Low Delta T between boiler Supply and Return

Chotu

Hi!

I'm noticing about 4-6 F delta T (depending on if more than one zone is calling for heat) on the boiler supply and return ports - as a result it is difficult to run the boiler in Max. Efficiency or Condensing mode.  Since the condensing mode requires around 120F return, and when it gets colder outside the ODR curve increases the sepoint causing the boiler return to increase.  I've tried a flat ODR curve but it leads to a colder house.

I'm wondering if the pumps on the primary and secondary are not matched right causing too much pre-mixing right where the primary loop joins the secondary.  First, is this delta T normal?

Second, if not, how do I determine the right pump sizes.  I have yet to do head loss calculations for each of my zones.

Here are the specs. to our recent Oil to Gas coversion:

1) Burnham Alpine 150 with primary secondary loops per I/O instructions. (I do realize that the boiler is too big and I should have gone with Alpine 105.)

2) Taco 0014 on primary loop (per Installation Specs.) & three Taco 007-IFC one for each zone + Taco 0014 for 60 Gallon HTP IWH on secondary loop.

3) I'm in the Boston, MA area and the heat load calcs. came to ~70,000BTU at design temp. of -10F; All three floors ~ 1700 sq.ft. have Sterling Petite baseboards total 118 ft.

4) ODR reset enabled with settings (Outside: -10 to 70; Setpoint: 170 to 80 with 126 Low Boiler Temp.) This allows the boiler to operate in condensing mode until about 29F.

Paul48

good reading

Look at some of the recent posts here on the main wall.Your problem has been discussed at length, and may save some time.Check out the post about the buffer tank.

Jean-David Beyer

First, is this delta T normal?

"I'm noticing about 4-6 F delta T (depending on if more than one zone is calling for heat) on the boiler supply and return ports - as a result it is difficult to run the boiler in Max. Efficiency or Condensing mode."



The one is not directly related to the other. You could run with an unmeasurably small delta T and still get condensing. I have a very small zone (needs 6500 BTU/hour when it is 0F outside, and design temperature here is 14F. It is 19F outside at the moment. My reset curve is requesting 128F water into that baseboard zone. At the moment supply is 123F and return is 122F. Chances are the circulator is too big (a Taco 007 IFC). It is supplying enough heat and is extremely likely to be condensing. In other words, for condensing, it does not matter what the delta T is; what matters is the return temperature and how much flow there is at that temperature. To get the heat I need at that low water temperature, I have about double the amount of baseboard I would need if I were running 180F water in it. So if all you want is to get condensing, adjust your reset curve to give lower supply temperatures. Of course you cannot go too far or you will not get enough heat. At that point, you could increase the amount of heat delivered by increasing the size of your emitters (such as more baseboard if that is what you have already). That is what I did. I have 14 feet in each room, where 6 feet would have been enough if I put 180F in there.



"Since the condensing mode requires around 120F return, and when it gets colder outside the ODR curve increases the sepoint causing the boiler return to increase."



You probably get condensing at 130F or so return temperature. The reason you want lower temperature return water is that the colder it is, the more condensing you get. A mod-con is especially good for radiant slab heating where the maximum supply temperature you need is usually 120F or less, and that only on the design day. For example, my slab would be getting 107F right now. It is a bigger zone and I am getting a delta T of about 9F.



" I've tried a flat ODR curve but it leads to a colder house."



I would never have thought of trying that, but I am not a professional heating expert. It seems to me that would defeat your outdoor reset completely. If I wanted to raise my delta T, I would either change the reset curve to run hotter supply temperatures, and getting a higher return temperature (which I would probably not want), or put in a slower pump. But that would cost money and if I went too far, result in uneven temperatures. That zone is two rooms with the baseboard in series, so the second room would run colder than the first unless I increased the size of the baseboard.



"I'm wondering if the pumps on the primary and secondary are not matched right causing too much pre-mixing right where the primary loop joins the secondary."



