Condensing, Modulating Boilers

Mike T., Swampeast MO

From all I can understand, a condensing boiler thrives on low return and the higher the delta-t the better.

By very definition a boiler with modulating control is trying to match the heat production to the heat loss.

With both of those in mind, I'm having a VERY hard time understanding how primary-secondary piping (which tends to increase the boiler return temperature) combined with a fixed rate of flow through the primary (boiler) side has a prayer of meeting these goals when the primary flow is fixed for a low delta-t at MAXIMUM boiler output.

Constantin

That was one point of the Viessmann training...

.... primary secondary piping is NOT the best solution for boilers that have no lower limit on the return water temperature.

Consequently, the Vitola does best with a 4-way motorized mixing valve, while the Vitodens is typically shown with a motorized 3-way valve IF you also happen to have a IDWH on hand. Without a IDWH, and only one water temperature demand, I suppose you could dispense of the 3-way as well.

So your question is right on. Have a look at the Viessmann literature for typical piping diagrams. You never see pri-sec kinda stuff on anything related to the Vitodens because it defeats the very purpose of the boiler.

Mike T., Swampeast MO

This is Not About Viessmann in Particular

It's about ideals that seem self-evident.

Dan Peel

Piping

The best excuse I know of for primary-secondary with a condensing boiler ceters around the flow requirements of the boiler. With typically tiny flow pathways the restriction of system resistance simply will not allow enough fluid to pass the exchanger. Even with a primary loop you can set up some pretty low return temps from your distribution - lots of low temp applications have much higher total flows than the primary flow rate. Enjoy.....Dan

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hr

But

condensing boilers are prime candidates for radiant, operating efficiently in that low emitter, temperature range. But radiant floor heat in not that do-able with real wide delta tees. Some designs strive for a 10- 15° delta tee in the floor. That's pretty tight.

I still feel DVW had the best application for a condensor on radiant. Run the sucker into a buffer with a 100° delta tee. Charge the tank from say the lowest possible temperature it (the floor) can stand, say 80-85° then charge the tank to 180! With a good size buffer tank think of the burner cycle available with that scernio

This would be a great way to settle down a non modulating condensor, connected to multi, micro zoned radiant systems. Although perfectly sized, fixed load, condensors would not benefit from that, say a snowmelt.

Hey, maybe that's (DVW)who had these RTI setpoint controls built with the 100° differential adjustment Wish I could get more. Maybe Mother Upo will revive them. The control I mean.

hot rod

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Chuckles_2

> From all I can understand, a condensing boiler

> thrives on low return and the higher the delta-t

> the better.

>

> By very definition a boiler with

> modulating control is trying to match the heat

> production to the heat loss.

>

> With both of

> those in mind, I'm having a VERY hard time

> understanding how primary-secondary piping (which

> tends to increase the boiler return temperature)

> combined with a fixed rate of flow through the

> primary (boiler) side has a prayer of meeting

> these goals when the primary flow is fixed for a

> low delta-t at MAXIMUM boiler output.

Chuckles_2

P/S piping gives you a steady flow rate through the boiler and thus a stable delta-T in given weather conditions (changes slowly with weather/heatload but not when the zone circulators go on/off). I think it's not the goal to try to make deltaT as high as possible. In mild weather, when the supply temp is low, it doesn't matter. In cold weather, even gravity (uncirculated) flow won't give you a condensing return temperature. So there's an intermediate window where gravity flow might make a difference, but at what cost? How many people do you know are eliminating their circulators?

The purpose of modulation is, as you said, to match the heat output to the heat load. But this heat has to be transferred from the burner to the water. As the circulation rate drops, this gets harder and harder and the heat exchanger has to be bigger. And if the flow rate fluctutes wildly because of zone pump coming in and out, it is very hard to modulate correctly.

Mark Eatherton1

That's a

Heat Timer set point controller, they're still made.

ME

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Unknown

Funny you should mention that

Brookhaven Nat'l Lab is studying gravity systems again, in order to cut the electric consumption of heating systems.

So, in answer to your question, Scientists are taking the circulators out of modern heating systems.

Noel

Mike T., Swampeast MO

Again, Thinking Ideally

Interesting that you EXCUSE primary-secondary with "tiny flow pathways" in the HX.

OK...

I'll play along.

If that MODULATING burner driving the HX is suited to picking up 80 mbh with 8 gpm flow, why not 40 mbh with 4 gpm?

Mike T., Swampeast MO

Floor Delta-t

With Warmboard (slab heat unavailable) I truly believe the attached would work with FHVs (Floor Heat Valves). Note high delta-t and low flow.

The large zones are LOGICAL spaces--not rooms. Were this done in real life, there would be multiple FHVs in these large zones. Would use a floor temp modulating device for the entry. Note the zone that WON'T work with floor heat alone--that's the sleeping porch with 8 big windows (5 facing N).

S Ebels

Welcome back

3" mains and 1 1/2" risers. Sounds like fun to me!!

