System design questions

MNSteve

I am getting ready to install a new heating system in a house I just purchased. The home is currently setup with an older fuel oil boiler and baseboard radiators.



Most of the radiators or in some state of dis-repair, so I have decided to change to under floor radiant heat for the main floor while retaining the baseboard radiators for the finished basement. I would also like to add the DHW with an indirect fired tank.



I plan to use two boilers for this system a 14kw Hydro Shark electric boiler (primary) and a Hydro Smart 170 Micro LP boiler (secondary). See <a href="http://www.hydro-smart.com/">http://www.hydro-smart.com/</a> for details.



The reason for the two boiler system is that I am in a rural area where I can get interruptible electric service for a much better price than LP.



Attached you will find the start of my design. I would appreciate any feedback about things that I am missing.



The plan is to have the electric boiler firing as the primary boiler and the LP boiler will only fire in one of two situations. First if the interruptible electric is off, or if the electric boiler has failed to meet the demand within a preset amount of time (20 minutes??).



Right now I am to the point I am trying to size all of the system components. The first thing I am working on sizing is the pumps. I have a copy of the "Primary-Secondary Pumping Made Easy!" and it does a good job of explaining the sizing, however I need a little help with figuring out the amount of head with the different loads.



For the baseboards I assume I just need to measure the total length of 3/4" copper pipe and multiply by some magic number??



How do I determine the required pump size for the DHW?



Again I assume for the under-floor radiant I take the total length of 1/2" PEX and multiply by another magic number? Also does the number of loops the radiant is divided into have an effect on this calculation?





One additional thing. I have been reading a lot about the virtues of one heat transfer plate vs another, however all of the comparisons I have seen are between various aluminum plates. Now I see that Nibco is making a steel transfer plate, any feedback on those?



For the radiant I have a heat load of about 32K BTU and I am going to be installing about 1200 ft of 1/2" PEX so that works out to about 26 btu per foot, how good of transfer plate do I need to achieve that?



Thanks for you help,



Steve

STEVEusaPA

You're on the right track

But your design really needs some help.  First, If you know how to install this complicated system yourself, I strongly recommend you purchase all your equipment from a reputable supply house.  They will better guide you with the sizing, and will even draw up a piping/control diagram for you.

First issue, if the power is off, you LP isnt working either, so I wouldnt go that route, unless you have a back up generator that can handle the load for the controls and pumps.  You may be better off sizing the electric boiler for the full load, and installing a generator that can handle the boiler, and a few essentials.  It would also handle central AC in the summer if needed.  This may end up costing the same for equipment, and give you a generator for peace of mind.

Ill try to give you as many pointers as I can.

"For the baseboards....".  The first thing you need to do is to see how many BTU's are required, then see how much baseboard/and size pipe you need.  Then you can determine flow and head.

The DHW pump, and piping size/pumping requirements will be determined by the manufacturer's instructions.

The radiant sizing will be determined by the complete heat loss, load for the zones it will serve.  This will then help you decide about the aluminum heat transfer plates, warmboard, number of zones, number of manifolds,lengths of loops, water temperature,  etc.  This is of the utmost importance. 1200 ft of pex just ain't gonna cut it off your primary loop with one pump, unless its a big one.  Those should be broken down and zoned according to a number of factors-temperatures needed, and floor finishes.

Your drawing is missing many things, whether omitted for simplicity sake or not.

I would move the expansion tank, air scoop and primary pump to the other side, downstream of the boilers.  I don't know what is installed after the radiant loop (flow chek?) but it's not needed there.  You could go with IFC's on the pumps, or in the isolation valves to handle the flowchecks.  Your missing the flowchecks, isolation valves, drain valves, etc.

You still need to design a complete control package.

I like that you educated yourself.  This will make it easier for you to make proper decisions.  Now do an even smarter thing, and bring in a pro to design the system, offer equipment/control advice, explain the newest and best technology, and give you a complete package-equipment, controls, wiring/piping diagrams.  You'll end up so much more happier, and comfortable.

MNSteve

Electrical service

Steve,



Maybe I didn't explain the electrical service correctly. The house is going to have two separate electrical services. One regular service at the regular rate, and one interruptible service at a better rate. The interruptible service is only used for the electric heating load. So when the interruptible service is disconnected. Everything except the electric boiler will continue to operate. The electric boiler is sized at 14kw because that is the limit of what I can run without having large expenses to upgrade the interruptible service. However that should be large enough for 95% of the heating season.



