Mod-cons in summer conditions

Paul Pollets

Most mod/cons will ramp up to the highest output temperature to recover the indirect tank. Why hold the boiler at a lower temperature? The tank recovery will take much longer. If the boiler is being used for DHW production only, a mod/con may not be the best application.

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S R Denny

Mod-cons in summer conditions

A customer asked how these systems perform on an efficiency basis in the summer on 90 deg. F days. I seems to him that using a mod-con boiler is overkill for domestic water only. Any thoughts?

tim smith

Regarding summer dhw production w/ modcon

I am not sure what the question relates to. A mod/con for just dhw use does not care whether it is summer or winter. You won't be using the outdoor reset function if just dhw. You will set it up just to use dhw function. You should size it in such a way that you can heat your dhw with lower temp water, i.e. 140 in 120 out on boiler side to maintain condensing temps. The boiler will modulate based on demand and temp difference between supply and return. I am sizing a large commercial job this way right now so it does sound feasible to me. Tim

Guy Woollard

Indirect efficiency

With a good quality indirect, it is possible to lower the boiler supply temp to the tank and continue to have good performance. In doing so, the return temp is brought down into a range allowing the boiler to condense in DHW mode.
Without thinking this through, it just occurred to me that perhaps a future generation of boiler control can allow a 2 temperature delivery based on outdoor conditions; one for summer when incoming water temps are higher, and one for winter .

John Ketterman

> A customer asked how these systems perform on an

> efficiency basis in the summer on 90 deg. F days.

> I seems to him that using a mod-con boiler is

> overkill for domestic water only. Any thoughts?



Overkill compared to what? It is not less efficient than a standalone water heater. But certainly it is less efficient than not having hot water at all, if your customer's feeling is that when it is 90F then cold water is good enough. In which case he can turn the mod-con off and everything will be fine until fall.

The outdoor temperature doesn't have any effect on mod-con efficiency.

Derheatmeister

Paul not so, On most of my systems i Adjust the DHW production to condense..I also give them a DHW disconnect Switch for the Unoccupied times that they are gone ..

Check the TT Prestige DHW production kit.(PSKIT 22). once you hook it up to the Boiler, they give you all kinds of Parameters to let the boiler work In a condensing mode! IT'is VERY efficient ! HM.

ALH_4

DHW Efficiency

Yep, by holding the boiler target temperature to a set differential above the tank temperature, you can maximize efficiency. That efficiency does come at the cost of some time to reheat the tank.

Boilerpro_5

My Experience

I have an older Dunkirk Quantum Leap, which continues to condense at 160 to 180 F return temps, connected to my Triangle Tube Smart 40 tank. This set up replaced my standard chimney vent 40gallon water heater (about 28,000 input). I use gas for space heating, cooking, clothes drying and domestic water heating. When I installed the indirect, my summer gas usage dropped about 30%. If you consider the other usage during the summer, the savings for using a condensing boiler for summer water heating is probably closer to 40% or more. The one difference between this set up and most mod cons is that most mod cons do not condense at these higher return temps.

Boilerpro

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hot_rod

dual coil indirects

are another way to leverage lower boiler supply temperatures for DHW (double the surface area, and pull the return to the boiler as low as possible.g.

Or if the DHW loads are such a plate hx can give near instantanous DHW production. Baxi and a few other use that method.

The small Triangle Tube Mini Smart works great also.

With legionella issues around more and more I see the Euros getting away from tank storage for DHW. even on the SDHW systems they use small or instantanous DHW production.

hr

Brad White_185

Some observations

Good thread!

I agree with the notion that most condensing ceases during DHW production, but you still get some when ramping up. As the temperature approach narrows, you lose condensing, but so what? The fact is, you are taking roughly 80-100 MBH input and directing, usually, 100% of the net output toward making DHW. That means fast recovery. (Andrew's point about using a fixed differential to DHW setpoint at the expense of faster recovery is worth considering here, even as Der Heatmeister makes the point that it does save on the margins.


But the question not asked is, does it make sense to mothball the ModCon for the summer (send it to the Hamptons?) and use a less-efficient, stand-alone tank type heater with slower recovery just because it is summer? Pay for it's installation and whatever maintenance one should do?

When you have a ModCon and an indirect, I cannot justify another heater unless you have a higher summer demand such as guests and an outdoor shower.

My $0.02

Brad

[Deleted User]

Use a reverse indirect..

and ALWAYS be in the condensing mode. My RI runs at 10 degrees hotter than my needed discharge temperature. So, if I need 140 (anti leginoarres disease temp) I only need 150 maximum boiler temps, which means I am returning back to the berler at 130 degrees F, WELL within the condensing range.

And as soon as the berler is done heating the DHW tank, and goes into post purge, the residual heat in the berler drops out FAST. Little to no waste.

Condensing technology and reverse indirects are an unbeatable combination.

ME

Tim Gardner

How do you compare

reverse indirects to regular indirects? Are you suggesting that reverse indirects are always better? Is there a limiting situation where you would prefer a conventional indirect to a reverse indirect?

When you say you only need 10 degrees above discharge, how many GPM is that supplying for what size tank?

Buzz G.

reverse indirect

Finally learned what IDHW is and now you have me stumped with a new term-reverse indirect. Please explain to us non-pros what and how that works. Thanks.

Robert O'Brien

Reverse Indirect

The tank contains boiler water and the heat exchanger contains the domestic,reverse of the usual

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[Deleted User]

Correct...

As Robert has pointed out, it is the exact opposite of the typical American side arm. It is essentially a HUGE, highly efficient (99%) heat exchanger with some storage (within the copper tubes), hence only requiring an entering temperature slightly above the required discharge temperature.

