Heat Exchanger - Non oxegen barrier piping - Aluminum boiler

jcw

Hello,

I am upgrading the heating system in my house. The existing system consisted of a cast iron boiler, staple up radiant with non-oxygen barrier pex, and multiple (12 - one pump per zone) cast iron circulating pumps. Obviously after 12 years this system has failed. The pumps are seized, the glycol is black and full of metal chips, and the boiler is shot.

I am self designing my new system. And I would like some help with some technical decisions.

So far what i have is: A Weil Mclain Ultra 310 boiler with an aluminum heat exchanger. I would like to reuse the staple up radiant (non-oxygen barrier) throughout the house. I would like to add a indirect fired hot water heater (currently electric) to the system. I would like to simply the system, instead of 12 circulating pumps I would like to add manifolds and only have 4 or 5 heating zones that are controlled by either zone valves or brass pumps.

The biggest dilemma i am having is deciding weather or not to isolate the boiler via a heat exchanger. With the non oxygenated piping I am concerned about air entering the boiler. I understand that the boiler is aluminum and will be more corrosion resistant that cast iron, however I am still concerned about this. The heat exchanger may also allow me to use a more diluted glycol mixture running throughout the house. I plan on using brass pumps regardless of final design. I see the down side of the heat exchanger being a loss in efficiency and another added cost that could possibly be avoided.

Should I install a heat exchanger or not?

RobG

How large is your home? An ultra 310 is a huge boiler. Have you done a heatloss on the structure? Lets start with the basics.

Zman

RobG said:

How large is your home? An ultra 310 is a huge boiler. Have you done a heatloss on the structure? Lets start with the basics.

Absolutely!

Aluminum is not the best choice for non o2 systems.
If you want to go without an HE, a firetube stainless boiler would be better. You do need to find non ferrous pumps and other components. Careful attention needs to be taken to clean the existing tubing and a magnetic dirt separator is a must.

Let's talk about that heat loss.
Oversizing the boiler is the worst thing you can do...

jcw

The house is 2,850 SF With an additional 700 SF of garage and mechanical space that will be heated. Typical 2x4 construction with wood siding and 1/2" gyp board. The entire house sits over crawlspace with no basement. The house is insulated but not extremely well. The house is located in the Colorado mountains at 8,500' elevation. Winter temperatures can drop as low as -40 with an average Jan temp of 25Deg F I have calculated a heat loss of 40,214 BTU/HR. The heating load without heat loss was calculated at 188,000BTU/HR. Total heat load for January is 228,214Btu/hr.

The boiler is 310,000btu/hr however it is only 90% efficent and so I figure an actual rating for this boiler is 279,000btu/hr.

The boiler may be a bit too large however I got it for a good price and it is in perfect condition. If I use a heat exchanger There will be additional heat loss to account for. What are the drawbacks of an oversized boiler.

kcopp

IF your house has a heatloss of 35 btu / ft2 (and that is a very drafty place) you are still over sized well over 200%....and that is on the coldest day of the year.
Just because you got a "great deal" on it does not mean its a good fit.
If you wear size 9 shoes and you see a sale on a size 14 boot you don't wear them do you?

The boiler will short cycle and waste a ton of heat. The idea of a Mod con boiler is that is adjusts it burner to match the load.

What type of tubing (brand) is it? Do you have pix? You May be able to clean and treat the system water and not do the HX...maybe. You will need to be diligent.

Rich_49

That is 98 BTU sf . My friend , you would be hard pressed to have a heat load that large in a cardboard box in Fairbanks Alaska . We all know someone at elevation in Colorado that has a home with a 100k heat loss at design and has sucessfully used a 50K boiler for going on a decade now .
Good thing you saved all that money because you will need to buy plenty of stuff to make it work right if in fact that is your goal . You may want to start by purchasing a Buffer / Storage tank . Assuming all these zones are similar in BTU output and the 25* average temp you state . The low end on the boiler is about 69,000 BTUh , your smallest zone ( if all are equal) is about 3000 BTUh . 69000 - 3000 = 66000 x 10 minute run time = 660000 / 5000 = 132 gallons . Don't forget to derate the boiler for your elevation either .You'll still need the 120 gallon tank . You can easily get a 120 gallon buffer and that will keep you burning for a 10 minute cycle per hour but what will you do for the rest of the season . That tank can be had for right around what a properly sized mod con would cost you .

You are 7 xs oversized for your design heat loss . The boiler will short cycle by any definition for 90% of the year .

You certainly will need to isolate the boiler from the non barrier pex , I have seen many an Ultra done by guys who swore aluminum was OK with oxygen rotted right to death . That may be a blessing though in your case because HXs have a penalty so that 120 gallon tank may be just right .
The chances that all your zones call at once further aggravates the issue . Others here may be a bit gentler than I but I tell you to cut your losses now and get some help here to design a proper system . Sell the Ultra to someone who needs to heat a 14,500 square foot home and wants to heat water for 12 baths with body sprays in all . Because that's who needs the boiler you purchased .

