Large delta t or low supply temp whats more beneficial

Franktg78

Im trying to understand the benefits and drawbacks to my experience with my own system.  I recently added rehau rauplates for under sub floor radiant and some raupanel in 2 rooms i renovated.  I doubled my baseboard in all bed rooms on my 2nd floor and replaced it all with rbi futera high output fin tube.  This was based on heat loss calcs and low temp supply water.   

Im piped primary secondary with closely spaced tees.  At some point id like to add a 50 or 100 gallon buffer tank/hydraulic separator.

   One room is all windows and we have 3 fan coil kick heaters.  This in in addition to the radiant i installed and original high output baseboard.    I added a nimbus vfd that controls the blower speed of all 3 fans on the kick heaters.  A 0-10vdc t stat in the room supplies the control sig for fan speed.  Fans dont activate till 5vdc.  This allows the radiant and original baseboard to be primary heat.  The fans are supplemental on those cold days.

I initially was hell bent on a 20 degree delta and utilized all delta t pumps.  I quickly realized the larger my delta t  the higher my supply temp needed to be.  At 20 deg osa, my supply needed to be 140.  You could feel variations in floor surface temp while walking around on the radiant heat.

I began playing around and set the delta t pumps to constant speed.  The delta t on all my zones is set at around 5-8 degrees.  My supply temps average 95 degrees at 26 deg osa.  I have a max temp set point of 110 degrees. My space temps indoor are 70 deg f

With my reset curve and constant circulation as long as osa is below 50 degrees the boiler never shuts off it runs 24/7.

With the low delta t  all my radiant heat has a consistent temp feel when walking around.  The rooms with raupanel above subfloor plates are by far the coziest.  When we did the heat loss calc i had them size the system to heat the room to 70 deg with osa of 4 deg f and max water temp of 110.  

Considering my supply water temp is always below condensing water temps. 

The boiler doesnt short cycle. 

Is there any need or benefit for a larger delta t?

I feel the lower delta t has made everything feel more consistent.

Thanks

EBEBRATT-Ed

Radiation , fin tube, radiant all is going to have a supply and return temp obviously. Heat emitters are sized by the average water temp the emitters sees.

So lets say the emitters need 130 to heat the space

5 deg delta gets you 132.5 and 127.5
10 get you 135 and 125
15 gets you 137.5 and 122.5
20 gets you 140 and 120


When you were running the 20 deg delta with 140 supply it was uncomfortable because you had 140 at the beginning of the loop and 120 at the end. Smaller delta keeps the floor temp more even across the whole floor.

I don't think you have a choice other than to keep the delta low will allow you to run cooler swt and condense more. There is nothing not to like about that except for pump electrical loads. Can't have everything

Rich_49

Was a room by room heat loss performed before you did this work ? How did you arrive at the temperatures you are using ?

Jamie Hall

Isn't thermodynamics fun? It's also completely unforgiving, and pays no attention to wishful thinking.

A heat transfer element... let's be specific; a radiator or radiant floor -- will transfer a certain amount of heat per unit time to the space around it. That amount of heat is dependent on the temperature of the element and its surface area -- and, to a first approximation, nothing else. Now if we assume that it is being fed by warm water, that heat transferred out to the space has to come from a change in temperature of the warm water -- the return will be cooler than the source. The greater the flow, the less the change in temperature -- our old friend the hydronic equation.

Bottom line: if you want a certain amount of heat output from your radiator or floor, you must run it at a certain average temperature. No options. If you want, for some reason, a large delta T, supply warmer water for the input and cut back on the flow. If you want a smaller delta T, speed up the flow and drop the supply temperature. If you want a specific return flow temperature, adjust the supply temperature and the flow to get the combination of average temperature and return temperature you want.

There is a gotcha in this. The above simple set of relationships works for reasonable delta T. It doesn't work for very large delta T, as it ignores the loss of heat transfer at the low temperature end. But close enough for reasonable values.

Zman

Do you have zone circs or zone valves?
Is the boiler being protected from condensation?

Steve Thompson (Taco)

At the risk of being "off-topic", what's the fellow Wallies opinion of the benefit of controlling return water temp to help a condensing boiler condense or conversely, saving a non-condensing boiler from condensing?