Good to see the powers that be still have a bit of common sense. I shake my head in disbelief when I see some "modern" systems with literally dozens of circs lined up in rows.

Mike T., Swampeast MO

You're still thinking in terms of zones and digital control!

If your burner is producing just the required amount of heat there is ZERO need for ANY digital thermostat!

The dead men designed this way because they had a VARIABLE FIRE! We again have variable fire but unless you ALSO have variable flow (either through variable speed pumps and/or thermostatically operated 2-way valves) the benefit of the variable fire diminishes as the outside temperature warms.

Cliff Brady

Modulating to a variable setpoint

I think you are on to something here in that modulation is all about modulating to a particular setpoint, preferrably variable based on outdoor temperature. At the lower temperatures in the lower percentage modulation you probably don't need all that flow rate in the tiny passages to safely extract the heat without risk of flashing. So the question is: is it more energy efficient to slow down the flow of water through the heat exchanger? Are there variations in heat transfer as the water makes it way through the heat exchanger? Does the higher delta-t really extract more energy?

hr

Noel, you mean

Slant Fin introduces 3" pex and fittings

Heck in another 10 years there won't be anyone around that knows how to thread and install large diameter threaded pipe!

hot rod

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Dan Peel

OK

Modulating the circulator and burner together? Could have some applications with micro zoning. Your chart (lower in the posts) may work with a flooded floor but i'm thinking huge variances across the surface with such sluggish delivery. I work mostly in constant flow radiant - pulsed flow modulation through a 3 way zone valve - really even distribution of temperature across the floor. Injection can work the same way. I'm not sure I want to be sitting close to either the start (too hot) or the finish (too cool) of the loops dropping temperature as in your example. What am I missing in your thinking here? Dan

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Mike T., Swampeast MO

It Comes From TRVd Standing Iron Radiators

and "wild" radiant floors.

Everything constantly circulating.

The delta-t is EASY to find at the supply and return, but you can't find anywhere near the low or the high in the radiator (or panel) itself.

This IS partly theory--I haven't yet used FHVs.

Mike T., Swampeast MO

Though You See LOTS of \"Zones\"...

...it's NOT about zoning! It's about meeting the current loss. Why on earth do you want your heat either "on" or "off" at any time that it's needed?

Those numbers are ideals--the actual might meet that at one point in one year. The object is coming as close as possible to those ideas the greatest amount of time.

Constantin

This is not about Viessmann...

However, Viessmann does happen to make a line of condensing, modulating boilers as well as the only inherently condensation-proof CI boiler that I know of. However, the issue is not who makes the equipment, but why they advocate using the equipment the way they do.

For example, if you want to minimize standby and jacket losses, the best way to achieve this is by modulating the outgoing water temperature to the lowest possible level while still meeting heating demands.

Quasi-Pri-Sec heating systems come into play with systems that do not suffer from low return water temps either when you have to gang multiple boilers or when the "primary loop" water flow exceeds the capacity of the boiler. That's where a "low-loss header" comes into play, acting as a hydraulic break between the boiler and the "primary" circuit.

Tell me, from an ideal point of view why one would not pair a super-efficient boiler to a IDWH? Seems pretty self-evident, no? Well, if you also want to heat a radiant space with that boiler, you'll also need a 3-way mixing valve to keep the radiant floor happy once the boiler fires on a 140°F demand for the IDWH while still supplying some heat to the rest of the home.

Where a condensing, modulating boiler only to supply heating loads of similar types (i.e. all radiant, for example) then there would be no need for any kind of mixing valve whatsoever. Just let it modulate down... the more it can, the more even the heat will be even during the shoulder months. To truely control temperature under all conditions, you'll need either a buffer tank or 3-way valve.

Constantin

Coming back to reality...

...also consider that even the most advanced modulating boilers feature a hump, a minimum turn-down. The most advanced make it to 5-1, IIRC. Thus, while some boilers might be happy to supply small loads with low input ratings indefinitely, other boilers will not achieve the turndown and consequently have to turn on/off whenever the load drops below the minimum input rating.

Under such circumstances, a pri-sec system might help a bit in terms of balacing out the heat "spikes" that every firing creates by smoothing out the input over a large amount of mass. A buffer tank would have the same effect though. Furthermore, the amount of time your system would face under said circumstances (where only a tiny load is calling for heat all by iteself) should be pretty rare in a home with constantly circulating systems.

Chuckles_2

sure

I am sure the Dead Men had gravity systems that were 100% efficient and needed no supervision and worked well, but now we've messed everything up with newfangled thermostats and pumps...

I've never lived with gravity hot water heat, but I suspect it gives the same level of temperature control that steam heat does (although slower variations)...and that's just not good enough today.

And as for Viessmann and variable-speed pumps and TRVs, I did try to price such a system for our house, but I would have to sell my firstborn. No doubt there's a nice profit margin there for contractors, but only people rich enough to have gotten a large tax cut can afford them.

jerry scharf_2

why the high deltaT

Mike,

I get the low return temperature, but I don't get why you say they want the highest delta T. If you are running an emitter that needs a high output temperature, then you will need the high delta T to get the low return. In applications where the supply water temperature is near 100F at design, the efficiency will be there with a 10 degree or a 30 degree delta T. With radiant floors, wide delta Ts have comfort issues.