You do not get a lot of choice about the circulator in the boiler loop. Generally, the boiler manufacturer tells you what pump to use. Mine even supplies the pump to be sure the right one is used. Now the circulators in the secondary loop are up to you, or up to your contractor. I believe my installing contractor was lazy and just used Taco 007-IFC circulators everywhere. The one for the Indirect water heater had to be like that, but perhaps not for the two heating zones. It has to be big enough to carry the flow necessary to deliver the required amount of heat. It must be small enough to not get noise, possible cavitation, an excess wear on the pipe itself. Generally you want the flow less than 4 feet per second and more than 2 feet per second. Other than this, I do not see what sense it makes to try to "match" the flow in the secondary loop to that in the primary. I can imagine systems where one is greater than the other, equal, or less. In my system, if both heating circulators are running, the flow in the secondary loop is greater than in the primary loop. If just the radiant zone is running, the flow is very slightly less in the secondary than in the primary. If just the baseboard zone is running, the flow is considerably less.



"First, is this delta T normal?"



I have no idea what normal is when running a modulating condensing boiler. I know what I get, and with my baseboard zone, I get much less delta-T than you are experiencing, yet the boiler runs in condensing mode until the outdoor temperature gets to 14F (design temperature). This with a delta-T of 1F to 2F.

Paul48

?

Everyone seems to focus on the boiler condensing. Imagine how efficient the boilers could be if we were sure 100% of the water heated by the boiler was available for radiation. If 50% of the water heated by the boiler is immediately sent back to the  because of the size of the boiler pump, but we are condensing,doesn't it seem like a waste? I can't find any information from manufacturers that give you a clue as to how much is actually being circulated through their boilers. A factor that I think is critical to the system efficiency.You can't seperate the boiler from the system, and then say....wow look how efficient that boiler is.This will probably start a very long thread, but that's not a bad thing.

Chotu

This is not a simple problem

I think the goal is to ultimately run the boiler at lowest supply temperature possible for the heat demand posed by the location.  For this the heat emitters have to be setup in synergy with the boiler to meet this demand.

But ... enough of high level goals - they are for the experienced contractors.

I now have a given design and need to tune the boiler and perhaps replace the pumps to get the best efficiency.  I might also consider adding radiant heat on the first floor (between the joists) to add more thermal output (BTU/hr) on Zone 1.

My thinking is there was no primary/secondary T connections - cooler return water would directly enter the boiler giving a better chance at running it in condensing mode vs allowing it to premix with hot water.  I believe this piping layout is to save the boiler from abruptly getting cold water causing undue stress on the heat exchanger.  But if we allow just enough pre-mixing with right pressure differences in the pipes around the primary/secondary connections we should get optimal chance for condensing even at higher supply temperatures.

Looking at the figure I would like to measure Ti and To the actual supply and return temperatures from the emitters.  I would like to keep Pm > Ps > Pe to allow most supply/hot water to go towards the zones vs return back to the boiler.  Also, Pm > Pr > Pi to allow most of the return water to go into the boiler vs come straight though into the supply side.

I think water recirculating into the boiler is not a loss of eff. as the thermal energy in it is not lost, the boiler simply adds more to it provided it's temp. is lower than the set point.  Otherwise it automatically modulates down so as not to exceed the supply temp.

Paul48

Yes

With the specifics of your particular problem, I believe you just re-stated the same thing I said. Aside from disagreeing that heated water, not used, is inefficient. You need to know the flow through the boiler to begin to determine the pump size for the zones.In my opinion.

Chotu

Boiler pump

So I looked into the I/O Manual for Alpine 150 and it recommends putting Taco 0014 (pg. 44, Table 12, I/O Manual) for 11 GPM @ 25°F Temp. differential through the boiler and a combined (boiler and piping loop) head loss of 11 ft (Boiler 8.9 + 2.1 for 75ft of piping etc.).  When I checked the 0014 pump curve (spec. sheet) for 11 ft of head loss - it is telling me the flow rate will be 18 GPM.  7 GPM more than required for design condition.  Lets look some more in the manual for answers ... so I found this:

Seems like the equation below is the key to get the desired (high) DeltaT.  It tells me that if the boiler output changes (to meet the ODR setpoint), if the flow rate is constant (like in my case), DeltaT must change ... which makes sense.

