Reverse Indirect (TurboMax or ErgoMax?) with condensing boiler

Mith

I'm helping a friend who's rebuilding a large house in the Boston suburbs after a fire.

It looks like the heat loss on the house rebuilt with modern insulation and windows will be in the low 200s. There's fairly high DHW demand -- my friend has 6 kids and 2 or 3 are often home, there will be fancy high output showers, multiple laundry machines, etc. The GC's preferred plumber is suggesting a pair of Buderus or Viessmann boilers for a total of 500KBTU of output, and a 119 gallon reverse indirect (ErgoMax I believe) for DHW.

I'm...skeptical. I manage large residental properties and we've done a lot of boiler and DHW upgrades. We had a proposal several years ago involving TurboMaxes and buffer tanks and the Mech E on the job finally intervened with a pretty convincing calculation showing that running our heating loops up to 180F to store energy in the buffers/TMs (which after all are just buffer tanks with "tankless" DHW coils in them, right?) would basically ensure we almost never condensed in cold weather -- eliminating the point of our fancy new condensing boilers (these were large commercial units -- Bryans -- not cheap). I have another property we "inherited" with condensing boilers and buffer tanks feeding DHW exchangers and it's a constant problem -- the buffer tanks can't store enough energy to avoid having to haul the HHW loops up to "storage temperature" all the time to charge them, and we get nasty overheating we can't lick and a lot of wasted fuel.

I suggested to my friend that he might want to look into a more conventional indirect setup, maybe a double-coil rig or just two single-coil tanks, if he really thought he needed that much DHW and he was going to buy a huge fancy condensing boiler for his house. Or just get a commercial condensing DHW heater (my "preferred plumber" is partial to the HTP Phoenix) and a more reasonably sized boiler for heating. But his plumber thinks the reverse indirect he wants to sell him is the best thing since sliced bread and swears even though the manual says you have to send it 180F water, the boilers will still condense.

What's the real deal here?

Zman

You are correct about reverse indirect and condensing. Unless the reverse indirect has absolutely enormous coils (they don't exist), it will not run at condensing temps.

As a rule, don't listen to anyone who is not willing to show you the math.
For this project, you would need a heat loss calc and hot water loads. Then you have something to talk about.

Mith

> @Zman said:
>
> As a rule, don't listen to anyone who is not willing to show you the math.
> For this project, you would need a heat loss calc and hot water loads. Then you have something to talk about.

There's always the question of "whose math?", no? Though it's really " whose assumptions" most of the time.

In this case the load calcs aren't too far off. I don't have them in front of me, but, the plumber's heat loss comes out to about 190 and his DHW calc, done very conservatively per the GC's instructions, is around 450 1st-hour gallons. Being as conservative as I can and doing both calcs myself, I get 205 for the heat loss and 390 for the DHW. Though the assumptions that generate this are a little silly - these are adult kids who are never all visiting at once, and though there's a huge soaker tub and 3 spray showers they're never really going to be used at the same time. Under normal conditions demand will be a tiny, tiny fraction of this. The DHW load is actually now driving the entire boiler sizing, which I find annoying but I think with a high turndown unit it'll be OK.

On the "but will it condense?", that calc may be beyond the capabilities of the plumber, particularly since ErgoMax doesn't publish the kind of detailed delta-T graphs for the HHW side that the major indirect brands do. I think I know how to do it if it's even worth bothering to do, but, I wanted to get a sense of whether others have seen this kind of design work in this kind of application, since intuitively it seems quite wrong.

Thanks so much for your response! I'm not a pro and I have no experience really with systems this small so I don't want to be...all wet.

Rich_49

You should let your preferred plumber have an HTP system designed using a commercial dual HX Super Stor and 2 cascaded UFTs designed and call it a day .

How large is this house that it could possibly be 200,000 BTUh ? Especially with modern windows and insulation . Have a room by room heat loss done because something sounds terribly wrong wit this picture . I can safely say that 500,000 BTUs of equipment is an astounding waste of money that could probably be better spent .

Nothing wrong with Phoenix water heaters with the exception that if you need boiler/s also why not use HTP matched stuff for best results / first cost . Buderus' aluminum HX can and probably will be a problem at some point , Veissmann is a better option than Buderus . HTP is a great local company though with an outstanding reputation

Mark Eatherton

My personal experience with Turbomax is that it doesn't require 180 degree F water all the time. Sure, the manufacturer rates it at 180 EWT, but in reality, you only need to maintain an approach temperature 10 degrees hotter than the desired DHW temperature.

So if you want 130, then maintain tank and boiler supply at 140 under load. This will result in a return water temperature to the boiler that is below the critical 140 degree threshold for condensing. Your assumption of it just being a heat exchanger is correct, but it is one BIG heat exchanger than can convert 99% of what the boiler throws at it into DHW. Unless it's a family of lobsters, they will never complain that the water is not hot enough.

I have MANY of these tanks in the field with modcon boilers, and have watches as the boiler has a return water temperature below 140, even during peak DHW loading, while maintain supply water temperatures between 120 and 130 degrees F.

