low delta T

craig_4

Thanks for your responses. The h.o. moved into the finished basement a few years ago. walk out on one zone. Used to have oil and it heated fine. She is bound to a wheelchair and he to oxygen hose. They replaced the oil with a LPG lochinvar 125 kbtu boiler. They said it would satisfy t-stat the first year but now it doesn't. I pulled piano and couch away from wall to allow better convection. I asked if any conditions had changed since it heated better; they said only a door wall had been replaced. The layout in the basement is: first on circuit is bedroom with about 20 ft. of element. Vents are closed because it gets too hot. The rest of the basement is reletavely open. Had some cob webs on element, but no serious restriction to convection. Boiler has built-in recirc pump for low temp return protection and 3 speed circ for space heating. Switched speeds up and down and could hear the difference so am assuming flow isn't the prob.Can't figure out why performance changed. If impeler was slipping on shaft, barely moving water, I'd have low retun temp. The two main floor zones are not used, but I opened zone valve and heard air and water moving. Thought my high temp readings on supply and return were due to gravity or laminar flow. Checked pipe temp throughout house and it was hot. I hate when I don't know why something changes but bottom line is I need to put more heat into space.

craig_4

I have customer complaining of not enough heat in occupied zone. The lower floor doesn't get above 68 degrees despite the boiler's firing and circulation. They have about 20 ft. of baseboard. I only have about 7 or 8 degrees temp drop accross boiler. They have a suspended ceiling with open truss style joists above. Having no luck finding a small and quiet fan backed hydronic coil to put more heat into the space. All I can find are large unit heaters. Need a ceiling mounted unit with decent appearance and about 5,000 btus. Boiler set point is 180f. Return temp. is about 173f. Any resources or direction would be appreciated.

Dave_4

Beacon morris makes a small unit heater.

its pretty quiet, dial the controler down to 100F and it can deliver 5 k to the room no problem.adding 12k to the room would be within its ability by turning the fan limit switch make on rise to 180 if you needed it.check what the manufacture specs it out on the lit. i think it isnt as sharp as making your own radiant panel though

clammy

pump size

Sounds with that low a delta t you may have to a high flow through your baseboard .You may need to throttle down the flow in that zone and increase your delta t also check around your baseboard is there furniture or high carpet limiting air flow across the baseboard element i ran across that also .keep checking sounds like too much flow but any thing could happen peace and good luck clammy

Brad White_19

Rather than go after the symtoms

can we take a look at things further up the logic tree?

Some questions:

1) Is the room in question at the end of a circuit? Might the entering water in that room be depleted by upstream radiation? How is this circuit performing re: delta-T relative to the system as a whole?

2) How is the element sized relative to the heat loss? Heating 101 question I know, but have to ask it. Also is the construction equal to the calculations? Was the insulation integrity maintained?

3) Is everything working on this zone in terms of establishing and maintaining flow? Zone valves or circulators open? Vented of air? Can flow be demonstrated independently of other zones/circuits?

4) The 7 to 8 degrees delta-t as clammy suggests does indicate a higher than average flow rate. I am not too concerned as this would tend to increase the average water temperature in the radiation. So long as there is no velocity noise, the turbulence in the tubing makes for good heat transfer.

If all of the above checks out ok then I would consider adding heat capacity.

If noise is a concern and there is room, consider a ducted unit heater (cabinet type). This will have the fan noise source remotely from the space. You may want to line the duct for noise control and use a return-side filter grille to keep the coil clean. Use a variable speed control to optimize noise versus capacity.

If the kind of heater that is exposed to the space and is deemed noisy, there is no easy fix in my opinion.

Good Luck-

Brad

brucewo1b

4) The 7 to 8 degrees delta-t as clammy suggests does indicate a higher than average flow rate. I am not too concerned as this would tend to increase the average water temperature in the radiation. So long as there is no velocity noise, the turbulence in the tubing makes for good heat transfer.

Brad while at face value this seems to make sense it has been explained to me that if the water rushes through the piping too fast then not enough heat transfer is taking place hence the low delta-t, or am I barking up the wrong tree.

J.C.A._3

Don't forget....

To check to see if the element is CLEAN!!!

The surprizing effeciency and heat transfer gained by removing the old crayons and Rover hair are amazing, to say the least. Chris

Brad White_19

It is one of the great paradoxes of

heat transfer, especially in small-tube emitters. Yes, retention time increases heat loss to a point. But fin-tube is predominantly governed by average water temperature and it's relation to inlet air temperature. No other variable comes close at design velocities. The higher the GPM, the narrower the delta-T hence the higher the average water temperature.

True, increasing flow rate from say 1 gpm to 4 gpm as you will see from manufacturers data will only increase output fractionally, ten percent if you are well-behaved. That was not my concern at the high end, it was exploring and attempting to eliminate the low-end of flow as a possibility.

