BTU’s! and Delta T! and GPM! OH MY!

jp_2

why do you need any delta T?

Dan House

BTU’s! and Delta T! and GPM! OH MY!

Lions! and Tigers! and Bears! OH MY!

Any wizards out there? Being in Kansas you think I could find OZ nearby but there is no man behind the curtain better than the WALL!

Maybe that should read: BTU’s! and Delta T! and GPM! OH MY!

Just so happens that is one of my favorite movie lines let alone the movie also has my theme song: “If I only had a brain”. But to get to the point I am asking for your thoughts and procedures about a concept that is making my head hurt.

Just the Facts: I installed two beautiful mod/con boilers piped in 1” parallel to each other with two 1 ½” crosses on the primary circuit. The TT Solo 110 have their own Taco circulators protected with strainers and check valves; each with their own ball valves and equal length piping circuits. It would make even the most picky fitter proud.

The primary circuit then has the air scoop with top auto air vent and bottom bladder tank upstream of the primary circulator (UP43-75) according to the pump head and gpm required. The house piping is 3” supply and 2” return feeding 19 cast iron radiators. Three story house.

On a call for heat the two boilers fire and the delta T in the primary circuit was 10F. For some reason I am thinking that it is best for a 20F rise so I throttled a ball valve downstream of the pump until I got a 15F rise and stopped.
What is right/wrong with this? Do I throttle more to get 20? Do I go back to full open and live with 10?

Presently the room thermostat turns on the primary circulator and closes a DPDT isolation relay to close the “T-T” circuit for each boiler. So the next question is since the boilers reset themselves according to outside temp why should I monitor the primary circuit supply temp in the thermostat controls?

Mid WEsT Head

Mark Eatherton

Which moves more energy....

1 GPM at a 100 degree rise, or 100 GPM at a 1 degree rise?

In my way of thinking, the delta T across the boilers is more critical than is the delta T across the system.

Yes, you can choke the living crap out of the main flow and "see" a 20, 30, 40 , even a 50 degree F rise (drop actually) across the system. But (and there's a butt in every crowd) the radiators WANT to be a s hot as they CAN be for a given condition.

Think "average emmitter temp" and you will want to see a lower delta, consequently a higher average temp across the emmitters. You are trading off flow for delta T.

Is this system originally gravity? Or has it been pumped up since conception?

Now the heat source, on the other hand, should have a low entering temperature, hence subsequent HIGH delta T.

The radiation will generate a 20 degree delta T when it is ready (spelled LOAD).

Now, back to my question to you... It's the same!!

G.P.M. times pounds times delta T = btuH.

ME

kpc_40

it is a condensing boiler...

lower water temps make it more efficient. Europe uses delta T of 30 40 dergees....kpc

jp_2

whys that mark?

"""In my way of thinking, the delta T across the boilers is more critical than is the delta T across the system."""

other than having enough flow across the HX.

I believe large delta T's are Inefficient! yep, I said inefficient.

larger delta T across the HX means a larger delta T in the combustion area, leading to higher exhaust temp which wastes energy. if your mod/con exhaust temp rises, your efficiency drops.
look at the visseman lit. the graph that shows efficiency proves my theroy, as delta T's increase, efficiency decreases.

added:

since we are talking about mod/cons with reset, the system should only be applying "small" amounts of heat constantly to maintain indoor temps, "small amounts". as outside temps drop, the boilers temp increases a little bit, once again small delta T.

[Deleted User]

My reasoning...

versus your reasoning...

Your reasoning assumes a conventional apppliance, in which case a small DT would keep the "heat exchanger" seeing a lower average surface temperature, hence more efficient.

The poster said he was using a modcon boiler, which NEEDS to see a lower entering return water temp in order to condense the flue gasses thoroughly. If the water is leaving the boiler at 180, and returning at a 50 degree DT, then it is returning at 130, which is in the condensing range. This assumes that there is 130 degree F water to be "seen". If not, regardless of what it is, the lower the return water temp to a modcon boiler, the higher its combustion efficiency.

Still waiting to hear his response as to whether or not the system was originally gravity, which makes a difference in the flow rates across the system.

ME

Steamhead (in transit)

If it was gravity

it's way too easy to over-pump. Mark and I have different ideas on this, but my experience tells me that on an old gravity system you want to mimic the gentle gravity flow by using a small circulator. This type of system has so little resistance that all you need the circ to do is move the water thru the boiler. Too much flow can actually decrease the radiators' output- the water goes in and out so fast it doesn't have time to give off heat.

On a boiler using a primary-secondary hookup, mod-con or not, this principle would apply to the secondary circ serving the old gravity piping.