Just because the manufacturers performance charts shows 180 degrees F doesn't mean it has to be operated that way, but most plumbers don't want to get "the call" and its really not their utility bill, so they keep the boiler cranked to 180 degrees so they don't get "the call". The manufacturer uses 180 to make his product look good. Too bad there isn't a standard by which all indirect and reverse indirect DHW tanks are required to conform, so that you can get an apples to apples comparison. As noted, it WILL convert nearly every Btu thrown at it into DHW, within reason.

Don't forget to use an ASSE rated anti-scald tempering valve.

ME

Zman

Turbo max should be able to provide specs for different temps. I would request it from them.

It may be that in your case you could run it at low condensing temps most of the time and increase it when you have a full house.

What type of emitters do you have?

Mith

Double checked and the proposed unit's an ErgoMax. They're basically the same, right? Gotta be some interesting story there!

I'm familiar with the idea that if you've got enough heat exchange surface, whatever input temperature will do (we use some pretty darned large plate exchangers in our buildings as part of a standard package when we do condensing boiler retrofits). But from the chart on the ErgoMax site, at 300,000BTUh input, the unit makes 391 1st-hour gallons -- at 180F HHW input temperature. Just about the lower of the two (I believe inflated) calcs for DHW load.

However. I asked my friend if he had any pictures or a list of the equipment that was taken out of his old, early-1900s house (same footprint, same square footage, old loose construction and some kind of retrofit insulation of dubious quality) after the fire. From his pictures, it was a pair of 75 gallon, 75,000BTUh gas fired DHW heaters and a very old 350,000BTUh *input*, 65% efficient boiler for HHW.

The only added DHW load is a larger soaker tub, albeit one with a 140(!) gallon capacity replacing one that from the insurance pictures I'm guessing was sub-100 gallons.

I've got to conclude the calculated DHW load is just wrong, and almost certainly the heat loss too.

Zman

How big is the house?
What are you thinking for heaters?
The fill faucet on those big tubs can be killers. 5-8 GPM is pretty common.
Everyone has a different way to approach a job. To me this is looking like twin mod cons ~110k-150k each depending on what an accurate heat loss looks like. I would do a 120 gallon indirect.

I think your assessment of the project is dead on.

Mith

The house is definitely over 10,000 sf -- I have to check the dimensions used in the heat calcs, that's one thing I'm suspicious of -- and is (was?) an old farmhouse that had decades of additions, you could say "not a rectangle at all any more", lots of extra wall area. It's an old close friend of mine and it was a beautiful house he's trying to put back together, so I've definitely got a sentimental attachment. I think I'm spending more time thinking about this than the last time we did a 100 unit apartment building (on the other hand, then we had a mech-e and a plumber I knew a little better... go figure).

The wife's very particular about the look of the rads. They had some kind of old recessed rads before, so recessed rads it'll be now -- looks like "Gov-Ray", cast iron. It looks to me like -- wrong heat load calc or not -- the plumber's appropriately derated them for low-temp use and he talked them into adding baseboard and radiant floor in a few rooms that would clearly otherwise have been problems. If the whole design's based on too much heat loss I don't think adequate radiation's going to be the problem.

I've got to think if a 119 gallon Superstor or equivalent doesn't do the trick, leaving some space for an extra storage tank ought to be good enough. But, of course, not my house. I'll have to point my friend at this thread and I guess how hard to push to get it done right is up to him (and whatever he thrashes out with the insurer; they might actually impose some sanity here too).

Thanks for all the help!

Rich_49

Don't forget to de rate 10% for those recessed rads .

Zman

Once you get a room by room heat loss you are comfortable with, Size the radiators based on the heat loss at the lower
supply temps. Run the system on an outdoor reset.
If you do Twin 150k mod/cons with 120 indirect and DHW Priority. The twin boiler will make 7.5 gpm on the fly and the tank should have ~90 gallon of storage to get you past big surges.

On the heating side the twin boilers should be able to handle anything from a design day of 270k to a minimum of 13.5k if you select a 10-1 turndown model.

Mith

You're talking 10:1 turndown on a single unit? What does that? Or 10:1 because it's a pair of 5:1 units with the controls set up sequential?

Zman

10:1's are available. Not sure who makes one that size.

Zman

This would be a good one.
http://www.lochinvar.com/_linefiles/WHN-05.pdf

Rich_49

10:1s , cascaded 20:1 http://www.htproducts.com/literature/UFTWallFloor-Brochure.pdf

Bobat2

I can't believe the amount of fossil fuel burning you are considering here. With all that roof area on a 10K sf house, why not install as much solar as possible, install a sensible AWHP (air to water heat pump) a decent sized reverse indirect water heater tank/buffer tank, a heat pump water heater downstream, and then sit back and enjoy the return on investment, and the cleaner air in the neighborhood. For details refer to John Siegenthaler, P.E., and go for it.
WE can do this, for our children and grand children, nieces and nephews.
Bob in VT

Bobat2

Slide deck on request - email me directly.
Bob