How to increase Delta T

Chris E_3

I currently have an Ultra 80 that has been running for 2 years. I noticed that the boiler usually only keeps about a 6 degree delta T. I have the boiler pipe primary secondary. The boiler is attached to the main loop with closely spaced t's. I'm using the taco 007 supplied to pump into the boiler and have a grudfos 15-58 on speed 2 pumping the main loop. The house is all baseboard with 3 equal zones that are not zoned off yet. So basically I know that I would have to increase the amount of baseboard to shed the heat. I was just wondering if there is another method.

Mike T., Swampeast MO

You might try lowering the heating curve and/or reducing (or disabling) the "boost" function.

Brad White_9

What is your flow rate and heat loss?

That is where I would start. The delta-T is arbitrary and incidental if:

1) Your house is warming evenly and correctly and

2) Your leaving water temperature is as low as will do the job and

3) Your return water if not your supply water is below the condensing (dew) point of your flue gasses.

The reason I would want to know your flow rate is, you could slow down your flow (the easy part of the equation to reduce delta-T). Cutting your flow in half -assuming you do not go below 0.50 FPS for laminar flow reasons- you will still get about 90% of your capacity. Your delta-T will almost but not quite double.

Try it and see.

Bernie Riddle_2

Brad I very respectively disagree, If you slow the flow in the emitter loop you will do less blending between the closely spaced tees and thus send more supply temp water back to the boiler and less condensing will occur. there is fixed flow in the boiler loop

jmo Brendan

Brad White_9

Which is another

reason not to use P/S piping on a condensing boiler, Brendan.

You are right, I was speaking in general terms of flow and temperature. Your point of defeating the primary loop and condensing is well taken. Nice to have an illustration of points made in prior posts!

Between Primary and Secondary circuits the princple holds though. Less flow, higher delta-T.

Glad you responded!

Brad

Mike T., Swampeast MO

Yep! That pesky fixed primary flow can really haunt you...

I suggested adjustment of the reset curve and/or boost setting only as a possibility. Depending on the modulation logic used by the boiler such might affect primary delta-t. It might however have little or no effect or might even reduce delta-t. Only some simple experimentation will tell for sure...

Kevin__Flynn

increasing delta-t

how about adding a little flat-plate H/X as your last secondary loop before your boiler return and use it to pre-heat the incomming cold water before it enters your DHW tank.

Kevin Flynn

Chris E_3

Delta T

Hi Mike,

I have lowered the heating curve and disabled the boost function thanks to Clammy's information. I figured out the heating curve last winter and have come up with the one that works for this house.

Chris E_3

delta T

Hi Brad,

Not sure as to the flow rate of the primary loop. The grundfos on the main loop is on speed 2. The heatloss for the house was 53,000 BTU. But have made improvements to the house like adding R30 in the attic and replacing windows.
The house is heating evenly and correctly. The house is a side to side split level. Bed rooms do get warm at night do to warm air in the house going up.
The leaving water temp seems to be doing the job. This morning with a 24 degree outside temp the boiler was running at 124 supply and 118 return. I did not check the flue temp.

Mike T., Swampeast MO

Did that have any effect on primary delta-t? Does primary delta-t gradually increase as load increases (outside temp drops)?

What sort of delta-t through the secondary?

If natural gas, have you clocked the meter to determine boiler input? If so, does it seem appropriate to your heat loss calculations for the indoor/outdoor conditions at the time. If t-stat(s) have been maintained at the same setting for at least two solar cycles and there has been no large, rapid change in outside temp then 50%-60% of your heat loss calculation at present indoor/outdoor conditions is likely a good guesstimate of the true heat requirement.

Are you getting nice long (say hours) heat calls during moderately cold nights or somewhat colder but fairly cloudy days? If not, you can try further lowering the reset curve, but if your b/b zones are one-pipe loops you may well get balance problems before you lower the curve far enough to produce such nice, long calls.

Without a variable-speed primary circulator and the boiler-integrated logic to control it, I suspect that there is VERY little, if anything, you can do to raise primary delta-t.

All mod-con manufacturers MUST be aware of this little problem yet only one seems to have addressed it

Not only will primary delta-t be quite low anytime you're not raising space temperature, but I still find it physically impossible for the boiler output to match the true heating requirement of any real-world structure. It won't be too bad with a single-zone system with few rooms but the problem grows as zones and rooms are added...

Brad White_9

Your best bet would be

if possible, balance your primary and secondary loops to be equal in flow and to reduce your secondary loop flow rate first. Primary flow in these tends to be to guarantee the boiler has enough flow at all times, not as a protection or savings strategy.

(When your P/S flows are equal, you essentially have direct series flow to and from the boiler yet the boiler has it's flow guaranteed, the real reason you would do this.) The boiler will see 100% system return water, the coldest it can be. This is why you want to drop that secondary flow. But what you have, as MikeT. says, is just fine for BBD.