We don't discuss price here but you'll probably spend low 5 figures to purchase what you need to use the bargain boiler . If it was free it was too much .

Sorry to be blunt . We will help you but what you want to do is beyond any good advice .

Ironman

jcw said:

I have calculated a heat loss of 40,214 BTU/HR. The heating load without heat loss was calculated at 188,000BTU/HR. Total heat load for January is 228,214Btu/hr.

This statement, at face value, doesn't make sense. I assume that you are saying that your calculation came to 40k buts for the structure and the 188k buts is for something else like the indirect?

40k buts for a 2850 sq. ft. house = 14 buts per sq. ft. Factor in the garage and your at 11 buts per sq. ft. Those numbers are way too low. With average insulation and a delta T of 110* (-40 to 70*) you're probably gonna be at least needing 35 buts per square foot. How did you arrive at 40k buts?

If you're adding the capacity of the indirect (the 188k buts), then that's not the correct way to calculate boiler size. The indirect should not normally be figured into the sizing of the boiler as it will have priority and will not heat simultaneously with space heating. You size the boiler for the space heating load.

You still would be no where near needing 279k buts; probably around 125 - 150k buts.

Over-sizing IS the worst mistake you can make in boiler selection. It will cause short cycling, greatly reduced life span and lowered efficiency.

Please share your calculation.

jcw

I calculated 60btu's per sf I added some additional SF to include the garage and mech room. That is where the 188k was derived. The 40k btu was additional heat loss that i factored from the house being poorly insulated, old aluminum windows, etc. My calculations were probably too liberal for the application and I appreciate this correction.

Thank you all for your input. I am open to cutting my losses with the ultra 310 and downsize my boiler.

Since it looks like I will have to fork out the cash for a new boiler, does anyone have recommendations? Would it be best to purchase a stainless steel boiler and plumb it directly into the non-oxygen barrier pex with non-ferrous pumps etc. Or, purchase a cast iron or aluminum boiler and still utilize a heat exchanger to isolate the non oxygen barrier pex?

Or, is the 120gallon storage/buffer tank option as mentioned above a viable system if used with my ultra 310?

Rich_49

I suggest finding an online heat loss calculator like the one SlantFin has . Enter the information as accurately as possible in that . I would not expect your home to be north of 160k for design heat loss , that is taking into consideration that you say it is a poor envelope .
is there any low hanging fruit as it were such as air sealing or insulation that could easily be done ? Many states have programs in place administered by the state or the local utilities to assist in identifying and addressing these type things , may also get you some money for your investment in upgrading the system . here is a link to Colorado database of programs , grants , incentives and rebates . maybe there is something there you could use .

http://programs.dsireusa.org/system/program?state=CO

In any case , use that heat loss program at Slant Fin and let us know how you make out . Think about how much infiltration of cold air you have also , maybe a blower door test is covered in one of the programs in Co .

Here is a unit that may very well fit the bill for your system with the non barrier tubing . Depending on your hot water needs of course .

http://www.htproducts.com/versaflame.html

hot_rod

Unless you can find a new home for that Ultra, I'd be inclined to add an 80- 120 gallon buffer tank to it. You are just a tank away from a versa flame

Many of the mod con boilers allow you to derate their output in the control. It really just a matter of slowing down the blower with negative pressure gas valves.

See what Weil will allow before you start over with a new boiler. It's probably a 5:1 turndown already. Run it at the min. flow they allow, probably around 15 gpm and a wide deltaT, charge the buffer tank, and draw it down with an ODR control.

The key is still in the load calc as mentioned above, to judge how much you actually need, compared to what you have.

"It's not having what you want
It's wanting what you have"
Sheryl Crow

jcw

The versa flame looks like a good option from a functionality stand point. I am awaiting a cost estimate to see if it fits my budget. Thank you for that suggestion.

I have recalculated the heat loss for the house and determined it is right around 140k btu's.

The boiler that i currently have looks as if it the minimum input is 62k btu's I am looking into seeing how adjustable the unit is and if will work for my application. It may work if I add a buffer tank as suggested. But then again I will still need a heat exchanger and an indirect fired WH.

hot_rod

How about this
The 310 Ultra you have feeds a 120 gallon indirect, via the coil, so there is your separation from boiler to non barrier tube.

Pull the heat from the tank, and DHW via a 30- 40 plate HX, with that much boiler power you should easily generate 7- 10 gallons of DHW instantly.

The tanks becomes a multi function device, buffer tank for the large boiler, non barrier tube isolation HX, and a high output instant DHW source. 1 tank and one HX and away you go. Everything downstream of the tank should be stainless or brass as it still "sees" the non barrier tube fluid.