Required Flow (GPM) = ** Output (MBH) x 1000/(500 x DeltaT) (Pg. 43, Table 11, I/O manual)

** Output (MBH) - Select Value for specific Boiler Model from Tables 2A or 2B

So I tried a little experiment, I tried to reduce the flow rate by slowly closing return side isolation valve (just before water enters the boiler) and see how the supply and return temperatures change in the boiler.  The DeltaT did increase by 2 F.  Obviously I did not want to close it all the way.

Need to do more calculations ...

Jason_13

Alpine pump size

Burnham sizes those pumps for 75 EFP.Less pipe you can reduce the pump size to equal the proper flow rate. Their newer Alpines they include pumps which are sized for 50 EFP.

JeffGuy

Similar

I had a similar question with a newly installed Alpine210 - link here which you might find interesting: http://www.heatinghelp.com/forum-thread/139467/The-best-Delta-T-for-a-mod-con



I have two zones in my 100+ year old house; one is an over-radiated cast iron radiator zone, and the other 50 feet of slantfin baseboard. Like you, I concluded that the pump that Burnham supplied with the boiler is larger than it needs to be. I have less than 10 feet of copper pipe in my boiler loop, way less than the 75 feet the pumps are specified for. I get a 20 degree delta T when both zones are calling, but only about 5 degree delta T when the slantfin zone is calling by itself.



You said you are getting 4-6 degree delta T - do you mean that you are getting 6 degrees when ALL the zones are calling at the same time? It would be interesting to test this if you haven't already.

Gordan

The pri-sec layout doesn't have to do with boiler protection

but with ensuring adequate flow through the boiler at all times. It's a combination of a low-mass heat exchanger, a (relatively) slow-responding control, and high pressure drop through the heat exchanger.



I've got a Triangle Tube Prestige Solo 60 that's not piped primary-secondary, but it has a (relatively) high-mass heat exchanger with a low pressure drop through it, and I don't use positive-shutoff zone valves. Yes, that means that I get undiluted return from the emitters, but I selected this boiler specifically because it was well suited to this application, and then designed my system to eliminate any concerns with inadequate flow through the boiler. All design is compromise.



Warm water that's recirculated to the boiler is not wasted! The boiler will just have to work that much less hard to bring it back up to the setpoint, so it will modulate down (if it's not already on minimal modulation.) The increase in the return water temperature will, however, impact efficiency somewhat for some of the heat exchanger designs (others are still quite efficient at low input even for relatively warm return temps.)

Chotu

Thanks!

Thanks for the thread it was exactly the kind of discussion I was looking for.  I think the search on this site is not very good.

Yes, 6F is deltaT when all zones are calling and it goes down to 4 when one of them is calling.

When I closed the inlet ball valve halfway on the boiler loop pump I noticed the deltaT increase to about 10.  The supply temp went up and return went down.  Which is I think proof that there is more premixing going on at higher GPMs.  I think I need to get a smaller motor.

furnacefigher15

Is this normal

Yes, the way this boiler has been applied, that would be a normal temperature rise.





If you are able to limit the firing rate to the load of the home, I would be okay with down sizing the pump. But if not, then leave it alone.



If there is no way to limit the firing rate to say 50% of the boilers capacity, then re-sizing the pump will be a disaster.



Personally, I think its a waist of money, but it's not my money



Also, why is the boiler so over sized? Future addition to the house?

Chotu

I should have listened to my inside voice

I thought the contractor knew what he was talking about and agreed.  I wanted to go with Alpine 105 - but he warned me that it's cutting close and for future additions would run small.  I should have done my homework - I know more about these boilers and fin-tube baseboard heaters.

Having three zones with 54, 51 and 13 feet of fin tube is just too small to radiate all the heat 150 gives out at minimum modulation and low condensing temps.  So now I'm thinking of combining 54 and 51 zones together and if either calls for heat both get heated.

Is there anyway (factory setting etc.) to lower the modulation on this boiler below 30MBH?

furnacefigher15

Turndown setting

I do not believe there is a way to reduce the minimums. But, the scenario you have seems to be getting more and more common.



You can make it work, but it will never be as efficient as what you paid for it to be.



Can you take some pictures of the piping?

