I am confused about delta t when charging storage tank

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goosegunner

Minister of Fire
Oct 15, 2009
1,469
WI
I have temperature gauges on my boiler supply and return. As my tank approaches 175 top and 170 on the bottom the delta t between return and boiler supply are very close. The supply does not seem to gain more than a few degrees. When return is 140 and after Danfoss is 150, my boiler supply to tank will be 15-20 degrees higher. It is usually about 165.

Is that normal?

Will moving more gpm help as tank temps climb?

gg
 
What is the temp dial set at on your boiler controller? If you have storage, you want to max it out at 195F. If you have it set to 175, your boiler is idling.
 
Hunderliggur said:
What is the temp dial set at on your boiler controller? If you have storage, you want to max it out at 195F. If you have it set to 175, your boiler is idling.

No boiler is set at 195.

gg
 
benjamin said:
It sounds like your danfoss is wide open just like it should be at that temp, right?

Danfoss is open, bypass is boosting temp a little above return temp. Boiler isn't adding much.

gg
 
Reading your original post, maybe your circulator is running faster than you need. What is your pump launch temp - 165 - 170F? Where are you measuring the return temp, before the Danfoss?
 
The numbers that you posted aren't making any sense (to me anyway). If your storage is 170-175*, and the boiler (with a high setpoint of 195) is loaded and has a good fire going, why would the boiler supply only be 165*? Is there also a big load from the house at the same time? Or, could you be getting unwanted flow to your pool loop?
 
Hunderliggur said:
Reading your original post, maybe your circulator is running faster than you need. What is your pump launch temp - 165 - 170F? Where are you measuring the return temp, before the Danfoss?

I have return temp gauges before and after the Danfoss. They are the push fit type in a 1/2" well, at times I do question their accuracy.

gg
 
willworkforwood said:
The numbers that you posted aren't making any sense (to me anyway). If your storage is 170-175*, and the boiler (with a high setpoint of 195) is loaded and has a good fire going, why would the boiler supply only be 165*? Is there also a big load from the house at the same time? Or, could you be getting unwanted flow to your pool loop?

No load from pool, it is winterized.

My next firing I need to watch the wood load more closely and see if it is related to that. It might be due to the fact that there is not enough fuel to left of the first load to gain temp.


The return is 165, off the vey bottom of the tank.

Maybe I am wrong here but I my boiler set point is 195 only for the very end of my charging. It has nothing to do with the first couple of hours bringing the tank up from 150. If the top is 150 and the bottom is 140 I don't think it would be physically possible to have a high boiler output with 140 degree return.

Anyone else with storage how does your boiler output relate to return temps?

gg
 
My boiler return is set at 150deg (3way thermostatic valve) it pretty much maintains a delta T across the boiler of 25deg (sometimes 30deg if I have some softwood burning). If the mix valve is supplying 150deg water to the boiler (my minimum) then i am getting 175deg water to storage, when the return water (from storage) goes above 150 I get hotter water going to storage (160deg from storage = 185deg to storage).

My boiler is set to 200, not that it matters, never idles, rarely goes above 195
 
I get a rather consistent 20-25 degree delta T, similar to Garth. The best thing I ever did to "tweak" boiler output was to install 3 speed Grundfos pumps. Having a Taco 007 as my primary circ was absolutely killing my output initially. My primary loop was is very short and 007's are rather strong pumps...way too much flow for a guy with his boiler, storage and loads all in the same room.
 
I have Grundfos 15-58 3 spd installed. From my head calculations it is actually undersized. I bought a 26-99 but have not installed it yet.

I guess I will try the 15-58 on medium speed instead of high.

gg
 
I am still finishing my storage controls but during initial tests I have a 20 degree delta T. I have a three speed Taco circ pump in addition to the zone pumps. I may bypass the Taco as every indication is that I have too much flow even at low speed. I didnt go with high level controls and its time like these that I see how much a flow meter and VF drive with a setpoint controller would make things a lot better.
 
My setup:

Dial Setting - 195F
Pump Launch - 170F
Pump off - 165F
Danfoss - 140F

I have radiant and actually sometimes (more often than I like) draw the system down to 100F. Let's say the return is at 110F. At initial firing, the pump will start when the boiler jacket reaches 170F. At that point, 1/2 half of the flow will go to storage, the other half will mix with the 110F return to make 140F into the boiler. The boiler will not maintain the 30F delta (140 to 170) unless the fire is going really well, so at 165 the pump will stop until the jacket reaches 170 again. The storage stratifies, but not perfectly. The top will be hotter than the bottom. When the return temp gets to 140 I have 100% going to the storage. I can maintain a 30F delta T on the boiler input/output. As storage increases in temp my max temp increases. My average storage temp increases about 10F per hour. When I get time to program my temperature monitors I'll have a better picture. Right now I have the temp control for the zone valve to bypass the backup instant water heater which gives me the top 1/3 temp, and a couple of Radio Shack wired temps which give me HX input and output (I have an in tank HX). I only run my loads up to 130F with a mixing valve.