Jamie Hall

I don't know about Wallies as a group (have you ever heard the term "herding cats"?) but I'd say that if it can be worked into the control scheme it's a very good idea. It does mean that you aren't controlling for or really even thinking about a specific delta T, though. If you are using outdoor reset, you will need, instead, to vary both the supply temperature and the flow rate to meet the demand. Not impossible to do, but not quite what seems to me to be usually done.

For non-condensing and, presumably, not modulating, the control scheme is a little different but the same idea.

Dave H_2

Designed temperature drop is a variable but also a guide based upon your heat emitters. When I design radiant floors, I like a tight temp drop, it provides a more even surface temp in the space, it may also depend upon the installation of the tubing (supply to outside wall, return towards the center of the room). As an underfloor system like you have, you don't have the luxury of the keeping the hottest water to the outside wall. So a typical design should be around 10 degrees as you have noticed.
When it comes to finned baseboard and such, design temp drops are 20F, these can be widened or tightened as needed. FlatPanel radiators are designed to have a 30-40 degree drop.
These temp drops are also safe design "rules of thumb" to accommodate many systems, components and installers all over.
It sounds like your system is working well.

Dave H.

EBEBRATT-Ed

@SteveThompson (Taco)

I don't know about condensing boiler but as far as standard CI bolers go I am in favor of a 3 way valve and a sensor on the return to the boiler to prevent condensation

Rich_49

Jamie Hall said:

I don't know about Wallies as a group (have you ever heard the term "herding cats"?) but I'd say that if it can be worked into the control scheme it's a very good idea. It does mean that you aren't controlling for or really even thinking about a specific delta T, though. If you are using outdoor reset, you will need, instead, to vary both the supply temperature and the flow rate to meet the demand. Not impossible to do, but not quite what seems to me to be usually done.

I like that . Not usually done would be correct . I do it on EVERY project , trying to get others to realize that you can maintain a Delta T and use ODR together is like you said , herding cats . Seems the cats are just too attached to the old school shepherd .


For non-condensing and, presumably, not modulating, the control scheme is a little different but the same idea.

hot_rod

Franktg78 said:

Im trying to understand the benefits and drawbacks to my experience with my own system.  I recently added rehau rauplates for under sub floor radiant and some raupanel in 2 rooms i renovated.  I doubled my baseboard in all bed rooms on my 2nd floor and replaced it all with rbi futera high output fin tube.  This was based on heat loss calcs and low temp supply water.   

Im piped primary secondary with closely spaced tees.  At some point id like to add a 50 or 100 gallon buffer tank/hydraulic separator.

   One room is all windows and we have 3 fan coil kick heaters.  This in in addition to the radiant i installed and original high output baseboard.    I added a nimbus vfd that controls the blower speed of all 3 fans on the kick heaters.  A 0-10vdc t stat in the room supplies the control sig for fan speed.  Fans dont activate till 5vdc.  This allows the radiant and original baseboard to be primary heat.  The fans are supplemental on those cold days.

I initially was hell bent on a 20 degree delta and utilized all delta t pumps.  I quickly realized the larger my delta t  the higher my supply temp needed to be.  At 20 deg osa, my supply needed to be 140.  You could feel variations in floor surface temp while walking around on the radiant heat.

I began playing around and set the delta t pumps to constant speed.  The delta t on all my zones is set at around 5-8 degrees.  My supply temps average 95 degrees at 26 deg osa.  I have a max temp set point of 110 degrees. My space temps indoor are 70 deg f

With my reset curve and constant circulation as long as osa is below 50 degrees the boiler never shuts off it runs 24/7.

With the low delta t  all my radiant heat has a consistent temp feel when walking around.  The rooms with raupanel above subfloor plates are by far the coziest.  When we did the heat loss calc i had them size the system to heat the room to 70 deg with osa of 4 deg f and max water temp of 110.  

Considering my supply water temp is always below condensing water temps. 

The boiler doesnt short cycle. 

Is there any need or benefit for a larger delta t?

I feel the lower delta t has made everything feel more consistent.