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jp_2

not ready mark

though I reference mod/cons in my post and not conventionals, I did not refer to return water temps but Delta T, so my post covers all heating devices. mods/cons use no special physics.
your comment also refer to only delta T as important.

yes I agree lowest supply temps equal highest efficiency, but if you still have a delta T of 50F, not sure, got any data?

Id say NO with a 50F delta T, but happy to change my mind with a proof!


what delta T's do you normally see in your installed systems?

Dan House

MOD CON Delta T

Yes the system was gravity flow-
the large main for most of the home is 4"; and 2" for the other circuit.

The return main is also 2".
The reason I used a large pump was due to some the other radiators had 3/8" iron pipe return bent down thru the structure to connect to 1/2" and 3/4" risers on the 2" return mains.

(maybe originally some kind of steam system converted?)

NRTDave

Own Taco circulators?

"The TT Solo 110 have their own Taco circulators protected with strainers and check valves; each with their own ball valves and equal length piping circuits."

I'm confused... The Solo 110s have an internal 15-58s. Have these been removed or are these Tacos running in series? Are the 15-58s on speed 3? Speed 2 or even 1 might be sufficient if you do the math.

Do they both fire together even during the shoulder seasons?

Dan House

piping ?

"I'm confused... The Solo 110s have an internal 15-58s. Have these been removed or are these Tacos running in series? Are the 15-58s on speed 3? Speed 2 or even 1 might be sufficient if you do the math. "

They are piped Primary/Secondary.

[Deleted User]

Actually Frank....

I'm starting to think more LIKE you than against you, especially as it pertains to these gravity jobs.

In fact, I think that there is only a need for a full blown pumping session for about the first ten minutes or so of operation, then the pump could be idled back to nada, and possibly shut off after a sustained call for heat. The biggest problem I see is getting the ModCon's hydraulicly interfaced into the gravity system so that gravity is allowed to do its thing.

In my minds eye, you'd have a large vertical chamber, with the cold return going into the bottom, hot out the top, and across from those two tappings, you'd have the mod con pulling its water from the bottom side and returning it to the top, side. Something similar to a low loss header, except the system tappings would large and would be on the vertical and the heat source tappings on the side. One problem is that nothing of that configuration exists off shelf, and the cost of building one would be an extra. But I am certain it woud work great.

JP, in response to your question about what kind of deltas I see on my systems, it depends. On low mass HWBB, typically 7 to 15 degrees F. On radiant floors, around the same. With cast iron systems, more like 20 to 30 degrees F, but it all really depends upon the conditions under which it is operating. So, as with any hydronic question, the only correct answer is "It depends!"

As for the parasitic cost (pump) of operation, where do we call the trade off of theoretical increase in combustor efficiency worth the additional power required to acheive it. Evidently, our German friends have taken this into consideration, and have determined that the trade off of combustor inefficiency is good right at around 30 to 40 degrees F across the appliance.

The Knight boilers I use typically see a minimum of 30 degree delta, but then again, they are modulaing their burners to maintain that condition.

I have tested this theory in the lab, and I found the increase in thermal efficiency to be only a few points when going from a 20 DDT to a 40 DDT. That was done on an 80% copper fin tube appliance. At todays electrical cost, one would have to take a real close look at the wire to water efficiency. But, as to your statement about it being more efficient, thermally you are correct, but then again, you always are. :-O

Peace.

ME

[Deleted User]

Grundfos too big...

Now that I've had a chance to mull over your system design, I'd replace the system circulator with a much smaller circulator, say maybe a 1558 on speed 1. All you really need is a "gravity enhancer", not really circulation. Mother nature will take care of moving the water for you. Especially in a 3 story house. In fact, if the dead men used restrictive orificies on the upper floors, adding a pump that is creating a large pressure differential across the system is going to create a situation whereby the upper floors don't heat well, but the lower floors do... The exact OPPOSITE of what they were atempting to acheive with the restrictive orifi'.

On a one pipe system that I was tweaking the other day, I saw a 50 degree DT when I shut down the distribution pump, and the radiators were smokin' hot. Don't be afraid to underflow, and obvioulsy, even with the 3/8" interconnects, it used to work before a pump was added, no?

As Dan has said before, and I will echo to my grave, "The urge to oversize heating components is stronger than the urge to have sex..."

ME

Steamhead (in transit)

Might have been

the pipe sizes suggest that. Can you take some pics of this piping? Maybe if we see it, we can tell for sure.......

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Steamhead (in transit)

Sort of like

a gigantic hydronic separator. You'd want the system return connections at the bottom, below the boiler connections as that's where the coldest water would be.

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jp_2

mark,

interesting,
what were the flue temps, supply and gpm reading for the 20 & 40 delta T's?

30F delta T on a constant circ iron rad system?