Chotu

Pictures

See attached pictures ...

Another idea I had was to somehow use the IDW heater as a buffer tank.  Its a 60 gallon capacity and should provide thermal mass to the system.

icesailor

Low Delta T:

Condensing is a beautiful thing.

That said, just one of those Taco 007 circulatore would give you more flow through the entire system than you could ever use. And you have three of them. If you want a lower return temperature, you need a lower flow through the system or circuit. Three speed pumps are nice. Tp prove my point, buy a three speed pump and put it on the biggest zone. Set it on #1 or the lowest speed. Your return temperature will drop. Set it on 33. Your return temperature will go up.

I have 6 Taco 571 zone valves with a TACO 007. I saw a demonstration of Wilo circulators. I saw the Stratos variable speed. I installed one. I can tgroll the speed of the pump because it senses the load.

In my opinion, your system was and is vastly over pumped.

If you want to slow it down, put on three speeds.

That's my opinion not shared by others. 

bob_46

Pumps

It looks like all four of the pumps in the photo are installed wrong. Motor shafts must be horizontal and electric box cannot be on the bottom.

Chotu

Keeps getting better

I feel like I hired a bunch of monkeys to do this installation.  Maybe the motor orientation explains why one of the pumps (Zone 2) starts making a rattling sound at temps. above 130F.



So I was thinking if I:

1) move indirect water heater to Zone 4 of Taco SR504

2) keep Zone 4 priority off

3) set the water heater thermostat to 130F

4) boiler ODR low temp setpoint to 126



So at low outside temps. when the boiler cycles down, since the water heater has not reached its setpoint - it'll keep calling for heat i.e. keep the motor running.  Thereby providing heat for the zones.  Once the water temp. drops below 116 (since diff. below is 10F) the boiler will come back on.  The question is, given the water heater is 60 gallon, how long will it keep the radiators heated.

icesailor

Install:

The installer did a REALLY nice job on your install. His insistence of the larger boiler is based on hos own personal experience. Wrong or right, it is a really professional job. I sometimes see this obsession with saving money and condensing. Sometimes it looks like stepping over a ten dollar bill to get to a dime to pick it up.

You may go out to dinner and leave a tip that is bigger than the amount of savings you get from a different installation. I'd be happy to know that the person who installed that system was a first class mechanic and not a hack. Who really gives a schitt about the quality of hos work. And will be proud to come to that site, 20 years from now and still be proud.

"Nuff Said.

CMadatMe

Not Enough

Reason for the small delta is you have no where near the capability to pull the btus out of the primary loop. The 0@14 is the right pump for a 20 degree delta-t on the boiler side. ?In yiur post you state you have 118 feet of board combined. The capable output of that board with 180 degree water doesn't even get you 70,0000 btus.



Only fix here is to reduce modulation rate of the boiler, add a buffer tank, change the

zone circs to zone valves, use a Alpha or Taco VDT as a system pump. I would also move the indirect to the primary side where it should be. You could change the boiler pump out to increase the boiler delta out to 30. Manual gives you the boiler gpm and head for a 30.

icesailor

Circulator Installs:

Bob,

I was under that impression until recently. But, I know that you can not install Taco wet rotor circulators can be installed horizontally and with the motor down.

I recently came upon a Taco 0011 that had the motor in the vertical position. I had to drain the boiler and when I drained and refilled it, the racket that came out of that pump was frightening. But I set the system pressure at 20# PSIG. I was going to flip the circulator but was going to have to rotate the flanges. It takes a different gasket than what fits the 007's. It was more like a Wilo gasket. My supplier was going to have to order it. I read the installation instruction manual and they plainly state that the circulator can be installed in any position but if installed with the motor on top, you must raise the system pressure to a minimum of 18# PSIG.

According to Taco, those circulators are properly installed.

FWIW

04090

On the pumps

Was reading over at the pump manufacturers site that you can reduce the speed of a 007 with a light dimmer switch - not recommended and might make the motor run warmer than without but works - and that might be a good way just to experiment with different velocities.



And shouldn't there be a ball valve on either side of the 007's for ease of service?

bob_46

Pumps

I must not be reading these right.