Thanks

I suspect if you monitor the system for a longer period of time you will see the delta varies based on the load of the spaces, as it should to provide the comport you are experiencing 

curious to why you were hell bend on a fixed 20 delta?

Rich_49

Franktg78

Why did you decide to use constant speed as opposed to lowering the Delta T setting to say 10*F ?

What orientation did you install the tubing in the RauPanel ? Did dyou supply all the loops with the supplies at the cold walls and allow it to cool across the room to the interior or another way ?

Rich_49

https://www.youtube.com/watch?v=6mNECDHgDrg&list=PLuuV0ELkYb5VE0I4evUZ30b5U78CRlRdg&index=31

Zman

Steve Thompson (Taco) said:

At the risk of being "off-topic", what's the fellow Wallies opinion of the benefit of controlling return water temp to help a condensing boiler condense or conversely, saving a non-condensing boiler from condensing?

Since the OP was a "one post wonder", why not go off track.

Delta t control, IMO preferable controlled by the boiler, is wonderful for mod/con boilers piped primary/secondary.

I am not sure I see the application for preventing condensation in a conventional boiler. Definitely not in a copper fin with minimum flow requirements. I guess you could configure a cast iron boiler to work, but why not just use a smart mixing valve instead?

Franktg78

Sorry for the lack of responses

My 12 year old has covid.  Her and my other 2 daughters are stuck in isolation with remote learning.  Bee real fun past couple days playing mr mom.

Rich_49
Heat loss calcs were done for some not all
2-rooms with raupanel above sub floor plates 
    I had rehau size the system for 4 deg osa 70 deg space temp and 110 mat water supply temp.  Each circuit ir set at .25gpm   This system is amazing and has the highest delta t of any of my zones.

All 2nd floor bed rooms


I used rbi futera baseboard with the highest output element they offered.  In the end the linear footage just about doubled in each room.  The baseboard replacement alone increased the room temps 3 deg on avg.  These were the coldest rooms 67-68.  I was able to change my reset curve from 1.5 osa-1 deg water to 2 deg osa-1 deg water temp and still hold 70 deg room temp.

The rest of the first floor i have a mix of thermofin extruded aluminum plates and rehau rauplate.  On all the underfloor i never did a calc just put it in every bay.

The room with all the windows i did a calc for and thats why i added the 3 kick heaters.  They can be noisy when vsd ramps them to full speed but the do the trick. 

For my outdoor reset 
It runs in a 15 degree setback at all times.  I have 2 stage tstats and if any area does not maintain room temp stage 2 is activated and that brings the controller out of setback.

Franktg78

Zman
I have a combination of circs and zone valves

My radiant is all first floor
One pump for rehau raupanel 007 always has a 10 deg or better delta t even as a single speed pump

One pump taco 00e delta t pump for thermo fin and rauplate set to 6 deg delta t

Both are pulled in by system pump output of my reset controller.  Osa below 55 pumps turn on

The other zones are grouped 
Master zone valve
3 bedrooms zone valve
Both share taco 007-vdt (delta t pump)

Room with all windows 
Dedicated 007-vdt pump
All 3 kick heaters go to a nimbus variable speed drive that responds to a 0-10vdc sig from the room stat.  I programmed the nimbus to activate at 5 vdc.  this allow the radiant and baseboars to be primary source of heat.  Once tstat goes past 5vdc then fans start to ramp up.

Franktg78

Dave h_2
All my raupanel was installed as feeds at the outside walls north and south.  Those circuits were also made slightly shorter than the other 3 circuits for the room.

What do you consider a tight delta t on radiant?  With the taco 00e on my exteuded plate and rauplate im set for 6 deg delta t

The fin tube ill never get 20 deg  delta im running my radiant temp through the fin tube.  My max water temp is set to 110 det f at 4 deg osa

Im running a condensing combi unit from intellihot.  The reset controller is separate control.  The boiler does not have the ability for remote set point so i change set point on boiler myself weather dependent.

