Tweaking Delta T

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Old 04-08-13, 12:01 AM
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Tweaking Delta T

Howdy All:

Last fall I replaced a monster 1978 vintage 299,999 btu boiler and 85 gal gas fired water heater. I installed an 85,000 btu Lochinvar Knight and Superstore 40 gal sidearm for DHW. Pulled a permit and had it inspected. Also had a licensed plumber there for the initial fire up, as my dealer said if I followed those 2 steps the guarantee would still be intact with a self install.

I redid all the near boiler plumbing, but I did reuse the zone valves and system pump (Secondary system pump is a Grundfos UPS 15-42 F). There are 5 zones: a 75' upstairs loop, a 75' downstairs loop, a 50' sunroom loop (all tube/fin baseboard), a fan/coil for the garage (all the above on a single system pump), and a plate exchanger on a hot tub with it's own circulator. Pumps/valves were on the return side, but I switched them to the supply side per Lochinvar diagrams. I used a Webstone near boiler plumbing set up, which was handy although expensive. I wired it up with a Taco zone controller.

I got it fired up on Labor Day weekend, just in time for the chilly fall weather. It's been working very well. Our gas bill has been one half-two thirds of last year, even with lower average temps. I've got the outdoor reset tracking well, set so that it just barely keeps the house at temp. My average burn cycle time since starting up is 43 minutes. I think it is sized just about right, thanks to some research done before purchase I avoided the common tendency of grossly over sizing-despite several local contractors recommending 110 or 150,000 btu units.

I've been trying to get my delta t's widened to take advantage of the lowest possible return temps in order to spend the most time in condensing mode. I have discovered that it is always recirculating heated water in the near boiler loop, thwarting my goal of low return temps. With one zone open, it raises the return water temp about 10 degrees, with 3 open about 4 degrees. Obviously, the primary pump is circulating more than the system pump. It's the pump supplied with the boiler-a Grundfos UPS 12-28 with 3 selectable speeds (a slip of paper in the box with the pump said to use speed 2 for my boiler size, but I've got it on 1).

I'm thinking of two possible solutions. 1-I wonder if a smaller primary loop pump could be safely used. I realize there is a minimum flow for the protection of the boiler, but I wonder if Lochinvar really overshoots this flow so they don't see returned boilers, not being overly concerned about the end users optimum efficiency. One source has stated that the pump is over sized. 2-I'm considering a secondary circulator such as the bumblebee, thinking it could be set at a large deta-t measuring output temp and return water of the primary loop. I would think it could vary it's gpm for the best compromise between lowest return from the radiation loops and enough flow to minimize the recirculation in the primary loop.

Am I on the right track? Any other suggestions for me?
 
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Old 04-08-13, 06:36 AM
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I would put the variable speed pump on the system not the boiler. As the zone valves open and close the pump will change the flow to compensate. You will always send some water back to the return when a single or even two zones are open and maybe even when all zones are open.
 
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Old 04-08-13, 08:23 AM
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You seem to have a good handle on things. I was a Hydronic moron till this winter when I realized my my newly installed Alpine 150 was not keeping the kitchen warm Since then I have read all I could and asked many questions on the forum and am somewhat up to speed.

Discovered big time short cycling because of it being over twice what I need. I had the installer come back and he is going to replace it with a Alpine 80.
I am also having trouble with lower than desired D/T's. I have 3 zones each fed wit its own Taco 007 pump. From what I understand its best to try and get your ZONE D/t's in line frst then address the boiler issue. Most if not all boiler makers way overpump the main on a P/S setup. It is a way of protecting the hx from overheating when being installed by intallers that are not up to speed on setting up these newer boilers. From what I understand you should shoot for 30-35* D/t on the boiler/primary loop. The flow should be in a chart with the manual for the boiler. If not you can always use the formula GPM=Output (MBH)X1000/500XD/T
For, lets say a 80k output at D/t of 35*, it is 80X (1000/17500)= 4.6GPM Now that is at max boiler output, anything less would be a lower flow requirement. I a sure this is far less than you are flowing now. The secondary D/T's are usually set at 20*. Slowing the flow will raise the D/T in the zones. Remember you cannot read the D/T's in the zones on the boiler. You need to get a IR gun or much better a Temp meter with clamp on sensors and it will read the D/T directly. You will need , of course, to read from mixed output to zone return water.