(Depending on if your boiler is oversized reducing the primary flow might increase cycling due to low water content but one hopes modulation will help keep this down.) You say this is an 80 versus a 53 MBH design loss. A day like you describe might be halfway there at 26 MBH for discussion. How does this square with the boiler modulation minimum rate?

Your temperatures are below the condensing point so far at least. When it gets colder you could lose it due to higher temperatures.

But I think you could stand to reduce your flows somewhat.

ALH_4

P/S

Obviously I have no idea what the specifics are for this system, so I cannot make a determination of whether P/S is needed or not, but the flow rates suggest it may not be needed, and I feel life is simpler without.

Why not install one of the currently available smaller dT-controlled variable speed pumps and a dP valve, and eliminate the P/S piping? It's probably not the cheapest way to go, but it would save the most fuel.

Unknown

Mike, where is the info you are using to back up that belief of yours, as I apparently missed the post.

Plumb Bob

Why should delta-T be 20F? Delta-T across the boiler will only be 20F when the water is as hot as it can get (say 180), i.e. on a design day. With reset, and with moderately cool weather, delta-T will be lower. That is how it is designed. The flow through the boiler loop is fixed, so if you have delta-T=20 now, it would mean that the boiler is running flat out. In this case the burner has been modulated downwards to match the current heat load, so deltaT is less than 20.

By the way with P/S piping there is no connection between boiler delta-T and radiation deltaT.

Mike T., Swampeast MO

Chris:

It sounds like we've had similar weather. Until about 2 hours ago, my boiler had been firing continually for about 44 hours. Outside temps averaging in the low 30s during the day and low 20s at night.

Vitodens boiler. Standing iron rads (all with TRVs). No primary/secondary, e.g. built in VS circulator in the Vitodens the ONLY circulator in use.

Delta-t throughout the entire period was very steady. 10½°F the vast majority of the time, 12°F max. (I'm not counting the brief interruption when the boiler conducted a self-test after 24 hours of uninterrupted firing.)

Supply temp averaging (mode average) around 90°F (89° min, 93° max). Flue temp maxed at 96° and was typically 2°F higher than supply temp. (Another interesting hour or so period when outside temp hit minimum--16°F--where flue temp was very slightly below supply temp however). Return temp (obviously) was typically about 79°F.

While I do not measure fuel flow and did not clock the meter, I did look at the exhaust plume numerous times and know from experience that the boiler was operating at or near minimum input (25 mbh) the vast majority of the time.

Space temp nicely maintained, but it did drop about 4° when outside temp rapidly dropped just before dawn on the colder morning. Temp back to normal by mid-morning. I consider this "normal" with my BARE minimum reset curve and the fact that I keep about 3/4 of the house about 7° below the rest.

While my system temperatures are significantly lower than yours, 124°F supply with 118°F return is certainly nice and low for a fin b/b system in moderately cold weather. While your 6°F delta-t is 75% (by magnitude) less than my 10½°F delta-t, I consider it quite reasonable since you don't have the benefit of variable-speed circulation.

From everything I understand, your system [appears] to be functioning optimally given its characteristics. Improvements in efficiency are possible but you would likely have to spend a LOT and greatly increase complexity to achieve an increase in efficiency that could, at best, be described as "modest".

p.s. As long as you're satisfied with the present system is would NOT consider activating the individual zones! Do that and you may well significantly reduce efficiency...

Mike T., Swampeast MO

My own observations and measurements as supported by literature from Danfoss and Viessmann and advice, experiences and anecdotes from Wallies like Mark Etherton and Bob Rohr.

I am NOT suggesting that boiler size should be reduced to such a degree below that of a good heat loss calculation.

What I AM suggesting is that true MAINTENANCE heat requirement is WAY below heat loss calculations and that you should NEVER, EVER, EVER, oversize a mod-con beyond heat loss calculations. If the sizing choice of the INPUT of a mod-con compared to a careful Manual-J heat loss finds one oversized by say 30% and the other undersized by 20%, I'd choose the "undersized" one EVERY TIME!!!!

Mike T., Swampeast MO

FAR from that "simple" in this system Andrew.

ALH_4

why

I have done and seen this done many times with small and large Vitodens (single speed pump with large vito) with no p/s connected to radiant floors, radiant walls, and/or panel radiators. Why is it not this simple? There is probably a bit more to the dT pump control scenario, but why pipe p/s when it is not needed?

If the system flow rate can be pumped through the boiler, why pipe p/s? This system is a boiler and 3 zones of baseboard. How does it get any simpler?

What am I missing?

Mike T., Swampeast MO

I'm presuming that each of these three baseboard zones have emitters connected in series as per standard practice.

Reduce the flow to maintain a steady system delta-t and your emitter balance will go kerflewey.

You won't see that problem with parallel (as per standard practice) radiant floors, radiant walls or radiators.

Christian Egli_2

Just scratching the itch for measuring things

Not to suggest this system should be converted to steam... but, these input - output heat relations are the bread and butter of problem solving in one pipe steam systems.