We use Turbomax on all of our commercial jobs.

Information at http://www.thermo2000.com/

They literally KICK BUTT.

ME

Darin Cook_5

Mark

I have never even heard of a reverse indirect till now. That is what I love about the Wall, almost always, information is gained! Is the Turbomax in the same price range as a quality IDWH? I LOVE the idea of being able to squeeze the fuel for even more energy savings! It sounds like a match made in heaven for a mod-con.

I look forward to meeting you in person at Wetstock! I hope you won't mind a few million questions from me!





Darin

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[Deleted User]

It is higher...

but worth the price of admission. If you look at their production numbers, under continuous draw conditions, they far outperform their closest competition.

And I too look forward to meeting you and thanking you personally for your time in Iraq.

My brain is here for the pickin'

ME

Mark Eatherton

Sorry Tim...

I'z asleep at the wheel...

A reverse indirect is only as good as the person sizing it. He must take into consideration ALL the parameters he needs to cover. In some cases, the DHW load exceeds the heating load, and the heat SOURCE must also be sized to the worst case scenario, which probably isn't the space heating load (think ski resort scenario). Be sure and address the large dump loads in your sizing, and any potential parallel uses and back to back uses, and back to back oarallel uses... It gets complicated, and it is a moving target. Hence the need for efficiency from teh gas meter all the way to the vent termination.

They are essentially a very efficient heat source and heat exchange. THe days of large tanks and small coils are limited. It's how the Euro's are handling large DHW loads.

The only thing more efficient than this setup would be a direct modcon hooked to a tank, like Lochinvar and HTP has. THe GPM and delta T is a function of sizing, and with Lochinvar coming out with their mil and a half beast coming out soon, there are very few limitations that it can't be used. I think you can parallel bus up to 11 of them together for the ultimate in control. Let's see, that would be a 55 to 1 turn down ratio...16,500,000 down to 300,000 btuh. Now THAT would be a fun job to do.

It's pure (relatively so) math. Gallons times pounds times delta T.

About the only potential draw back I am aware of is lime scale accumulation and that can be treated with magnetic water conditioners in most cases.

ME

ScottMP

Mark

I'd never heard the term reverse indirect either, until this thread.

Apparently I've been using them . Mark I've a couple these tanks out there and I totally agree they kick but.

I think a Mod/Con with an indirect (reverse better) is a fine setup for our customers. Even if not in condensing mode its still a hell of allot better than the efficiency of a 40 gallon standard tank. And a longer life span in most cases.

Scott


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Andrew Hagen_2

Reverse Indirects

I could be missing something completely here, but given two identical indirects, wouldn't they both have the same capacity whether run as a reverse indirect or as a conventional indirect?

The main benefit I see is that the tank side has very little head loss so the dhw pump could be sized much smaller. In some cases, that is not a trivial amount of electricity.

If there is glycol in the system, I would rather buy glycol to fill the coil side than to fill the tank side. Plus if there is glycol in the system, the heat capacity of the stored water is less than with pure water. I tend to think a reverse indirect would be the worse choice with a glycol system.

ScottMP

Andrew

I think the idea here is more surface area for exchange. These tank are able to handle alot of btus. Its possible to use a smaller tank.

Scott

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Andrew Hagen_2

Agree

I agree with that completely. The more heat exchanger surface area the better.

Tim Gardner

is there are limiting case

where you would use an indirect instead of a reverse indirect? For example, if you need a high GPM for a short time period, aren't you better off with the large reserve of a conventional indirect? Or are you saying that even in that case you are better off with a reverse indirect?

upsellovoto

10 to 1

Mark, the 1, 1.3, and 1.5 will all be 10 to 1 turn down.
Just a "FYI" -Greg

hot_rod

but, if that reverse

indirect has paper thin insulation, like some brands, I'd bet the standby loss at 150F tank temperature would negate any savings over a regular indirect and a 180F DHW operating temperature at DHW demand..

I think the small capacity flash tanks like the mini smart or the new Prestige with a small DHW tank on board make the most sense. Treat the mod con as an instantanous. If it has the HP to handle the load of course.

hr

Bob Forand

Devils advocate

So let me ask this, if the indirect tank is less than say 10 feet away, why would you want lower temps to heat the water. How is it more efficient to run a boiler longer at lower temps than to heat the water quicker at higher temps. I have asked this question and still have not found the answer. To take it a step further, lets say in the heating mode, you have 300 feet of 3/4" copper with fin tube. That is approx 10-11 gallons of water and your mod-con holds a gallon of water. How or why is it more efficient to heat the water to say 140, take a longer time to satisfy the space, than to heat the water to 180 and take less time to heat the space ? That has been my question of the day lately !

hot_rod

the theory is

to keep the return temperature back to the mod con boiler, below condensing temperatures. If the dew point of the fuel is, lets say 135f then the boiler will condense and allow higher efficiencies via the secondary heat exchanger that extracts additional heat from the flue gas.

Above the dew point the boiler does not condense and extract the additional heat from the condensate.

They claim the cooler the return the more condensate, and hense more heat extracted from that condensate.

I know for a fact condensing boilers "pee' a lot more condensate when you run them below 130F then at 190F in the high fire, high temperature mode.

If you have enough HX surface you can leverage that "close approch" temperature from your DHW production for example. This is why ME and others prefer the reverse indirect with lots of HX surface.

I discovered a tank that is stainless with a corrugated stainless coil assembly. Looks to have even better HX surface and water distribution through the coils.

REX from www.myaic.net.

hot rod