I have a bumblebee that I will install after the plumber replced he boiler. I am going to use it in a zone and see how well it works then perhaps replace the other two circulators. I would be hesitant to use it in the primary loop because you cannot program min flow rate, I believe, and I would be too worried about it lowering flow too low to get the D/T you programed in.


Please keep in mind I am a beginner here but have read much of the plethora of good informaton here and this is what I understand at this point. Others will hopefully add more info.


Tom
 
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Old 04-08-13, 12:19 PM
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Yeah, as stated my #2 plan is to put the bumblebee pump on the secondary loop, which is what I'm calling the pump serving the radiation loops. And the primary loop is the the boiler loop, which I think could use a smaller pump for less flow as in plan#1. Is my terminology incorrect?

I'm wondering, is it necessary to always recirculate water in the primary loop? Or can the flows be optimized to mostly eliminate this?

Tom, your formula is interesting. It's not possible to have more delta t in the boiler loop than in the radiation loops, is it?

I could not get my IR gun to ever indicate the same temp twice. Rudimentary, but I'm using a couple of digital thermometers-the kind with the lead to stick out a window for outdoor temp. I'm placing the sensor on the end of the lead on the pipe surrounded by pipe insulation. Probably not an accurate read of the actual water temp in the pipe, but should be ok for comparing delta t's.

I get around 12-15 deg delta t on the radiation loops, but this is reduced significantly by the recirculation in the primary loop. Right now I am measuring the return water from the loops just prior to the closely spaced T's and then right after the T's, thus indicating the temp rise from recirculation. With one radiation zone open, temp increases by around 10 deg! With 3 zones open about 4 deg. If I slow the secondary (system) pump, it recirculates more and gets worse-despite a widening of delta t in the radiation loops themselves.
 

Last edited by c9s; 04-08-13 at 12:53 PM.
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Old 04-08-13, 05:17 PM
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The best way to get somewhat consistant readings with an IR Gum is by putting black friction, plumbers, hockey tape, whatever its called over the pipe where you want to measure. It will give the gun a much better reflective surface to read. Yes you can get higher D/T's on the primary than the secondary. I know what your thinking "ain't gonna happen" but at full blower and full load with the correct flow in the primary it is supposed to hapen.

The reason the D/T on the primary get worse is because your primary pump is WAY overpumping the primary zone. Find out what the min flow is for 30* on your boiler and shoot for that flow. A pump that will allow you to dial in the GPM will work the best.

Tom
 
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Old 04-08-13, 05:18 PM
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As you have said, you feel your boiler is correctly sized.
So the load matches the source well.

So if you get your system loop to flow at 20 degrees delta t, with all zones calling under full load, then you can easily get a 30 - 35 degree delta t thru the boiler. As zones open and the load comes off your flow in the secondary loop will drop, giving you some nice lower RWT's. But now the flow thru the system loop reduces to keep the 20 degree DT right. Unfortunately at some point your boiler injection flow may be larger than your system flow. At this point you will have mixing at the return point of your injection tee's. It really is unavoidable with a zoned system. The only way to get around this ,to some degree, is with a mass tank (buffer, storage, cushion...). There are many names for it.

Ideally, get a globe valve in the boiler loop piping and set the flow for as high a delta Tee you can with in the man. suggestions. As long as you have enough flow for high fire (and there are times it will fire that high), set your system loop for 20 degree's and you will be as good as you can get it.
One thing to be aware of, your old system may have been sized for 10 degrees to keep the rads at the end of the loop small enough and still heat the space. Your 20 degree DT now may make theses room cooler. You may need to adjust the dampers on the BB rads (if you have em).
 
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Old 04-09-13, 08:32 AM
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Why do you want a higher delta-t in the boiler?
 
