Condensing Boilers in Baseboard applications

Uni R_2

The buffer tank is piped so that it can convect hotter water back to the heating circuit - unfortunately the logic in the design was done before anyone had told me that Prestiges could be piped direct. I had my installer add the bypass so I could hedge my bets, cause direct sure wasn't in the manual then! ;-)

Anyway, long story short, buffer tank will sit below the piping so that warm water convects out while the coolest water displaces it between heat calls.

On a heat call, the buffer tank is essentially a monoflo branch so part of the return draws through the cooler water in the buffer tank mixing down the return temps. Ideally, it'll mix down temps at around 4 gpm from the buffer each 10 minute firing cycle emptying the cold water from the buffer and leaving 40 gallons of warmed water to convect out during the off firing cycle.

Too bad the only thing I found with P/S was that I had to buy 1.8 KWH of electricity every day to keep the dedicated circ running - I don't feel a comfort difference.

Still, with the system valved so that it is piped direct, having an extra 330 pounds of water to absorb each 10 minute 6000 BTU heat replacement injection can't hurt, right?

Robert O'Brien

Condensing Boilers in Baseboard applications

Does anyone have the BNL paper on condensing boilers in fin tube applications?Thanks

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Brad White

Brad White

Here you go, Robert

Enjoy.

Robert O'Brien

Thanks.Brad!



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Weezbo

Thanks Robert and Brad *~/:)

today we had the condensing Boiler's senior Tech rep here giving a class at the Cold Climate Research Center.

there was a definite "Showing' of interested techs ,sales , business owners,General contractors.. had i had this at my disposal it would have helped in the interpretation of my somewhat unique way of delineating the applicability *~/:)

mtfallsmikey

Very interesting study

I'd be interested to hear about some experiences with different piping schemes, especially fin-tube/Monoflo systems.

Brad White

Of course, dear Weezbo,

your neck of the woods has LOTS of hours where condensing boilers can work with 180-degree-sized baseboard.

July and August come to mind.

Uni R_3

I have fin-tube/Monoflo

Mid-70s backsplit originally built with electric baseboard - youch! - retrofitted a few years later with 1.25" black iron monoflo main with 0.75 branches feeding fin-tubes which was fired by a crude boiler from the 50s. Single zone, continuous circ, about a dozen branches above the main and three below the main with double Monoflos. Was Series 100 pumped, but switched to a 15-58 on speed 2 before modcon changeover. Manual J was mid-50s, boiler was 0.65 GPH fired with simple W-R ODR.

Had Prestige 110 installed and piped P/S with a bypass and closing valves so that it could also flow direct if desired instead of P/S (just uses the internal pump although it has to pump through the idle distribution circ's volute - but it does).

Was very surprised at how low the required temps were. True heatloss was actually just under 40K. Worked fine P/S with continuous circ on distribution loop, but turned out to have no perceptible loss of comfort running direct and using minimal post purge. Running direct more than halves electrical consumption, especially in the shoulder seasons.

The only downside is the ΔT. Big pipes, a smaller load and a medium speed 15-58, result in a ΔT of around 6°F or less even at design, unless it's doing a huge temp recovery. ODR curve runs supply temps from well under 100 to just under 140 at design, so the boiler condenses well enough but 10° cooler water would obviously make that water far more attractive to the heat flowing through the HX.

Short cycling has never been an issue but I do plan to install a buffer tank to try and increase the ΔT a slight bit by taking advantage of the boiler's 10 minute fire cycle (4gpm from a 40 gallon tank using a mono-flo to draw in cooler water from the buffer into the return for the boiler).

Mark_46

Uni R,

After a boiler repipe to P/S with ODR I too am (early in the heating season of course with only moderate outdoor temps) observing a very skinny delta T.

At the beginning of a heat cycle it is wider at maybe 17 or so. But as the heat cycle wears on the delta narrows to maybe 7 (eg supply 130, return 122) or so and the boiler rolls on at the lowest firing rate (based on fan speed observed through control interface). This is with only one out of 2 zones calling. Of course I have more measuring to do and am waiting for lower outdoor temps.

But why is what's on my mind right now. Why is the Delta so skinny? Secondly, is the delta T even relevant? Return temps are critical but Delta T?

Weezbo

well , as usual , new ? 's arise as we go along....