Thus, thinking in terms only of system sizing and real home heat losses, here is a way that may be helpful in diagnosing a potential problem. Hydronic balance is not easy.

The twice undersized scheme for a gargantuan home?

Could it be that both the boiler and the radiators are sized too small in relation to the home heat loss? And that the further ratio between boiler output and radiator shows that the radiators are a bit small for the boiler output (baseboards with possibly shut dampers?).

Your return water is somewhat hot, which can be explained by a lack of radiators to boiler output. They can't keep up with the boiler.

Next, the small delta t you reach at steady state indicates the home voraciously eats everything you put on its plate, the boiler only stays six servings ahead of the game and isn’t really keeping up.

This should all suggest a cold home, but with inertia and long cycle runs, the dips in indoor temperature could possibly be forded without you actually getting your feet frozen.

If you can’t find an obvious flow valve to adjust, perhaps redoing all the calculations might show something.

Perry_2

Is there any heating problems? Or are you just concerned with

the low delta T.

If the system is heating fine - then it is probably not a problem.

I installed a Vitodens 200 with a Low Loss Header. The boiler typically runs in the 8 to 10 degree raise range too.

It is obvious that the Taco 007 pump is circulating more water through the system than the old B&G Series 100 as the Monoflo T system temperature usually only has about a 5 degree F temperature drop. I have noticed that the boiler rise is almost exactly twice of the system drop which indicates that the boiler circulation is 1/2 of the system circulation.

Up to today (when it got below 10F) I had never seen the Vitdens 6-24 run continously. It normally cycles on minimum fire rate - but holds the house rock steady in temperature. In reality, I would guss that the boiler is almost twice the needed size based on that information; but smaller ones do not exist (and I am not impressed with the Munchkin).

So, who cares if the boiler rise is "low" as long as the house is being heated. Primary/secondary loops have their own strange magic in how they work.

Now if you have heating problems - then you have other issues.

Perry

Chris E_4

temps tonight

Hi Mike,

I went down and took some readings this evening. I was getting a 126 supply 120 return with an outdoor of 26. The Flue temp was 122. I do get long run times on the boiler and so far I am very happy with the performance of this system. I knew that being it is baseboard that the performance gains of a modcon would not be as great as if it were a radiant floor system. I also knew that the boiler was over sized but I knew that the Ultra could turn itself down. I'm planning on an addition to the house in 3 years and wanted to have enough boiler to satisfy the extra load.

Plus I wanted the Ultra and they do not make a 50,000 BTU version. I have turned down the settings to limit maximum fan speed to 3000 RPM. My reset curve is 146 degrees @ 0 outdoor and 96 @ 70 outdoor. The Ultra really does not have a curve, It's more of a straight line.

I was just wondering about the delta T because I know the holy grail it 20 degree delta T. It just seems like the ultra will not allow more than a small delta T. I wish I installed a ball valve between the closely spaced tee's on the primary loop where the boiler makes it's connection. If I did I would be able to stop the primary loop's main pump and run on the boiler pump only. I guess I could disable the primary loop pump and use the taco 007 to run the whole thing.

Chris E_4

No real issues

Hi Perry,

I have no problems with the system. I do want to split the zones out at some point. The only T-stat is in the living room which is over the basement and next to the dinning room and kitchen. When everyone is over the t-stat turns off with the amount of people in it and the down stairs (six steps down being that it is a split) gets cool. So I do want to zone out the system which will probably cause some problems making the boiler work on a small load.

But everything is working good just wanted to vent some nagging questions in my mind.

Brad White_9

20 Degree Holy Grail?

Not at all, Chris. 20 degrees is convenient, sure. It also reflects a reasonable compromise between flow energy and the size of emitters (coils, radiators, panels, what have you).

In my commercial/institutional design, I might size baseboard for 20 degrees (it needs a higher average water temperature typically for the lengths of wall I may have to work with). In-duct or terminal box reheat coils can be sized for 40 degrees, radiant floors for 10 to 15 degrees -and mixed down to lower temperatures. Convectors 20 degrees and unit heaters 30 degrees. It varies.

On average, the system Delta-T's range from 32 to 35 degrees depending on reheat coil dominance.

These are all on the same system, each has it's rationale.

Point is, nothing magic about 20 degrees!

Mike T., Swampeast MO

All mod-cons can most likely be used without primary/secondary, but you have to pay very careful attention to the flow rate through the boiler. Both too much flow and too little can cause operational problems if not actual equipment damage.

Multiple on-off zone control complicates this matter GREATLY! With "typical" American hot water space heating systems, primary/secondary seems to be a necessary evil. While it minimizes the chance of improper operation due to improper flow through the boiler, it does not maximize fuel economy.

Two-pipe systems using TRVs or FHVs, differential pressure bypass, careful design attention to delta-P and (hopefully a VS circulator) are really the way to go if you want eliminate primary/secondary and maximize efficiency.