Franktg78

Hot rod

Hell bent on a fixed delta t

This came from my work in commercial boiler rooms.  Where new systems are going into existing radiation and high supply temps were never addressed.  Or new buildings with condensing boilers go up and architect still spcs slant fin standard baseboard.  In these settings we are at minimum looking to achieve 20 deg delta t.  But we also have a fairly agressive reset curve with a min supply of 140 and max of 180.  In my own home i am trying to design around low supply temps.  This through me a curve ball in the way i look at things.  When initially trying to get a 20 deg delta t my supply temps went up and so did my stack temps and stack losses.  Then I decided to experiment some.  I started to focus on low supply temps and not pay much attention to delta t.

Im trying to get these taco vdt pumps to hold around a 5 deg delta t.  This seems to provide a nice consistent floor temp.

I plugged the vdt pumps into a watt meter.  Its amazing how much more they pull compared to the 00e.  Vdt at min speed 47 watts and the 00e hovers aroumd 8-9 watts.

Would love to eventually get the vdt pumps out and all 00e pumps in.  One of these days

Hot rod you will appreciate this.  I installed the caleffi 3/4 in quick setters.   Just because the piping was 3/4 i ordered the 3/4 ones to make install easy.  Never bothered to check the gpm scale till i went to set them.  Thats when i discovered my oversight.  The 3/4 units are 2-7gmp scales all my zones are around 1gmp.  I was very disapointed in myself.  Now all i have are fancy shut off valves.  Wont do that again thats for sure.

hot_rod

To my way of thinking you chose a delta t to design around, for comfortable floors maybe 8-12°,there really is no set in stone number. Commercial shops, warehouses may chose a larger delta as comfort and consistent floor surface is not as critical as covering the load. Other types of emitters can run various deltas, maybe 30- 35 on panel rads for example. I have yet to see any documentation stating fin tube of panel rads are only efficient at one specific delta? In fact many or the panel rads show as wide selection of deltas and the change in output associated. and/ or you can calculate that also.

With a properly sized, piped and installed system you may on occasion see the system running at that exact delta you designed to. With a fixed speed circulator what you will in fact see is that delta changes, moves as the load on the space changes. That load can change minute by minute, even with consistent outdoor conditions. Things change inside the structure, cooking, laundry, showers, exhaust fans, occupancy load, etc.

Since the controls and emitters are inside your home, hopefully The system responds to what is going on inside. The wall thermostat, while it could have some onboard intelligence, is mainly a high limit.

My opinion based on plenty of installation, calculations, observations, is that a system where the SWT can change based on both indoor and outdoor input, a modulating heat source, and a circulator that changes based on flow rate changes, delta P, if the system is zoned gets you as close as possible to an ideal comfort system. No need to over complicate or put artificial constraints on the circulating.

If I'm reading your post correctly you are discovering the same?

Yes, as you discovered control valves of any sort need to be sized to flow rate not pipe size, balance valves, thermostatic, pressure regulators, check valves, zone valves, etc.

Zman

I think that a delta of 20 became the standard because the math was easy, you just slide the decimal
I like to get the delta closer to 10 for radiant.

I remember back 10+ years ago when Taco only made Delta T circs, their sales trainers would tell you they were so good that you should put them on every part of every system. I had to walk out of a seminar in Chicago because it was getting so thick.

Now that they make Delta P, that message is quieter but they refuse to back off some of their earlier claims.

I have never agreed with the claims that Delta T belongs on a system with zone valves, especially high mass systems. The circulator is totally blind to what is going on in the individual zone and is reacting to the average return temps only.

Franktg78

Zman 

The taco vsd pump is currently on my radiant manifold and so far performing and responding as desired.  One of the vdt pumps handles my master as one zone and 3 other bedrooms as other zone.  Weather one zone or both are calling the vdt seems to respond well and maintain 8-10 deg delta.  I can see it modulate up and down via thw watt meter is plugged into.  On mild days the delta narrow to 2-4 deg.  I do not believe it turns down as low as th 00E Pump.  Right now the weather is also so mild here in nj getting good data is a challenge.  What advantage would a delta p pump have in place of the delta t?  I would think delta t would respond more to heat load conditions as apposed  to head pressure based on a single or both zones calling.

Franktg78

Hot rod is there any way to replace the gmp scale on the 3/4 quick setters?

hot_rod

Franktg78 said:

Hot rod is there any way to replace the gmp scale on the 3/4 quick setters?

Nope, the orifice inside is different, sorry.