Select a tank with a good sized HX coil, or surface ares. The Caleffi for example has an 1-1/2 coil and can easily handle 20 gpm flows. There are others if you want stainless, the SS HeatFlo has large coil size also, or the tank in tank type

Or a plain steel insulated tank and two external heat exchangers

Rich_49

You will need a Stainless Steel buffer with a coil heat exchanger to isolate the boiler from the system . You will need stainless or bronze pumps , stainless at present is less expensive than brass . Then you will need an indirect for DHW also .
I have an extensive database of current or recent pricing for all these things if you'd like to do some math . You could PM me to receive the numbers

hot_rod

Rich, why a stainless tank? A glass lined steel indirect would work fine and probably less $$. This same tank would provide DHW via an external plate HX. Why the two tanks?

More tanks, more heat loss lower= "system efficiency" No need to store DHW in a tank when you already have that boiler HP to leverage.

Glass lined tanks are readily available from Lochinvar, HTP, really all of the WH tank manufacturers have glass lined indirects. Lochinvar is very popular and available throughout Colorado.

The tank becomes a sealed system and should last a good long time with no fresh water being added.

I'd guess a tank and plate HX well below 3 grand.

hot_rod

jcw said:

The versa flame looks like a good option from a functionality stand point. I am awaiting a cost estimate to see if it fits my budget. Thank you for that suggestion.

I have recalculated the heat loss for the house and determined it is right around 140k btu's.

The boiler that i currently have looks as if it the minimum input is 62k btu's I am looking into seeing how adjustable the unit is and if will work for my application. It may work if I add a buffer tank as suggested. But then again I will still need a heat exchanger and an indirect fired WH.

There are some good formulas in this journal for sizing buffer tanks with On/Off, or modulating heat sources. This will let you run all sorts of what ifs. Do you know then loads of the individual zones? If you know then load of the smallest zone, you can really nail down some tight buffer numbers.

The trend in the industry is to get away from storing DHW in a tank, especially under 140F. Tankless water heaters are becoming more popular and no storage is one of the features. No tank, no standby loss.

So with a hydronic system, properly sized you have the same ability. A flat plate heat exchanger is basically a tankless heater, just hydronically powered.

You will see quite a few options for that detail within this link.

http://www.caleffi.com/sites/default/files/coll_attach_file/idronics_17_na.pdf

Rich_49

Bob,

I was writing in between phone calls and did not see your post until just now . I just prefer stainless , especially in a non barrier application . Explanation being , I have had to tear out too many glass lined tanks because of accidental integrity loss of the glass in several buildings where people are not so careful . A few of them were not addressed right away and the metal and terrible garbage was rampant through the system by the time it was noticed .

I like your idea . But what about putting that flat plate for DHW right to the boiler ? No doubt with that power there would be plenty hot water , No ? This would keep it away from the possibility of an early demise and also allow lower temps in the tank and cooler return water to the boiler for a longer period each year .

hot_rod

Maybe this is a good compromise. I'd rather not fire the boiler on every DHW call. With this piping, if the tank has enough energy stored, say 120F or higher, no boiler fire. if the tank drops below that low limit 120F, the boiler fires. 7- 10 gpm to the hx, the balance of the gpm to the tank. So this should assure a good boiler run cycle in both heating and non heating seasons.

The bigger the better for HX, a 40 plate would allow a close approach and provide 115- 120F DHW with the tank about 5° warmer. Gives you DHW at best boiler efficiencies.

jcw

The link provided from callifi.com was extremely helpful and informative. I Like the idea of only utilizing a single tank for both DHW and as thermal storage for heating. Has any one ever utilized the single storage tank that contains a stainless steel inner tank within a carbon steel outer tank. The inner tank contains domestic water, while the outer tank contains system water... pictured in image 7-2 on page 33 of the below link. ?? Any brand recommendations?

http://www.caleffi.com/sites/default/files/coll_attach_file/idronics_17_na.pdf

Bob, the second diagram you drew... How will the DHW draw draw heat when the boiler is not firing? Would i have to run a second pipe from the storage tank to the DHW HX for times when the boiler is not firing?

The first diagram you drew makes more sense to me. In the first diagram, wouldn't the boiler only fire when the storage tank drops below the "low" set point?

hot_rod

jcw said:

The link provided from callifi.com was extremely helpful and informative. I Like the idea of only utilizing a single tank for both DHW and as thermal storage for heating. Has any one ever utilized the single storage tank that contains a stainless steel inner tank within a carbon steel outer tank. The inner tank contains domestic water, while the outer tank contains system water... pictured in image 7-2 on page 33 of the below link. ?? Any brand recommendations?

http://www.caleffi.com/sites/default/files/coll_attach_file/idronics_17_na.pdf

Bob, the second diagram you drew... How will the DHW draw draw heat when the boiler is not firing? Would i have to run a second pipe from the storage tank to the DHW HX for times when the boiler is not firing?

The first diagram you drew makes more sense to me. In the first diagram, wouldn't the boiler only fire when the storage tank drops below the "low" set point?

In the second drawing the tank temperature is maintained, probably as a minimum 125. With 125 or hotter in the tank you can still generate 120F for DHW, if the heat exchanger is sized adequately.

The second drawing allows full boiler output to the HX without first heating the tank. So a large DHW draw the HX gets whatever flow the pump sized for the HX can supply. Sort of a priority DHW piping.