There is a drop-off in performance below certain pipe velocities.

My comment was directed at the possibility that flow could have been so low (below 0.3 to 0.5 feet per second) in which case laminar flow results. Specifically the film of water along the pipe wall abutting the fins essentially give up it's heat and stays there. (Not really, but it does not mingle as well with the primary flow now in the center of the pipe). When velocities are higher this film is continually disturbed. The pipe walls are scrubbed with fresh hot water as it travels down the pipe, thus preserving capacity.

In summary, my point was that this low-flow condition probably was not happening and that the average water temperature was higher as a result of that. But I also asked if this under-performing zone was representative of what is happening in other parts of the system.

Does that make sense?

Thanks!
Brad

brucewo1b

Brad Makes sense to me but not being as versed as I should I had to ask, and thanks for the details.

brucewo1b

I have also seen the element truned 90 degrees in the housing therefore blocking the flow of air through the baaseboard.

Bruce

J.C.A._3

I came across....

A local condo development where just over 1/2 of the units have the element turned 90° to proper. Thanks for the reminder! JCA

Brad White_19

Thanks Bruce

You could find better answers elsewhere but it is fun to share. I always learn from everyone else. This stuff is fun, eh??

Brad

Wayco Wayne_2

I once had

a cast iron baseboard job that was underperforming. I was out there measuring and figuring and barking up every wrong tree for 1.5 hrs. I finally dawned on me that the customers had installed a shag carpet that was blocking the air flow from entering the bottom of the baseboard. I found 2 little blocks of wood and elevated the baseboard an inch. Boy did the heat come rushing out. I felt a little sheepish about not finding it sooner, but was also relieved that it was working. WW

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craig_4

tried pressure

drop accross zone in question?

hr

Maybe nothing has

changed? Suppose the homeowner just has different comfort levels. If you measure the temperature going into the BB and the temperature leaving the baseboard, and know the air temperature in the room, then you should get some pretty accurate output numbers.

Siggy's HDS program has a Series Baseboard Simulator. You can input your actual data, board brand and model, measured temperatures, etc and it too will give you the BTU output data.

Really not a lot of smoke and mirrors here.

The bottom line is making your customer happy, and warm. Boost the supply temperature to the board, add some supplemental heat, whatever. Doesn't sound like you are that far away from their comfort temperature.

I know as I age I like warmer living spaces. I'm up to 72F at my living room stat. Some nights when I come home from working in the cold all day that's what it takes! 68 used to feel just fine years back.

hot rod

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Dave_4

just a quick question. i know pump sizing is very important but are you saying that to much flow is less efficient then a 20F delta t

like is 7deg temp drop less efficient then a nice 20F temp drop between supply and return?

thank you

Jason

Brad White_21

Delta-T's and Other Thoughts

Hi Jason,

(More than you asked in hindsight, I just felt like writing tonight, hope you do not mind.)

There is nothing magic about a 20 degree delta-T aside from the fact that it makes the math relatively simple (1 gpm = 10,000 BTU's per hour).

Raising the delta-T decreases the cost of moving the water because you move less to do the same work. Of course your emitters have to be selected for whatever temperature they will receive and send back.

Some jobs such as radiant floors require very low flows, hence higher delta-T's particularly on the primary side. A trickle of hot water flow makes a lot of low-temperature water.

Most commercial projects I design use a lot of terminal hot water coils which have a 40-degree delta-T.

What is good to note is that when you run a higher delta-T (40 instead of 20 degrees for example) you only need to move half the water. On a large job this adds up to some significant horsepower. For a homeowner it still saves cost, both in Watts consumed and in first cost for pipe sizes. A house requiring 80,000 BTUH or 8 GPM can be heated with only 4 GPM. Your pipe size may drop or be more comfortably at 1" instead of 1.25 inch pipe size. Runouts can be 1/2" instead of 3/4". It all adds up.

Some other thoughts:

1. When using high delta-T's/Low Flow Rates, water balancing/management is critical. Use multi-turn metering valves, not quarter-turn type ball valves. That little overage adds up. Meter water wisely.

2. The only benefit to lower delta-T's is that it can increase the average water temperature in the emitter (radiation in this case) and therefore the rated output. If I supply 180 and return 160 (delta-T =20) the average water temperature is of course 170. If I halve that to ten degrees (180 to 170) I pick up five degrees and have a 175 degree average water temperature.

3. It is entirely permissible to mix delta-T's on one system so long as they are controlled separately. For example on a recent performing arts center, I ran 20 degree delta-T for radiation, 30 for convectors and unit heaters and 40 for terminal box coils and radiant floors. My heat exchanger was sized for the proportional mix of all flows, averaging 35 degrees over the system as a whole. Point is, it can be done.

Again, nothing magic about a 20 degree delta-T.