Help with delta T

SelectAV

I installed this system in 2005, and I have never been able to get my delta t to be where I would like it. I am only getting 7-10 degrees. Perhaps someone can take a look and see what might be the problem. The system is 4 zones of radiant and a zone of low temp baseboard hot water for the basement. That will probably change to high temp at some point. I just have not had time to change it. All zones are running the same temp water and all my tubing lengths are within 20 feet or so of each other. Thanks for any ideas on how to make my system mor efficient!

Zman

Where are you measuring?

Are you measuring delta T at the boiler or coming off the radiant loops?

What type of radiant panels do you have? What model is the knight? Which circulator is on the boiler? How is the alpha set up? How many gpm does it typically show?



Sorry for all the questions. That is a very nice install. The black background makes it look like art.



Carl

CMadatMe

Radiant or Baseboard

You want a 10 degree delta for the radiant. As for the baseboard you could change the pump to a Taco VDT or better yet Bumble Bee.





http://www.taco-hvac.com/en/products/Bumble%20Bee%20Wet%20Rotor%20variable%20speed%20circulator/products.html?current_category=405

Jean-David Beyer

You want a 10 degree delta for the radiant.

As  a homeowner, I have been unable to understand a comment like this. I have a large (for me) radiant heat zone: concrete slab at grade with copper tubing in it.. The boiler is mod-con with outdoor reset. The thermostat is set at 69F, the heat loss must be pretty low. So until the outdoor temperature goes below 50F, the supply temperature is 75F and that is enough. There is no way I could get a 10F delta T from that, no matter how slow the circulator would go.If it would ever go to 6F outside (design temperature here is 14F), the supply would go up to 120F and perhaps the delta T would go up to 10F then. But I have never seen that.



It was around 50F a couple of days ago and I looked at the boiler. It was putting out about 75F water, and the return was also 75F. There must have been some temperature drop, but not enough to read. The boiler controls have a precision of 1F, so I should be able to see a temperature drop of about that.



It seems to me it would be a mistake to diddle the reset curve to put out 79F or hotter water now in the hope of increasing the delta-T.

Gordy

Varying delta t

Jean,



Delta t is a design tool. You may not see a 10* delta t on a radiant loop until a design day condition. You are at a 50* outside temp so yes you may have a very narrow, or very wide delta t on a loop depending how your controls are set up because its the shoulder season, and the house just needs a little bump with heat now, and then. So if you are on constant circulation, ODR you would have a narrow delta t, if its bang bang control it could be wide because the t stat gets satisfied before the loop may get the full supply temp through it, and the boiler shuts down. It also depends what type of radiant. In concrete insulated, or not. Staple up, suspended tube, over the top sandwich, plates no plates all play a role in how a loop reacts in how quickly it gives up its btus.





My house is a combination of ceiling , floor radiant. I have LOOPS that will run anywhere from 5 to 15 degrees depending on their location in the structure, and the heat loss inflicted on the loop.



As far as a total radiant system delta t. I'm always at 15* plus or minus a couple depending on the outdoor temp. I should also note it's a bang bang setup no bells, and whistle controls original taco panel troll mixing valve from 1952.





So in a nutshell design delta t is a design tool to size a system. Lets not get wrapped up in the number when a system is online, cold start from a cold structure is different.





Delta t could also be used as a trouble shooting tool. Example to narrow a delta t may indicate over pumping a loop, or that loop is not giving up much heat. High delta t could mean the opposite.



Gordy

SelectAV

more info

I am measuring delta T at the boiler.   My problem exist all times of the heating season.  I live in the great white north, and my design temp is minus 11 degrees.  even when it is minus 11 I may get a delta T of 13 degrees at the boiler.  The radiant panel is staple up onix tubing.  The boiler is an 80K btu.self mod with 5:1 turndown. I am using outdoor reset. The boiler circ is a grudfoss ups 15-58 set at medium.    The Alpha is set to Auto and typically runs at 3gpm with all radiant zones running.  I have been told by a couple HVAC guys that these boilers like cold return water, and the higher delta T that I can get the more efficient it will operate.  I would really like to see a 20* delta T at normal operation, an then perhaps higher Delta T at design temp.