Gordy

Not a good idea

 What circulator Manufacturer did that come from??



Gordy

icesailor

Motors Up:



Is this what you are wondering?



1. Mounting position – Circulator must be mounted with the motor in a horizontal position. It may be mounted

vertically with the motor up, provided that the system pressure is at least 20 psi (138 kPa).

icesailor

Ideas:

I don't think it is the greatest idea. Taco just says you can do it.



1. Mounting position – Circulator must be mounted with the motor in a horizontal position. It may be mounted

vertically with the motor up, provided that the system pressure is at least 20 psi (138 kPa).

bob_46

Up ?

Motor up

icesailor

Up:

Yes, like it says in the first line of the Taco installation manual.

http://www.heatinghelp.com/files/posts/11580/00%20cartridge%20circ..pdf

Chotu

Boiler pump

So I closed the inlet side of the boiler pump halfway and noticed the delta increase to about 12 at minimum modulation and set point of 126. This was possible after I combined zone 1 and zone 2 pumps into a single zone 1 thermostat (simple wiring change on the Taco switching box). At higher set points I do see larger delta T of 30-35. Note that I still have separate Taco 007ifc running for the zones.

Is it okay to run the system with ball valve half closed?



I also did some calculations via the equivalent length method to determine the head loss in the primary loop and it did suggest taco 0014 as a good match. But then I'm not seeing the right deltaT per the I/o manual.



I also noticed that there is some flow in the IDH water loop even when the pump is off. This loop is missing a flow check valve. So some of the return water is taking this path. When the closed the ball valve in this loop I can see the boiler return temp go down about 3F.

Chotu

Pump orientation

Does anyone know why the axis have to be horizontal? Does this affect life of the pump or it's operation, energy consumption etc?

Chotu

Found the answer to pump orientation

Quick Google search ...

"It is preferable to install Grundfos circulators in a vertical pipe

pumping upwards. This position ensures that the pump shaft

is horizontal, which reduces the thrust bearing load and ensures

positive air purging from both the rotor chamber and impeller

housing.

Pumping downwards in a vertical pipe is not recommended as

this may lead to air locking of the pump, with resultant loss of

performance. However, pumping downwards is acceptable

provided an effective air vent is incorporated in the system,

before the pump.



Where pumps can only be installed in horizontal pipework, it is

imperative that the pump shaft is horizontal, or slightly higher

at the vent plug end.

The shaft must not fall below the horizontal plane, even by a

few degrees, as this causes premature wear of the top bearing

and shaft.

Pumps should not be installed with the shaft in a vertical plane,

as this may lead to dry running of the top bearing, noise and

possible pump failure.

To avoid cavitation noise and risk of damage to the bearings the

minimum pump inlet pressure should be 1.4m at 82°C (water

temperature)."

http://www.inspectorsjournal.com/forum/topic.asp?TOPIC_ID=8115

Gordan

One problem...

I'm pretty sure those circulators are not supposed to have their motors in a vertical orientation like that.



Edit: Never mind, should've read the thread to the bottom.

Gordan

Notice that none of them are showing...

...the motor hanging down.

Chotu

Quality work ... Lol

Just because the work looks clean doesn't mean it performs well too. I hope I had found a contractor that you're describing here. After I pointed out to him that the motors were mounted incorrectly he refused to believe me and actually hung up on me. He has not replied to any of my emails and tells me I'm spending too much time in the basement.



I had a totally different image of American quality work - this does not even come close. Nuff said!

Chotu

Had the pumps turned

Okay - so I had the pumps turned - the motor is now on top and pump on the bottom.  This is not ideal either but that's the best the plumber thought he could do.



After he restarted the system it went into hard lockout (Error 25) - Internal Control Failure.  So he reset it and it seemed everything was okay.  Don't know why this happenned - but whatever.



Later after he left it went into another hard lockout and I had to reset the Sage 2.1 controller.  After that it has been going into soft lockouts with return temps higher than supply - Lockout# 15.



The three TACO 007 IFC have arrows pointing in the right direction.  There does seem to be air in the system as I can hear it whirling and clanking in the house and boiler loop.



Any help is greatly appreciated.