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Old 04-09-13, 11:52 AM
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I wondered that also? My goal is to lower the input water temp to maximize condensing efficiency. The only way I can imagine the delta t in the boiler being greater than that of the radiation loops is if the secondary system loop flows more than the primary boiler loop, thus mixing cool return water with the boiler output. I can't see how this would help efficiency at all. The return water can not be further cooled, so the only way to stretch delta t of the boiler would be to heat it up more, which seems wasteful and contrary to the goal of increased efficiency.

Am I thinking correctly? Or am I missing something.
 
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Old 04-09-13, 04:57 PM
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not sure where that question was directed RBeck..

I try to get a decent delta tee on my boiler piping with one zone running. This means LESS flow thru the boiler. When all zones call, the boiler will ramp up but the flow thru the boiler will not increase. This will lead to a higher delta T in the boiler, it will mix at the injection point and end up where we want it for ODR (hopefully). Thats my logic anyways.
Too much flow thru the boiler and we end up mixing the return water back to the boiler and increase the RWT to the boiler. Not ideal.
 
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Old 04-12-13, 12:13 PM
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So...there's no advantage to increasing delta t in the boiler. It's just done to reduce the amount of heated water recirculating back into the return?? I guess that makes sense.

How is it that you slow the flow through the boiler loop? I've been considering a smaller pump. I've also heard of people using a ball valve, but I'm not sure how appropriate that is.
 
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Old 04-14-13, 09:53 PM
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Smaller pump and a GLOBE valve.
Ball valves are for isolation not flow control.
 
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Old 04-24-13, 04:16 AM
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Too many zones. The problem is, you have a design day flow through the boiler. Say that's 8 GPM. The design day flow is for design day - when ALL zones are open. Any time you are running when one or more zones are not open, you will be returning some water directly to the boiler instead of sending it to the system/secondary loops.

You can calculate exactly how much is returned by looking at each zone seperately - calculating the flow for each zone independently. If only one zone is running at 4 GPM and you are pushing 8 GPM through the boiler, you are sending 4 GPM directly back through the boiler, raising your return temp and lowering your delta-t.

What you are looking for to solve this problem is a variable speed pump for the boiler, that would be controlled by the boiler - as it modulates, it modulates the pump flow. That doesn't exist.

If you want a more consistent, and higher delta-t, then combine some of the zones.
 
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Old 04-28-13, 08:08 PM
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Well, there are just 3 main zones, 2 with 70' of tube/fin bb and one with more than that. So they are not micro-zones by any means. Even with 3 open zones (sometimes 4 as there is a plate exchanger for a hot tub) the primary zone over pumps.

I suspect that the pump sent with the boiler for the near boiler loop is very much oversized. I would like to figure out a smaller size pump for the primary (near boiler) loop, then combine it with a var speed system pump set to maintain the highest delta t across the primary loop. That way if it slows too much (which would give you a high delta t in the radiation loop) causing more recirculation the measured delta t would decrease and the pump would speed up sacrificing delta t in the radiation loops for the overall good. Make sense?
 
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Old 04-29-13, 05:25 AM
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I understand your problem. I have a Triangle Tube boiler and would like the flow at the boiler to match that moving through the zones. Triangle Tube requires 5 GPM minimum for my boiler.

They don't necessarily have to be micro-zones. Have you calculated the heat loss for each of your zones, as well as the flow rate at a certain delta-t?

For my house, I have three floors. I have a design day heat loss:
- 70,000 btu's for the first floor.
- 28,000 btu's for the second floor.
- 15,000 btu's for the third floor.

My design delta-t is 30-35 degrees.

My approximate flow rates (don't feel like looking up my specs) for each floor are as follows:

- 5 GPM for the first floor.
- 2 GPM for the second floor.
- 1 GPM for the thrid floor.

Any time the first floor zone is not calling for heat, most of the water is returned directly to the boiler, and I'm not going to see design delta-t in the smaller zones.

You could also eliminate the zones altogether and run with constant circulation.
 
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Old 04-30-13, 04:34 AM
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Ivanho,

What you really need is a buffer tank to stop short cycling on those small loads.
Both are well below the min. fire rating on your boiler I bet.

While it does add some capital cost, it should pay for itself eventually with fuel savings and less maintenence costs. Short cycling really kills parts.
 
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