Gordy

Boiler supply and return

What are those temps at design conditions? Typically if the boiler is seeing return temps in the 90's or lower that is optimal condensate range.



Another question is how well insulated is the structure?



How well insulated is the staple up radiant? Is there plates? Typically staple up requires higher supply temps then a sandwich type of RFH. So that may hinder really low supply temps to the boiler.





You could try low speed on the boiler circ, but the boiler may not get enough flow through the HX and lock out.



Gordy

SelectAV

Design temps

If memory serves me correctly the system starts at 80* and go all the way up to 150*. The staple up is stapled at least every 8" and it is insulated with double bubble and r15 insulation. The house is fairly well insulated but was also built in 1932. We have remodeled throughout including new windows. I have tried low flow, never got locked out, but seemed like I had worse delta t. One concern I have always had is the close proximity of the return of my secondary to my closely spaced t's. could this be causing ghost flow. It always feels like I'm just recirculating the water in my primary loop

Jean-David Beyer

design delta t is a design tool to size a system.

OK. And since my house was built in 1950, I have no idea how it was designed. I to not know the spacing of the copper tubes in the concrete slab. I do not know if there is any insulation under the slab (I suspect not), but I do know that oil was $0.44/gallon when I bought the place in 1976, so it must have been a lot cheaper when mid-east oil was around $1/barrel.



I do not know if the system was even designed or not. It had a GE oil fired residential boiler that may have put out 70,000 BTU/hour bang-bang. I calculated the heat loss at about half that. It bounced between upper and lower limit very rapidly. The former homeowner had the aquastat set very tight, less than 5F apart. I moved it up to 10F apart. Any more and the pressure relief valve on the makeup water line would relieve.  The boiler did not have a pressure relief valve of its own.



And the upstairs never got enough heat because the baseboard up there was way too small with the 140 or so water temperature supplied. The whole house got the same temperature hot water.



I now get enough heat up there because there are two zones, and I increased the size of the baseboard in each room upstairs from 3 feet to 14 feet. Now the maximum temperature I can supply (unless I change the reset curves) is 120F downstairs and 135F upstairs. These temperatures are so low that the circulators run a lot of the time. Yesterday, the downstairs (radiant slab) zone ran for 11 hours 27 minutes. So the reset curve is mighty close. I did not dare set it any closer because I want some margin for high winds.



I think the downstairs is designed right (by accident). There is a Taco 007-IFC circulator it. Unknown amount of copper tubing; 745 square feet. Upstairs has 28 feet of Slant/Fin and is 450 square feet. There is over 70 feet of 1/2 inch copper tubing in that loop. It also has a Taco 007-IFC and I suspect the flow rate is higher than it needs to be. Even when it is very cold outside, I get quite a bit less than 10F temperature drop through that loop. When the air is out of that loop, it is very quiet, so I guess I am putting less than 4 feet a second in there. But I do not think I will put in a smaller circulator until this one fails, and I hope that is a long time from now.

Zman

I think your system is optimized

I don't think you are going to improve on what you have. It looks like you are running between 4 and 5 gpm on the boiler loop. If you are running 3 gpm on the heating side, you are likely in the 8 -12 range on the infloor when your boiler is at 7.

As suggested, You could experiment with lower flow rates. You could go to low on the circ and experiment with throttling valves. I suspect you will lock out the boiler.

It is true that you will get better efficiency by lowering return temps. You would have to have different emitters to do that. Hugely oversized baseboards and tons of surface area on the radiant floor would help with that.

I think you have designed a nice system that is running well. Aluminum plates are the only thing I would have added.

Carl