Mark Eatherton

Too much attention being paid to delta Ts?

Are we designing these systems to satisfy the equipment, or the people inside the buildings?



Are we willing to have to put on sweaters just to satisfy our need of providing for a low temperature return?



Remember, most existing hot water baseboard systems were designed with AVERAGE water temperatures of 180 to 200 degrees F. This will only be necessary for roughly 2 % of the time north of the Mason Dixon line...



The ONLY way to (properly) increase the D.T. is to increase the load, or increase the heat emitters, neither of which makes sense after the fact. Performing significant conservation efforts (windows, caulking, insulation) will reduce the loads to the point that the existing convectors CAN be more effective with lower average water temperatures, but conservation efforts should come FIRST, not later..



Fact: In my 36 years of tromping through mechanical rooms, at design conditions, in systems that I know for a FACT were properly designed, installed and operated, I have seen a true 20 degree F delta T on (wait for it.....) ONE system. To be fair, that was a brand new system, that was being cold started with a completely cold house inside, at design conditions. When I went back to the job to commission it a few days later, the D.T. had dropped to a 7 to 10 degree differential, and the house was completely comfortable, wall to wall.



If the flow is correct, and the heating elements correct, delta T is WYSIWYG.



If there is TOO much base board on a given zone, creating a HUGE D.T., then I can just about guarantee you that any rooms near the end of that circuit WILL suffer from low temperatures. Been there, seen that, had to split loops to make it all better.



The trade off of not keeping a modcon boiler in the condensing mode all the time is insignificant. If you really are concerned about keeping it purely in the condensing mode, get ready to buck up Bunky, cause the only way you are going to be able to achieve this AND deliver good comfort is to increase the surface areas of the emitting surfaces (read adding high mass or high efficiency heat emitters to every surface available, i.e. WarmBoard EVERYWHERE.)



Quit worrying about stepping over dollars of comfort for pennies of efficiency. If properly applied (outdoor reset) you will realize a minimum reduction of 30% just in the change in appliance efficiency.



'Nuff said??



ME

icesailor

Delta "T's"

Never seen it said better.

A cool, condensing exhaust is a fine thing. Unless it leads to domestic potable water not being heated as needed. All that condensing exhaust can be put in a dark place when loved ones are in the shower with wet hair full of shampoo and only cold water to rinse it out with.

I turned my boiler water down slightly because it didn't need to be so high on the high limit and lowered the operating control to 135 degrees. I have a storage tank. I NEVER run out of water. The next day, my wife was complaining that the water wasn't as hot in the shower. Raise the limit stops on the shower handle or move the operating control back to 140 degrees.

I turned the boiler back up.

I have tried about everything that one can try. Tightening up the structure is the #1 best value. Clock thermostats on low mass systems (copper baseboard) is next and insulating the heat pipes is third. All else is a distant also ran.

While everyone is swooning over Mod-Con boilers, our market is going to hell in a hand basket. By the time one gets done adding all the crap to the system to make it work in the perfect world that some try to get it to, the price goes through the roof. So, when prices are compared, the scorched air installers get the job  and the customer gets a third world system and AC.

I work in one of the wealthiest counties in the country. In the last 15 years, it has gone to almost all scorched air, Air handlers and AC. It's cheap. Flex duct and duct board.

Cheap, cheap, cheap.

icesailor

Pump orientation

I think you misunderstood what was written. And that is for Grundfoss pumps.

If you look at the Taco instructions, they deal with what you have. And they are Taco 00* series pumps. Taco says that any orientation is OK but the motor body placed on top is the least desirable. But allowable as long as the system pressure is kept above 20#. There must be circulator instruction sheet there for those Taco circulators.

When in doubt, read the manufacturers installation instructions.

Note this submittal sheet for 00* circulators:



http://www.taco-hvac.com/uploads/FileLibrary/100-1.8.pdf

If you aren't running 20# at the circulators, you may be hearing air. And the bearings grounding.

That site you pointed to is not a professional heating site like this one. It is a home inspectors advisory site and giving non-professional opinions to lay persons on some topics they have no business giving.

What you read here in answer, you can be reassured that advice is correct. Or else, someone will call the person out for being wrong.