For the baseboards. I am familiar with the formulas to calculate the required length of baseboard, however I have not been able to find the formula to calculate the head pressure once the length is known.



In regard to the DHW, you are correct. With a little more digging, I was able to find the manufacturers recommendations for pump sizing.



For the radiant, I understand that the heat loss is the starting point. I have calculated the heat loss of the space to be about 32,000 BTU as shown in the drawing. I know that if I run one "loop" in each joist cavity I will be able to fit about 1200ft of pex under the floors which works out to about 26 btu per foot does this sound like it is in a reasonable range? I also understand that the head pressure would be very high if the 1/2" Pex is all in one loop. I plan to divide it into 5 loops returning to a single manifold. Again I am looking for the formulas to calculate what the head pressure so I can determine pump size.



I appreciate the recommendation to move the expansion tank and primary pump the the other side of the boilers. Is there a reason for this? or is it just a standard practice?



After the radiant loop, that is a strainer. Is there a better location for this?





I was planning to use IFC's in the pumps for the check valves.



I appreciate the recommendation to add isolation valves.



For drain valves, is there a recommended location?



I know that I am missing the control package, I have a relatively simple system in mind. However I will get to that shortly.



In regard to "bringing in a pro". I have tried this several times and most of them seem completely unfamiliar with concept of interruptible electric, so I have had very little success.





Steve

STEVEusaPA

ahhhhhhh

I missed the "interruptable" or demand meter.  Ok that does make the 2 boiler option seem better.

Sorry about no luck with a pro.  That's always a bummer for our industry.  As far as a competent design person, you could try some of the resources here, specifically a few people, in my opinion, who really know the most, are Rob_NRT, world famous Mark Eatherton, and Tim McElwain.  They really would do the best design work.

As I'm not familiar with the Hydro Smart (I just looked it up), I can't comment on the near boiler piping, although it looks fine, but their installation manual will definately give you the exact way they would like it done. They may even have a built in circ, and will tell you what size piping for the btu's needed.

For the radiant, you're actually running 2 pieces of pex per joist bay (joists 16" on center, pex 8" on center).

The expansion tank, is better there, I think.

The strainer, can be combined with air eliminator at the expansion tank connection on the primary loop (see TACO's site).

Other design ideas.  I usually come off the closely spaced tee's, with isolation valve, circ, isolation valve.  Then on the return, right before the closely spaced tee, boiler drain, shut-off, then closely spaced tee.  Most people put another check valve, but I haven't had much problem with ghost flow or thermosiphoning, to add one. The ball valves between the closely spaced tees, there's 2 schools of thought.  One being thats a great place, helps you bleed each loop quicker, and the other,  it affects the pressure drop and could force some ghost flow.  I've done it both ways, but am leaning toward just doing it on the radiant loops (lot more piping to bleed). 

For the radiant loop(s), I like using injection mixing.  For more info, check out TEKMAR's website, looking at the essay on injection piping/mixing.  Depending on how you control your system, the 374 will do 2 boilers, high temp, 2 mixing temps, a setpoint (if needed) and domestic hot water.  You can even program it to not rotate the boilers, and put a longer stage delay to fire the second boiler.

Some formulas

btu/h load/10000=flow rate in gpm for a 20degree temp drop.

Then take your total run and multiply it by .006 to get feet of head (quickest method).  Thats for the copper tubing.

The DHW tank manufacturer will tell you what you need for piping/circ for their product.

Just remember, your gas boiler should be slightly larger then your electric boiler (electric is 100% efficient).

For pipe sizes, 3/4" copper will carry 40,000 btu's @ 4 gpm, maximum flow rate.

1"-- 80000 btu's @ 8 gpm max.

1 1/4--140,000 btu's @ 14 gpm.

For the radiant loops, the calculating is a little more involved.  It will ultimately depend on spacing, installation method, finish floor coverings, water temp, etc.

Those formulas I don't know off the top of my head so I'll have to get back to you when I get home.

quick edit:  Watts Radiant is one manufacturer who will design your radiant system for you.  Look them up for a local distributor, or on their website.