Your opinion on piping layout please

Tom_35

We are putting together a proposal to tie 2 large AO Smith Legend 90% boilers into a single loop in order to get better performance from them.

This home is nearly 18,000 sq. feet and the engineer speced a 750,000 and a 500,000 BTU boiler. The boilers are on opposite ends of the home and not tied together in any way. They handle hydro-air fan coils, domestic hot water needs, and a snow-melt.

These boilers are significantly oversized and do not run enough to ever get to the condensing stage.

Here are our plans: Trench through the lawn area and tie the 2 boilers together with 2--- 2.5" single Ecoflex piping. We will add buffer tanks, and set up the controls with a Tekmar control system that will stage the 500K boiler as the first stage heat, and the 750K as the 2nd stage.

I have attached a piping layout for you to look at and provide appropriate advice.

Thanks,

Tom A

hr

Why

tie them together? Anytime you bury piping in the ground you will have some heat loss, even with insulated piping! So you will lose some efficiency there.

What are the actual load calcs for the two ends now? Could you simply buffer the two systems the way they currently stand without connecting them? Tanks may be cheaper than that large bore insulated underground tube

You could run some "what if" deigns with the Buffer Tank module in Siggys HDS. Then you could size the buffer tanks to the load and firing times you want.

I guess installing the correct size modulating boilers is out of the question?

Seems the larger one should be at the snowmelt end, depending of course on the load.

hot rod

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Tom_35

hr

hr,

The reason we are wanting to tie the boilers together is that neither will run more that 2-3 minutes before cycling. There are no buffer tanks now, and only room to add tanks in 1 of the mechanical rooms. We realize that there would be some loss with the pipe in the earth, but there is just no other way to get from 1 end of the house to the other.

The engineer sized the boilers based on the load of the home with all zones calling at one time, plus the domestic hot water, and then must have thrown in a huge safety factor to cover his butt.

The house has hydro-air systems and each system has zoned areas, which comes out to almost 30 thermostats. With the addition of buffer tanks, we feel that we could provide all the heat for the home with the 500K boiler.

I'm going to get the plans and run loads, but the fact is that we have to work with what we have...and these boilers are not modulating.

Having the boilers tied together would also give us some safety in the event that the lead boiler failed for some reason.

We are planning on proposing outdoor reset via the Tekmar controls as well.

I really appreciate your input.

Tom A

Mike T., Swampeast MO

Digital STORM Needs to be Tamed

A number of guest suites typically unoccupied and set back in temp? Possibly at the 500 mbh boiler end of the house?

Since you're dealing with many hydro-air units many of which are sub-zoned, you're still probably going to have problems with severe short-cycling, need a LOT of buffer and have to move a LOT of water a LONG way underground and keep it continually hot to boot!

How about divide and conquer to try to turn that storm into a wind?

Use a number of small, staged boilers at each location--with the first sized for loss in the area at 45 degrees outside or so with some kind of rational indoor conditions based on actual experience of the owners. If not too small, or too many boilers, keep adding boilers of the same size until you meet design loss. If too small or too many, make the added boilers the next size up.

Constantly circulate your primary loop serving the each general area--even if the AH coils aren't "home run" to a central location. Only shut it and the boilers down via a warm-weather shutdown control.

Install a reset control for each primary loop--not too much reset, but as much as possible given condensing concerns and problems with "cold air" from the AHs.

As the load on the first boiler increases, so will the delta-t. Use a controller that adds a boiler each time the delta-t increases beyond some threshhold.

As additional boilers come on-line, either increase the speed of a multiple-speed circulator or add additional circulation power via multiple (paralleled) circulators. [Not sure about parallel vs series--have heard good and bad about both.]

Since you probably won't have much reset, DHW may well be able to just "drop in" the primary loops as needed. Not sure--you may need a boost in temp but that won't be much of a control challenge.

Personally, I'd try to keep the snow melt on a dedicated boiler. Have no idea how much snow melt or if the smaller existing boiler could be reasonable for the purpose.

