Will this work?

[GCTerpfan]

I was recently given an outdoor furnace by a family member. It is a 1996 Central Boiler model SB 17. It is still in good shape with no leaks, so I have decided to try to get a few more years out of it. I am trying to heat a 2,200 sq. ft. house (total including finished basement) and my DHW. My house apparently is well insulated as I only burned 500 gallons of fuel oil each of the past two winters that I have been in this house, including last winter which had several weeks of sub-zero temps. I also plan on running the return line through my garage for the potential to add a heat source there in the future. I am only hoping to be able to heat the garage to about 50 deg. or so. After a lot of head scratching and reading post on this and many other forums on how to hook it up, I have finally came up with the plan that is shown in the attached picture.

So hear are my questions:

-Is heating the garage a pipe dream with this boiler? It only adds about 30' feet of trench to run the return into the garage, is it worth it?

-Is there a benefit to using 1 1/4" lines, or should I save the money and install 1" lines? I believe the Taco 009 pump only pumps 8 gallons a minute. I am considering using 1 1/4" lines in case the current pump is not enough then I can upgrade to a larger pump and the lines will already be in place. Does that make sense?

-Do you think the current pump is large enough?

-Any other advice or suggestions? (just an FYI, I do plan on spray foaming the lines in place like shown in this forum)

 

[Clarkbug]

To help with pump sizing you would need to know your loads, and the rating of your heat exchangers.

I'm going to take a guess and advise to go with the larger apex, just to cut down on the pressure drop. Pumping through three heat exchangers in series might really add up.

Be sure to add a tempering valve to your hot water heater.

 

[GCTerpfan]

Thanks for the reply. I am struggling to figure out how to calculate my loads. I have found a few on line calculators that are based on square footage but they give me vastly different results. I also have a friend who used to do this for a living who told me to figure 75,000 btu/hr for my house and 40,000 btu/hr for my DHW. Both of these seem high compared to other numbers I have seen.

How much variation is there between brands of heat exhangers? These are 5"x12" HX with 1 1/4" ports, I have found some charts for other brands of HX but the manufacturer of the brand I planned on buying simply listed this:

"Below is a guide to help you. If there is any question, ALWAYS buy the next size larger. It is always better to have more than enough heat, than not enough! Again, all items above are based on a 12" length and 1" ports.

A. 10 plates are used for domestic hot water or small in-floor heating

applications of less than 1100 sq ft. These will work with the largest of

domestic hot water systems and should never need to be larger unless you

use your domestic hot water for more than 3 hours straight at one time.

B. 20 Plates are used for smaller homes from 1100 - 2500 sq ft.

C. 40 Plates are used for larger homes from 2500 - 4500 sq. ft.

D. 70 Plates are used for areas 5000 - 7000 sq ft."

 

[maple1]

How far apart are the proposed boiler location, and the garage? And how far apart are the house & garage? Just wondering if you can run your boiler piping through the garage to the house, rather than boiler to house with a T to the garage. That would also give more options for the future if you end up replacing the OWB - you could maybe put something in the garage or turn it or part of it into a boiler room for an inside unit.

40,000 btu/hr seems crazy high for a DHW load. 70,000 for the house sounds pretty high too given your past oil consumption. How is your DHW heated now?

I would do 1-1/4" lines - that's seat-of-pants based, but with the same general thoughts as Clarkbug.

 

[GCTerpfan]

I have attached a picture showing the approximate distances. I wasn't planning on adding a T to the garage, I was just going to loop the return line to the garage and back out as shown in the picture. I thought about running the line into the garage first but I wanted to get the maximum heat load to my house first, the garage is just a luxury. Maybe it doesn't matter which comes first, just the total load on the line?

I currently have a 40 gallon electric hot water heater. I thought 40,000 btu/hr seemed high also.

 

[Clarkbug]

Quick and dirty method... what size is your existing boiler? That's a ballpark for load for the house.

Your DHW heater probably has a 4500 watt element. 40,000 btu/hr is about 11700 watts.

Consider a sidearm for your water heater. With an OWB the lines are always pumped, so the sidearm has flow, and low pressure loss. But make sure the potable side of the system has an adequate expansion tank and a working T&P relief...

You need a real load and real data on the Hx before you buy anything. Ballparks can be awfully far off....

 

[GCTerpfan]

My understanding is that there is nothing hurt, other than your wallet, by going bigger with the HX, is that correct? Both of these HX are considerably larger than the "ballpark estimates" say that I need, yet they only cost $200 and $166, so I am not going to save much money by going with a smaller one that has been sized correctly. Does the system work less efficiently if you have an oversized HX?

Not that I am against sizing it correctly I just don't know how to figure out the load data, and the 'professional' I consulted just says "don't worry about it, it will work fine"

 

[Clarkbug]

A bigger heat exchanger has higher pressure drops, so you may have to increase pumps on both sides of it to compensate. Bigger is definitely better than smaller.

 

[heaterman]

With everything in series like that you should definitely use 1-1/4" pex.

Given that you heating load is very low you don't need a lot of GPM but the "head" or resistance to flow illustrated in your drawing will still be significant given the total length and the heat exchangers.

 

[GCTerpfan]

Thanks for the input, you guys have convinced me to go with the 1 1/4", I am thinking that since the Taco 009 pump is a high head/low volume pump that this set up will work. The heat exchanger in the garage may never happen, I will decide after I see how well the house and DHW work. Now If I could just get a spray foam contractor to call me back.....