delta T for indirect DWH loop

tbone52

I'm sizing the circulator for the loop from a mod con to a indirect DWH. What delta t should one use for a DWH loop?

Simply Rad

Delta T

I try to size my DHW with a larger delta T. I do this for two reasons, first is to less energy by using a smaller pumps.  Less flow = less pressure drop = less energy.  I commonly size for for a 30 + degree DT.  The other reason it to get that mod/con to spit water(condensation).  I design for a supply with 160 and return at 130 for a nice drip of  condensation. 



As for tanks I like the Euro tanks with much larger HX coils for easy pumping.  They also have big access ports for service.



Jeffrey

Brad White

As an additional idea

Here is a diagram I developed for a specific project using a condensing boiler with an indirect but I added a brazed plate heat exchanger in the mix.



If you follow the diagram, the way this works is, the entering cold water (50F) is diverted to the brazed plate exchanger (BPX) cold side to pre-heat the water to the tank.



The hot side of the BPX receives returning boiler water from the indirect, diverted from the boiler return. This wrings out another 10 to 15 degrees, depending on the selection of the BPX and increases condensing.



This arrangement would add several hundred dollars to the installation, but the HO wanted to go for it as an enhancement.



The diagram does not illustrate every device, just the concept; there are details to be added and I just wanted you to get the basic process flow. The thermometers are instrumentation you may or may not want to include, but are shown to illustrate representative temperatures.



I agree with Jeffrey regarding wide delta-T. This is another way to do it, but it does add a slight increase in pump head, that of the BPX and diverting valves/tees. But the benefit is measurable.

Gordan

Isn't that sort of like using an indirect with a larger coil surface?

In essence, in a well-stratified tank the bottom of the coil would get the "pre-cooled" boiler return water and act as a "pre-heater" for the incoming cold domestic water. When you add the cost of the brazed plate exchanger to that of an indirect, how does the price/performance look compared to, say, the cost of an indirect with a larger coil?



What I'm curious about, is what's the best strategy to use with a reverse indirect. It would seem that, if I'm not overpumping, I should likewise be able to get decent stratification on the boiler side (tank volume) - and these things are known for their huge coil surface. What throws me a curve ball when thinking about this, is how post-purge on the DHW side would impact this, especially with a boiler like the TT Solo which has a relatively large heat exchanger content compared with the volume of the reverse indirect.

Brad White

That pretty much

says what it is, but with a difference.

With the brazed plate exchanger, your coldest incoming water "goes after" the heat from what would be going back to the boiler anyway and outside of the tank, once it has done all it can do. Maximum delta-T.

Absent this, your coldest water could only dilute your tank temperature. This gives it a leg-up, by 10, 15, 20 degrees. (When the DHW and boiler are "cresting", just before the tank is satisfied, the effect, the delta-T, is as high as ever, so it keeps the condensing going that much more.)



I would also think that this arrangement would lessen stratification in the tank, or at least the gradient top to bottom. Even so, if the top of the tank is 140 and the bottom is 70, that is not going to force the 70F water to rise no matter what I do. But less potential to stratify, I would say.



This of course presumes that the HW is running during this time of course and in a house with a shower going, teenage girls being how they are, it can make a difference.



Now, sometimes you have an indirect already, so that is where this detail shines, you are not going to replace the indirect with a larger one, not likely anyway. But a larger HEX also requires, most of the time, higher flow rates to drive it. I was told not to over-size the HEX by a vendor, but did not challenge him on why. Another discussion. I think, as you do, that this would be OK most of the time. Short of over-drawing the boiler (if CI in particular), I cannot see a reason not to. Again, another discussion.

CMadatMe

Where are you taking the indirect from?

If you are taking it from the boiler/primary side I believe the boiler mfg wants the same size pump as the boiler pump.If from the system side it makes sense to try to get a wider delta-t to keep the boiler condensing. I'm struggling with this my self.



Getting ready to install a Vitodens 200 next weekend. I currently have 2 zones but I'm going to eliminate zone 2 and let the system rip on ODR with a Vitorol 300 providing indoor feedback.  Got myself a BubbleBee to try out. House is all baseboard and after doing the heat loss and measuring emiitters I can use 140 degrees as my needed water temp at zero degrees. I will be condensing every day for heating but to ruin my quest to condense every day is domestic. So I came up with a solution I'm going to try. Added storage.



I'm going to use a 50 but may change to a 79 gal indirect. A 40 gal would work but I'm going to play around with my domestic supply temps. I'm going to start making domestic with only 140 degree water and see where that gets me. I am using a Viessmann tank so the larger coil and the ability to draw 95% of the tank before recovery should take care of me. The nice thing about the Vitodens 200 is I can run domestic on time and for periods of time then shut it off. Can push a button remotely on the Vitorol if someone wants a shower off normal  times. We pretty much have a set schedule.

Mark Eatherton

Why doncha...

Drop a small reverse indirect (~30 gallons) and be done with it. At 140 degree F boiler temperature, it will give you boo coo gallons of hot water with a return temp below that of the minimum required for condensing, between 110 and 120 F.



I don't understand why these aren't the standard...



ME

CMadatMe

Are you refering

Mark I'm assuming you are talking about ErgoMax. Never seen them in action nor have I ever seen a rep for the come in the door.  Their web site is pretty blah.

Simply Rad

Reverse indirects

Mark is right.  Reverse indirects really make a lot of cents.  I have used the Thermo 2000 tanks.  The recovery is tremendous with a small amount of storage.

Jeffrey

tbone52

Use a delta t pump

My understanding is that for a fixed boiler supply temp an  indirect DWH's heating loop delta T will be dependant on the DWH demand and will shrink as the DWH recovers. My goal is to keep a 30deg delta T to support condensing. Assuming I'm correct, does using a delta T pump make sense for this? My DWH loop is a secondary with its own circulator, so the min boiler flow is covered by the boiler primary circulator.

Brad White

The way I would think about that

is this: When in DHW priority mode, the boiler will fire to limit; it has a job to do and quickly.



If your boiler limit is at least 170, 180, 167 for a Viessmann, chances are you are out of condensing range anyway. Your indirect will have a stated output based on a stated input, that much is clear. Lower that temperature and you lower your DHW output.



In a typical firing sequence, with boiler water at room temperature and maybe 100F in the tank to start (after a draw-down, keep it hotter than that!), will start as a reverse heat exchanger. Your first returning water to the boiler will be hotter than it left, until the boiler heats up.



Your second passes will gradually cross the temperature of the tank as you have it and only then will you begin to heat the DHW side. You will be condensing all through this process, so you have that efficiency anyway.



But pretty soon, the boiler temperature rises to above the tank setpoint, ostensibly to the boiler limit and holds it there until the tank is satisfied.  To widen your delta-T will prolong the process. I would not mind a ten degree delta-t at the end because the temperatures are approaching equilibrium and shorten the process.



To heat a tank from a cold start takes what, maybe 20 minutes. To top it off, half that or less.  I see the short time as the benefit, not stretching it out. But give it a try and see what you get! I am curious, but I just do not see a benefit, myself.