Alta Combi into an Indirect Tank?

Jeepdoc

Hello. We're currently having a new boiler installed as the previous, 30yr old boiler cracked.  We are also making the switch to propane.  We went with our installers recommendation, an Alta Combi and an indirect water tank.
Now that the install is partially completed, plumbing the combi into the indirect water tank seems odd.  The indirect tank will be receiving hot water from a 5th "heat zone" into the coil as well as hot water from the DHW outlet of the boiler and into the cold water supply port of the indirect tank.  This seems like it may be an inefficient way to set the system up but I don't know enough about these systems to know.
Any guidance would be greatly appreciated.  Thank you.

hot_rod

A combi and an indirect? What’s the thinking with this setup?

Jeepdoc

The setup has us scratching our heads.  From what I've been reading the indirect would typically be used with a boiler and not with a combi.

We're worried we won't be getting efficient use of the new system.  I'll be asking the installers about it today and reaching out to the sales guy for some answers.  I was hoping to get some thoughts from this forum to make sure my question isn't way off base.

Hot_water_fan

A combi is a boiler, but the point of a combi is space savings with the downside of low DHW capacity. The indirect gives you the high DHW capacity, but now you’re using something probably very oversized and with more parts for nothing. 

Jeepdoc

Here a pic of the install so far.  Inlet to the indirect tank's heat exchanger from the heating side of the combi and inlet to the supply side from the DHW part of the combi.

Hot_water_fan

Did they explain why they didn’t just install the heating only Alta?

Jeepdoc

They haven't explained it yet.  The company that designed the system and is doing the install is a pretty big name in Southern NH.  We went with their recommendations.

The only reason I noticed this possible issue is that the cold water in one of the bathrooms wasn't working last night after they left so I went down to the utility room to see if I could turn it back in.  As I was looking at the install it didn't seems to make sense, hence the question I posted.  

When they get here this morning I will ask them to walk me through the system and voice my concerns.

My nonexpert feeling is that we have more gear than we need and they are making it work (what hot_water_fan said).  Which was is not what I was expecting.

Hot_water_fan

It’s something so out there, the installation manual probably doesn’t even say not to do it! Because why would anyone even think of it. 

SuperTech

I like the use of the indirect tank.  In general I usually prefer see one installed instead of a combi. But I've never seen the two together, very strange.  The Alta is a fine boiler. They should have installed the heating only version with the indirect as a priority zone with its own dedicated circulator. 

Jeepdoc

Yeah, if it's correct or not it's certainly unique! This configuration is not mentioned in the Alta install manual and a fair amount of Google searching didn't turn up any similar installs.

hot_rod

I think it will wok alright, but not sure of the reasoning?
so the indirect is a “hot” preheat tank to the combi?

Jeepdoc

Not quite.  The way it's connected the combi is putting hot water into the storage tank.  So the combi is preheating water going into the tank and then the tank will be maintaining the temp.  

The way you wrote it is an interesting idea, it seems like that would provide nearly limitless hot water at a pretty high flow rate.

Hot_water_fan

Not quite. The way it's connected the combi is putting hot water into the storage tank. So the combi is preheating water going into the tank and then the tank will be maintaining the temp.

The way you wrote it is an interesting idea, it seems like that would provide nearly limitless hot water at a pretty high flow rate.

An indirect attached to a boiler does the same thing without this extra labor and parts.

Jeepdoc

That's what I suspected.  I should have paid a little more attention to what was being proposed.

Other than added initial cost of equipment and install do you think this will cause a loss of efficiency long term?  It seems like preheating the water to be stored and then maintaining that temp of that heated water might use more fuel (albeit a small amount).

Thank you for the feedback.  It is greatly appreciated.

Hot_water_fan

I think efficiency losses for DHW will be lost in the noise. It does mean you'll have a larger boiler than you need, so short cycling will be worse.

yellowdog

you are basically paying to heat your hot water twice with this setup.

Jeepdoc

The installers are here now.  I asked them if they install a lot of systems like this... Combi and Indirect tank.  They said the do not and that this was unique.  I've got to call the sales guy and get an understanding for the system.

SuperTech

Jeepdoc said:

The installers are here now.  I asked them if they install a lot of systems like this... Combi and Indirect tank.  They said the do not and that this was unique.  I've got to call the sales guy and get an understanding for the system.

At least they were honest with you.  It sounds like the sale guy made a mistake and the installers cobbed together what was given to them. Maybe they will do the right thing and put a properly sized heating only Alta in. Although if @hot_rod thinks it will be OK then perhaps it will.  He's more of an expert than I am. 

Jeepdoc

The supervisor came by for something so I asked him about the system.  He says they install them like this so the DHW out of the combi boiler is a backup in case the indirect tank fails.  We won't be with our hot water.  That sounds a bit far fetched to me.  Plus, it's not plumed that way.  The hot water out of the DHW port on the boiler is going into the Superstor tank.  More investigation needed...

Hot_water_fan

Is it so hard to follow the manufacturer's instructions?

SuperTech

Hot_water_fan said:

Is it so hard to follow the manufacturer's instructions?

I'd like to think that the engineers who design boiler and make the installation manual know better some supervisor about how the boiler should be installed. 

Jeepdoc

So I spoke with the sales rep that sold the system.  He said they install them like this so there is back up hot water in the event the indirect tank fails.  The DHW loop on the combi would be normally off until the needed when the bypass valves would be switched and hot water would come from the combi DHW port.

The installers were a bit confused so I'll need to confirm with them that it is plumed properly to achieve the desired result.

As far as the back up hot water goes, if I had caught it earlier I think I would have opted for a standard boiler and indirect tank.  I don't know how often these Superstor units fail but I'm not sure I really need a contingency for that.  

Thanks for all the feedback.  I certainly learned a lot during this process.

GGross

I'm not shy, I'll say it. that is crazy talk. You don't sell someone a combi boiler as a backup for an indirect that is fired by the same boiler. The most likely component to break is the boiler, and if that goes you don't have hot water with either method. Indirect water heaters are so rock solid most of them come with a lifetime warranty, and generally the boiler (not sure on this specific one) has programming to more efficiently handle an indirect, that you would have to lose due to the programming difference being a combi

Jeepdoc

GGross, I completely agree and I'm definitely disappointed that I didn't catch this sooner.  To complete the picture the boiler is an Alta Altac-136b tied to a 60 gallon Superstor.

hot_rod

So a pump runs to circulate from the combi hx to the indirect?
Or hot water only flows into the indirect when a faucet runs?

Every time you flow 
through a HX you take an efficiency hit.

GGross

Yeah I am pretty curious to see the whole install at this point

Alan (California Radiant) Forbes

I've never seen this kind of installation. Novel. I think someone made a boo boo.

I would try to disable DHW on the Alta and document how you did it in case you want it back.

hot_rod

You take a pressure hit across those combi plate exchanger also. They are purposely sized for low flow for the boiler to keep up. And cost.

The higher the flow the more pressure drop. Plus occasional delime maintenance 

Alan (California Radiant) Forbes

So, they piped the combi and indirect in series? If they wanted to safeguard against DHW interruption, I would suggest a parallel installation with appropriate valving.

Jeepdoc

So the install was finished last night.  We have heat and hot water.  With the additional piping that was added the DHW ports of the combi boiler are in parallel with the indirect hot water tank.  Here is the full system and a close up of the DHW piping.