BTW - I use a Taco-007 3 speed on medium (I'll have to go check that, may be on high), with 160 feet of 1 1/4" from the boiler to the storage one way (1/2 PEX, 1/2 Copper).
 
Hunderliggur said:
My setup:

Dial Setting - 195F
Pump Launch - 170F
Pump off - 165F
Danfoss - 140F

I have radiant and actually sometimes (more often than I like) draw the system down to 100F. Let's say the return is at 110F. At initial firing, the pump will start when the boiler jacket reaches 170F. At that point, 1/2 half of the flow will go to storage, the other half will mix with the 110F return to make 140F into the boiler. The boiler will not maintain the 30F delta (140 to 170) unless the fire is going really well, so at 165 the pump will stop until the jacket reaches 170 again. The storage stratifies, but not perfectly. The top will be hotter than the bottom. When the return temp gets to 140 I have 100% going to the storage. I can maintain a 30F delta T on the boiler input/output. As storage increases in temp my max temp increases. My average storage temp increases about 10F per hour. When I get time to program my temperature monitors I'll have a better picture. Right now I have the temp control for the zone valve to bypass the backup instant water heater which gives me the top 1/3 temp, and a couple of Radio Shack wired temps which give me HX input and output (I have an in tank HX). I only run my loads up to 130F with a mixing valve.

BTW - I use a Taco-007 3 speed on medium (I'll have to go check that, may be on high), with 160 feet of 1 1/4" from the boiler to the storage one way (1/2 PEX, 1/2 Copper).



Ok thanks that helps.

The next question is what is better,

A 30 degree delta t at 10 gpm

Or

A 10 degree delta t at 20 gpm

gg
 
How well insulated are your pipes? The higher delta T means more potential heat loss. The lower velocity means better stratification and lower pumping costs. I personally would go with the 30/10 but someone may be able to give a super duper computationally derived answer ;-) 20 GPM also has the potential for more long term pipe wear due to the higher velocities.
 
Update,

I switched the gauges around and the supply gauge must have been off, It now reads a higher output. It was a solid 20 degree delta T for most of the burn. When load was near the end and tank was over 180 the delta t was closer to 10.

The problem I find is the huge variance in the fuel load. Sometimes I am not moving enough gpm and the temp will climb to the point of where the low fan kicks on. Other times in the beginning of the burn My forced air heat will drop the boiler temp 10 degrees when the load kicks in.

It is definitely more difficult to get to the burn clean mode vs running with no storage. With no storage everything was always nice and hot so whenever you throw in wood it will gasify right away. With storage it takes awhile to get everything in the boiler nice and hot.

gg
 
I have an effecta lambda 35 boiler and use a laddomat 21-60 loading valve.

This loading valve has an integrated circ pump and three seperate temperature gages for boiler bypass, boiler return from tank and boiler return to boiler (this is a combination of the hot bypass and cooler water from tank bottom).

This laddomat has a 72C thermostat and thus throughout most of the burn the boiler temp is 72C.

However, once the bottom of the tank gets higher than the 72C (and goes to 90-92C) the delta is usually 10-15C.

Brian
 
Effecta Boiler User said:
I have an effecta lambda 35 boiler and use a laddomat 21-60 loading valve.

This loading valve has an integrated circ pump and three seperate temperature gages for boiler bypass, boiler return from tank and boiler return to boiler (this is a combination of the hot bypass and cooler water from tank bottom).

This laddomat has a 72C thermostat and thus throughout most of the burn the boiler temp is 72C.

However, once the bottom of the tank gets higher than the 72C (and goes to 90-92C) the delta is usually 10-15C.

Brian


Thanks, I think that confirms that boiler setpoint means little compared to return temp. My problem was a bad gauge. There is a balance between gpm and setpoint. My guess is if you can hit your set point that is 30-50 degrees over return there is not enough gpm moving.

Gg
 
Hunderliggur said:
How well insulated are your pipes? The higher delta T means more potential heat loss. The lower velocity means better stratification and lower pumping costs. I personally would go with the 30/10 but someone may be able to give a super duper computationally derived answer ;-) 20 GPM also has the potential for more long term pipe wear due to the higher velocities.

Be sure to avoid confusing delt T in the boiler HX (being discussed here) and delta T between the pipes themselves and the outside world (not being discussed here). An increase in delta T within the boiler is technically not related in any way to the heat loss after the water leaves the boiler. The difference between boiler room air temp (assuming indoors) and your exterior pipe temps is what determines heat loss from the pipes. Like you're saying, I think, as pipe temp inscreases, assuming pipe temp is above room temp, so too will loss increase. But not because of boiler delta T.
 
the formula for heat transfer is btu/hr= 500 x delta T x GPM

this can be looked at like this
- for a fixed flow rate
heat into storage is proportional to delta T

So- as your delta T shrinks, you are adding less to the thermal storage.
In the beginning, the delta is large, then as it approaches the boiler supply temp, the delta decreases.

To get more heat in, your boiler supply water temp has to come up.