Uni did you lash up an injection bridge to your mono-flow with the mono flow its own dedicated circ?

or a mix valve ,or set point control, or is it like direct drive off the condensing unit?

the idea of a mono flow system is somewhat like having multiple "mini-stations" except each station is its own by-pass as it were. that means it is always presenting the 'Boiler At the Emitter', right? sorta like on demand hot water at a tap. it would be "Unusual to Not have a skinny delta T " ....

to further dial down the "Boiler at the Emitter" and increase the delta T return temp, seems you would have to know which of the zones was doing what when and ramp up and down the temps according to that information ,along with the minimum water temp possible to maintain flow through the main loop with no emitter open and Ramping when it noticed a reduced flow lower than a "Satisfied" condition of all emmitters , from that minimum temp set point on the heating curve for the main loop .

i suppose i should amend this also, as if you designed each emitter to just meet the heat demand for the heat loss then it would be a different set of variables to master. they would be fewer however then you would not have individual control of the rooms they would always be "ON" requiring a specified temp across the board for all emitters.that would likely have out door reset of the system, and some minimal low supply water temp at each emitter .the last emitter's "satisfaction" being the lowest minimum water temp ....


on a relatively huge system of office spaces that did not have heat,too much heat, large bounces rather than redesign rip everything out and go some other direction, i changed and eliminated some returns to the emitters , changed the room with the control(/s)(there were many ), lengthen some interconnecting them with others, shortened others as a "quick fix" to a large set of minor technicalities....that is a comfort solution when i had no time ...

Uni R_3

Mark C.

In my very non-professional opinion, your ΔT sounds quite good actually! A condensing boiler's efficiency increases as return temps lower since a basic fundamental is that heat moves to cold.

Lower the return temps and it helps more heat move across the HX and fewer BTUs end up going out the vent. You need the heated up supply temps to warm the house, and the cooler return temps to suck the heat from the HX and the ΔT just shows how well that's happening.

What kind of piping arrangement do you have?

Uni R_3

How to explain...

Weezebo, both... see the attachment. There are 3 valves involved. My crude diagram shows the valve positions for running direct - flip the 3 valves and it's P/S with constant circ. The piping can go either direct or PS.

Any attempts to widen the ΔT basically deprive heat from the last of the "mini-stations" and make the middle ones pretty random in their heat delivery. The system's always been balanced and comfortable with sufficient flow - even with my old dragon boiler attached.

Monoflo fittings makes for a very nice simple relaible system but high GPM /low ΔT is what they need to work comfortably and predictably and a low ΔT is not the way to squeeze the last BTU out of a modcon. There's not much left for savings there, but it would still be nice.

TRV'd panels from a direct-piped manifold would have been my choice if money were no object, although my better half might have still asked if its just for efficiency why we were spending another $15K plus to save maybe $30 a year! LOL

Mark_46

Piping is 1.25" copper Primary/Secondary running on ODR with DHW priority.

Theres a Grundfos 15-58 on the Primary set to Med speed and a Grundfos 15-58 on the Secondary also set to Med speed.

It's zoned with valves (2).

Theres a Caleffi Pressure Differential Bypass Valve in the Secondary set to 4 psi but I have some research to do into that setting.

Weezbo

real quick....

that last remark is so genuine it deserves its own reply *~/:)

Weezbo

ok *~/:)

Yup. wifer good help keep focus

ok perhaps you could dial more than two rooms emitters into a single take off. the trade off would be then dialed to the control within the room of the first take off or depending on the size and heat variations within each room,

i forgot to ask you if you have a straight shot to and through the wall to the next emitter on the monoflow system.

the return system with a buffer, like you are thinking, is to some degree a means to dial colder water temps down to be honest it is not exactly quite that simple though. in a way, what you would want to do there is create completely different flows through the buffer, one at the top for "incoming and cooler mixed return to the temp going back into the system And another off the two lowest tappings of the water heater say at the drain and the lower element taps to mix two entirely separate cooler water temps back down out of a mixer back to the return of the boiler.and if that isn't enough ...:) there would need to be some form of check valves in place in order for all that to Happen and as you know whatever goes into a T must leave a T.that is what is somewhat cool about the low loss headers and electrical, design, flow, emitter,piping ,boiler and pump selection...