P/S is all about allowing the boiler to keep enough flow through it to prevent problems. In the world where boilers have 3:1 or 5:1 turn downs, there's a minimum flow you need. Simply said, P/S is safer for the boiler. Given the things that manufacturers see done wrong in boiler installations, I can't blame them for recommending piping that is safest for the boiler. They put having the boiler operate successfully above the efficiency gains.

As for lowest return temps seen by the boiler with P/S, as long as the flow through the load loop is higher than the flow through the boiler, any heat injected by the boiler will not be seen by the boiler. It will see the system return temp, which is as good as it gets.

In at least one modulating/condensing boiler, the boiler controls assume constant flow through the boiler, and use the abilty of the boiler to maintain a given delta T through the boiler to drive the modulation. Muck with the flow to change the delta T and their boiler control software gets very confused. You have to be very careful in the world where you have several computer controls dancing together, often making simplifying assumptions about other parts of the system. There are no standards and few incentives for having interoperable controls in today's world of heating systems. Ask any of the boiler manufacturers for a published document describing thier bus communication protocols.

jerry

Dan Peel

Theoretic Heat

Your trail is leading away from comfort with conservation into the realm of minimized input. For comfort delivery across the demand range of building you need regional delivery proprtional to local losses. You can introduce enough BTUs from a corner fireplace to heat most homes,just not evenly. Boiler range shift through reset, boiler output management through modulation and thermostatic delivery regionally is getting very efficient without sacrificing comfort. In your cast iron rad setup example reducing delivered temperature at a constant flow rate will produce more stable dissipation rates under a wider range of piping options than will general system flow reduction. Enjoy......Dan

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Mike T., Swampeast MO

High Delta-T

Perhaps it's a misconception on my part, but fairly high delta-t throughout the system as a whole seems to indicate the efficient transfer of heat--both from the boiler to the water via the HX and from the water to the space via the emission device. Most emission devices are capable of extracting quite a lot of heat--how much depends on the flow rate. Only bare PEX systems seem to be sorely lacking in this ability to transfer heat.

Since the average temperature drops with increasing delta-t and a large portion of heat transfer deals with the difference in temperature between emitter and collector it would sure seem that efficiency in the initial transfer from flame to HX to water increases by some measure with lower average temperature of the water.

Increased delta-t also allows a given flow to produce more work. If that flow must be induced by forced circulation you can actually save energy by moving less water and extracting more heat from that water.

The unfortunate thing is that if you are able to produce just the amount of heat required by the structure at any given time, either flow or delta-t MUST change with the weather. If you keep a constant flow, delta-t will drop with warming weather. If you keep a constant delta-t, flow will drop with warming weather.

If you can figure out a way to make this not true, please let me know! I've tried and tried to no success as you start hitting "infinites" or other "undefined" numbers. Do remember however that I'm thinking in terms of fully constant circulation and space temperature maintenance. Yes, I realize that is an impossible perfection but you can get awfully close with TRVs or reasonable close with an extremely well-designed digital system.

That's where I just don't "get it" with a modulating boiler with primary/secondary and a constant rate of flow in the primary. With that constant rate of primary flow if you modulate the burner to attempt some arbitrary delta-t you are NOT modulating the burner to meet the heat loss! When I look at it this way, I FULLY understand why many consider the benefits of modulating flame to be questionable and likely not worth the added complexity.

If however you allow either delta-t OR flow to change through the HX you CAN modulate the burner to meet the heat loss.

If you choose to make flow constant you wind up with a bit of a problem. Since delta-t will decrease with increasing outside temperature you have to design for extremely high delta-t at design conditions to keep such reasonably high in warmer weather. This may well result in a supply temperature much higher (and much further away from condensing temps) unless your emission devices are quite oversized.

In most instances it seems preferable to vary flow. TRVs make this easy. It's a bit more difficult with variable speed circulation. When TRVs and variable circulation are combined, the variable circulator is mainly about using the least amount of electricity required to operate the TRVs.

With high delta-t and radiant floor I'm a bit out on a limb. I do know that with TRVs and constant circulation on standing iron that you can get an extremly high delta-t between supply and return yet almost no delta-t in the radiator itself. I do know that with constant circulation but no flow control that radiant floors [seem] to operate reasonably depending on their design. Using the same supply temp and exact same length of tubing I have one floor producing a 20° delta-t and another 40°. The one with 40° does have a higher temp difference across the surface, but certainly not objectionable. Also, while my heat transfer mechanism is efficient (copper in T-Fin) it must still drive through an insulator--¾" wood. Were this tube in slab, fin on top of the floor or Warmboard, I suspect that the floor would achieve a closer average temperature. I could easily be VERY wrong here--that's why I really like to hear from anyone who has used FHVs.