As for the flow rates a lower delta T will give a higher average temperature. For instance, suppose you are supplying a radiant heat floor, the supply temp is 120. With a 10 deg delta, the average temp of the floor is 115. with a 30 deg delta, the average temp is 105. Generally, the design calculations for heat output are specifies for average temperature, not max temperature.

I prefer lower delta t's because floors tend not to get cold spots. Also it leaves some design room if a zone was sized a little under, the delta will go up, but it will still work.
Generally, high deltas indicate low flow.

I hope this helped.
 
I don't recall your plumbing setup, so my comments will be theoretical. Your boiler is rated at 200,000 btuh. I will assume that is maximum at high burn, delta-T=20F, return water at 160F, supply at 180F, gpm=20. I also will assume that average output over a burn load is about 150,000 btuh, which at the same parameters = 15gpm.

An appropriate setup (look at your manual for specifics) would be return water protection no less than 140F, and up to 160F would be good and might be better. Mine is about 145F minimum no matter how much below this is the system return water temperature. Also, boiler circulator should be set to not turn "on" until boiler temperature is 160F or somewhat higher, and should turn off if boiler supply drops below 160F. I have a sensor fixed to the boiler supply pipe just as it exits the boiler, and my "on" point is 165F at this location, which means at initial "on" actual internal boiler temperature is somewhat higher.

I think a reasonable goal for delta-T between boiler supply and boiler return after return water protection should be 20F, at least until system return water before protection reaches about 165-170F, assuming for your boiler 20 gpm flow rate and assuming a high medium to high burn rate. This means that at 20 gpm your circulator can move 100% of boiler output all the way up to the point when the boiler will shut down (assume 190F). For example, supply 160F and 140F after protection, supply 170F and 150F after protection, etc., until supply 190F and 170F after protection (which also means that return protection valve should be wide open because system return before protection is above 160F).

As system return before protection rises (in the above example) above 170F and a high burn rate continues, a "problem" begins to occur, because at boiler shutdown at 190F, delta-T is now less than 20F, and if gpm = 20, flow no longer can handle full boiler output. Assuming continuing high medium to high burn, your boiler will start idling cycles. At delta-T=15, you only are moving 150,000 btuh with output as high as 200,000 buth, thus boiler heats to idle point and idling begins until boiler cools down. There are at least 2 solutions to this "problem," the easy one being loading your boiler so that as system return (which also would be return from storage) approaches 170F, the boiler wood load is burning down to medium burn and below, which means that boiler output is less than 200,000 btuh and falling, and at 20 gpm full output is still being moved from the boiler. Experienced or careful loading (as I achieve with weighed wood burns) can result in the entire storage tank being brought up to 190F (no system load) as the wood load burns down and then out.

A second solution is to increase gpm as delta-T is less than 20F. A variable speed circulator might do this, or installing in parallel a second circulator can also do this. A second circulator could be set to "on" at 185F and "off" at 175F, for example. Actual increase in gpm will depend on pump head at the increased flow rate. I used the second circulator solution in the past and it worked well, up to its limit on additional flow rate.

You don't necessarily need 20 gpm flow rate, although with your boiler that is what I would plan for with plumbing and circulator selection. If your flow rate is less than 20 gpm, idling will become more likely because boiler output can be greater than the system will draw and boiler temperature will rise, and also delta-T will be greater than 20F. This is not an issue at lower return water temperatures: If your flow rate is 13 gpm and delta-T=30F, you still are moving 200,000 btuh. Example: return water after protection 150F, boiler supply 180F, gpm=13, system will move 200,000 btuh. But you can see this becomes an issue as boiler supply approaches your idle point of 190F, and idling may start to occur at system return temperature of 160F.

Similarly, if your flow is greater than 20 gpm, that too is not necessarily an issue. If your flow rate is 25 gpm, idling will become less likely because boiler output will be less than the system will draw up to a higher temperature, and also delta-T will be less than 20F. This too will not be an issue at lower return water temperatures: If your flow rate is 25 gpm and delta-T=16F, you still are moving 200,000 btuh. Example: return water after protection 150F, boiler supply 166F, gpm=25, system will move 200,000 btuh. And the greater flow rate will allow full boiler output at 190F up to 174F system return water before idling will begin.

I would think that with your system, a base flow rate of much less than 20 gpm is unsatisfactory due to the extent of idling that is likely to occur. Also, a base flow rate of much more than 20 gpm is inefficient, except as suggested with a variable speed or parallel circulator design. In all cases you will want to load and fire your boiler to minimize idling based on system demand and actual flow rate. Hope this helps.
 
My comment on 160F start temp is directly related to gasification boilers. Most mfr's recommend return water protection no less than 140F, and often as high as 160F. Typical delta-T temperature rise is 20F, which means at 140F minimum return water protection, supply temperature needs to be at least 160F. Therefore, a start and operating temp less than 160F will not allow the boiler to operate within its recommended operating range.

As a practical matter, boiler operating temps below 160F are inefficient, can cause creosote, corrosive condensation in the hx tubes and/or flue, smoke and pollution.
 
I know my boiler starts the pump below 160.I get gasification within a few minutes.I dot not get smoke or creosote.My danfoss does not let return water to the boiler below 140.
 
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