Mike T., Swampeast MO

You Might Want To Resize Your Drawing

Either that or remove the check box for "display with message". Just select the red "edit" button that appears only with your own message.

Big photos displayed with the message cause horizontal scrolling problems. When you leave them to be "clicked" to see, they automatically re-size to your screen.

Tom_35

Thanks Mike

I was not aware of "unchecking" on the attachment---it's done now and certainly does make it easier to view. I had thought about re-scanning it today at a smaller scale in order to see it without having to scroll.

I wish we could go with smaller multiple boilers, but the owner won't go for it. There are flue issues with that anyway. The boilers are in a basement and from the point they enter into a chimney space, they rise 46' to the top of the chimney. From what I've been told, most of the ventor motors are not able to push the flue gases that far, and horizontal venting is out of the question.

We can get 2--65 gallon buffer tanks, and possibly 1 additional tank, but it would be tight. The 2.5" pipe will also be about a 125' run, so that will give a total of 250' of volume as well, that could be considered as "addtional buffer"---although as Hot Rod said--not ideal.

We're just not dealing with an ideal situation.

Tom A

Mike T., Swampeast MO

Vertomat VSB-17. Your diagram. 3" loop.

Mike T., Swampeast MO

Is the problem poor comfort, high fuel use or both?

hr

You are making it tougher!

I agree on recalcing the home. Rough guesstimate-18,000 sq ft at 25 BTU/ sq foot = 450,000. May be out of reach of the 500,000 alone, on a design day.

Sure the piping could be considered buffer. the system doesn't know or care where the capacitence is

I wonder that you can derate the large AO? I know when you muzzle down a Munchkin with FACTORY approved vent size reduction the output drops off. A 140 vented with 35 feet of 2" drops the output to 121,000 and 109,000 at 85 feet of 2" (So I've heard

Good question for the AO Smith boys, maybe.

hot rod

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Tom_35

More info

The flues on these boilers are a 6" stainless single wall pipe.

I will check back with A O Smith to see if they have a way to de-rate the boiler. When we checked a couple of years ago, they said they didn't.

Mike, the reason we are looking at doing something is that the constant on-off of the boilers is not good for the boilers and they are just not getting the efficiency that they are capable of. Don't know what the wear and tear of the short cycling is doing, but I would think that it wouldn't be good. They have never had any comfort issues.

By the way, the snow-melt is less than 400 square feet. They use it on a patio outside their master bedroom where they have a large hot tub.

Tom A

Christian Egli

Unasked for advice.

Hmm, convert everything to steam, add vacuumizers for efficiency, and all the complications go away... The steam finds its own way to the coolest places in your system.

This may be a stupid question.

In the drawing, I see no pump on the secondary loop for either of the boilers. They are drawn for the other loops. Is this an oversight on the drawing? Is the pump built into the boiler itself?

Or, is there no way for the hot water to leave the boiler except by gravity? This could explain the short cycling.

If there is a pump, could it help your system if it ran a longer time than the burner? It would purge the hot water and make for a longer cycle on the next start.

I think tying the system together is good for all sorts of reasons. I suspect the trench is the shortest way to go. What if you keep your link all within the home. At least any loss will remain inside. Also, the longer path might give you more buffer water volume to do without the storage tank.

Just thoughts.

Good luck

Tom_35

Boilers have pumps built in

Each of the boilers have 1 hp B & G pumps built into them and they do run for a specified period of time after the boiler has cycled off.

These boilers are both located in a finished basement. The homeowners didn't allow any furrdowns in the ceilings.
If they were to allow us to run the piping inside, we would have to cross through an exercise room, sauna, wine tasting room, and wine cellar to get into the other mechanical room.

Not hard to understand why they would choose to go underground when you consider all that would be involved keeping the pipe inside.

Tom A

Christian Egli

No Holy Grail, too bad.

With exposed ducting being all the rage nowadays, it should have been a great opportunity to have the pipes go everywhere through the living quarters.

It's strange how home owners think sometimes. They'll be thrilled by hot air ducts, but will avert their eyes from hydronics.

Oh well, nothing to get steamed about.

Ecoflex makes-easy-to-use stuff.