style is mo better with a cast iron beastie to keep water temps above 140 in addition to stopping short cycling. WW

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Constantin

A drawing?

... I guess it's true, a picture is worth a thousand words. I'm puzzled.

Are you saying that the tank in your example is piped in series with the zone returns or the boiler return? Plus, how is the heat "harvested" whenever the boiler cycles off?

Jamie_5

I can't draw

So, I'm sorry, but no picture. I'm assuming that the boiler is piped as a secondary loop as illustrated in many of the Munchkin diagrams. In my system and, if I understand him correctly, Wayne's design, the buffer tank is in series on the return side of the primary loop, on the return side of the tees tying the boiler loop into the primary loop. The contents of the buffer are thereby available to whatever zones or other secondary loops come off the primary. (So there's no confusion, I use "primary" to mean the loop with the expansion tank, not the boiler loop.)

Your low-loss header approach does essentially the same thing; it just requires a four-port tank while the return-series approach replaces two of the ports with tees. Piping the buffer as a zone, in contrast, is different. It is not in series with the primary loop, but either in parallel or as a secondary loop.

Clear as mud?

JimmyJam

And the heat loss is!!!!!!!!!!!!!!

At design temp the radiant heat loss is 60,689 BTUH.

I will be proposing for a heating source change as I don't like "band aids"!

Thanks for the input from all who responded.

71Gibby

Wayco Wayne_2

Let me see if I can

draw something to illustrate. The parallel picture is not entirely accurate but hopefully will give you the idea. The Series picture I took a drawing I was working on and threw in a buffer tank so it's not exactly accurate either. Lets see if I can attach the pictures

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Mike T., Swampeast MO

NO EXCUSE for such boiler sizing!!!!!

And never forget that you can "microzone" to your hear's content if you use TRVs for rads and FHVs for floors. While the customer sees individual zones as defined by room, the boiler only sees zones as defined by temperature requirement--usually one and little rational reason for more than two.

Constantin

Thanks for the pics!

I'm still confused, but ask my wife, that's quite normal! I have enclosed three drawings of my own, omitting the IDWH, which is just a source of distraction.

In the top drawing biggest benefit is that the munchkin pump does not have to run for circs to get hot water. The zone circs do their own thing, the Munchkin simply keeps the tank warm. Conceivably, drawing #3 would also allow that (very little ΔP across the tank), giving you some minimal electrical savings.

Drawing #2 is what I think you meant. I can see how implementing it with a boiler that wants to have its circ on whenever a zone is calling for heat makes a lot of sense.

Drawing #3 at the bottom was just having some fun... why not "closely space the T's" across the tank... have the munchkin run as a counterflow heater, getting all it's needed water either from the system or the bottom of the tank, where it's coolest. Any "overage" gets added to the tank?

I dunno. Just having fun and it eliminates the need for a second set of tappings.

Jamie_5

almost drawing 3

Drawing one will work with the Munchkin if the boiler pump is operating whenever a zone pump is. It will not work with boiler reset without that, I think. The Munchkin needs to know, first, that there is a call for heat and, second, what the *system* supply and return temps are. It can get the first piece of information from a switch through its TT connections, directly from a thermostat, from an end switch, or from the X1 and X2 connections on a relay panel. It will never get the second piece of information, however, unless its own pump runs, because the supply and return sensors are inside the boiler while the supply and return water are at the buffer tank. You gotta move the latter to the former.

Drawing 2 will work fine, I think, though it's not what I meant and (again) works only if you assume that the boiler pump (which is also the primary pump in this diagram) runs whenever a secondary zone does, for the same reason as before: the Munchkin doesn't know what the water temperatures are going to and coming from the system (zone) unless the primary pump runs.

If you moved the boiler return to between the buffer tank outlet and boiler outlet (above the buffer tank but below the red line to the boiler in your drawing 3), so that the boiler connections were a pair of closely spaced tees to the primary loop, then you'd have what I meant. But again, the boiler pump has to come on at the same time as any zone pump for the same reason it did in drawing 1.

The more I look at it, the more I think there's no inherent advantage to one over the other, except that 2,3, and modified 3 save on tank ports.

JimmyJam

Sorry for my lack of \"wetness\"

I am somewhat embarassed to ask what an FHV is Mike T?

We have done a couple of retrofits with TRV's of which works really well except with space limitations for horizontal mounting applications for second levels but I can honestly say I have never heard the term FHV.


71Gibby