FPHXer dumb question (you know you love dumb questions!)

toasty-toes

Sorry, I can't seem to find an answer for this.
So if I have a ci boiler with an output of 38k BTU's and a design loss of 28k btu's and a flat plate x-changer between the slab and the boiler. How much of the 38k is lost in the transfer of heat to the slab side? (btu's that never make it to the slab). Ballpark. 1k btu's? 5k btu's?

Basically am I still going to be way over the design loss or will I be closer because of the xchanger?
Or am I confused and this doesn't matter?

(I have found a stellar deal on a 4 month old minigas-50 (derated to 45k input for elevation) only 300 bucks!
Am I barking up the wrong tree?

Thanks!

Ironman

To know the answer to that, you would have know the size, make, model of the HX; the flow rates on both sides and the temps (in and out) on both sides.
All their manufacturers have formulas and programs for sizing them. In other words: it requires some engineering.
I use a lot of flat plate HXs and if you give the width x leng x number of plates, I can ball park it for you.

toasty-toes

Ironman,
(now I have Black Sabbath going through my head!)

Yikes! I wouldn't have a clue at this stage what size the HX would be.

Basically I am trying to figure out if I should pull the trigger on this boiler deal or walk away. I know its a bit over the top in output, but I would imagine there are not a lot of perfectly matched load to boiler output installations out there.

If there are losses I am not accounting for, that make this boiler suitable for my needs, I'd sure hate to pass up the deal.

So yes there are losses in the HX (just not easily ballpark-able)

hmmmm what to do? what to do?

Ironman

That's about the smallest btu boiler that you're gonna find and it should be fine. It may need a bypass installed depending upon the return water temp.
Give me the dimensions of the HX and I can help you. Also, a diagram and/or some details on the radiant floor:
1. In slab or staple up?
2. Square footage of the floor?
3. Tubing lengths, spacing, number of loops?

STEVEusaPA

toasty-toes said:

Ironman,
(now I have Black Sabbath going through my head!)

I think more like Tony Stark....IMHO Bob is a brilliant guy.

toasty-toes

Well I just bought it!...so either way I have a shiny, almost new Boiler for super cheap!
Ok Ironman, Thanks.
(still have Sabbath in my head)

(you guys should set up a beer fund we can all contribute to!)
KickStarter BeerStarter! Good thing I am far too lazy to actually create an "app" and make a bajillion dollars!

960 sq feet of slab on grade
One zone
4 circuits
Non O2 barrier pex (1/2") in 230 foot loops spaced at 10" - 12" apart.

This is the boiler:
http://alliedboilers.com/products/mini-gas-boiler/
(MG-50)
Still need to determine pumps, but I would/like to stick with very low power models like Alpha's if possible.
As far as HX I am clueless. I just know I need one to isolate my boiler from the devils Pex.
Not looking at domestic water at this point as I have an On Demand that works well.

ok, I should get back to actually working so I can pay for all this!

Thanks everyone.

Ironman

"Ironman" is a monicker for someone with a lot of endurance. When I was 17, four of us, including the service manager, were carrying a very large cast iron rad. I never had a grip on it and consequently lost it all together on my end. The service manager kept riding me about it. After a few days, I'd had enough and told him to put his arm up on the sales counter at the shop. We locked up and in about 10 seconds or less, I slammed the back of his hand down. He was stunned since he out-weighed me by at least 60 pounds. He started calling me "Ironman" and it kinda stuck.
When I joined the army and volunteered for airborne duty, I learned that it was also used there to describe those who had gone through advanced training that required great endurance.

@toasty-toes
Do your have any insulation under the slab and around the perimeter?

toasty-toes

Ok, so now I have an image of you arm wrestling while Black Sabbath plays in the background!
Oh...now you are jumping out of a plane. DA-na--na-na-na!!!!!
That's cool you have a legit username! Now I want to light my feet on fire so I can claim mine! hahahaha.

So, yes the slab has 2" foam board under the slab and around the perimeter 2' down the 7' frost wall. Yes I live is a frozen wasteland.

Ironman

The truth is, that at 60, I'm feeling more like aluminiumman. But I'm blessed with great health and can still hit a ball better than most of the young bucks.

As far as your original question: a 5" x 12" x 30 plate with 1" connections should be more than ample. It never hurts to over-size a HX, so go larger if you like, but it shouldn't be necessary.

A couple or more things to watch:
1. Since the slab is high mass, you need a smart mixing valve or variable speed injection mixing with out door reset to control the water temp in the slab or else it will over-heat. Don't use a fixed temp (dumb) mixing vale. You can go to Tekmar's site and look up their essay on injection mixing for a detailed explanation.
2. With sustained return water temps well below 140* from the floor, the boiler should have a thermostatic bypass installed to protect it from flue gas condensation. That's not a condensing boiler, but it will be without the bypass. It will also be a dead one very soon without it.
3. You're gonna need somewhat higher than normal water temp to get 29 btus per square foot from that floor on design days. Without doing any calcs, I'd start around 105* design temp and adjust it slightly up or down every few days as needed on the smart control until the water temp just keeps up with the heat loss. You don't want to go above 125* SWT at design days and you don't want your floor surface temp above 85*. Avoid or minimize using carpet and rugs as much as possible as they resist heat transfer and will decrease the floor's output.
4. Make sure you use a bronze or stainless pump on the floor side. All components on that side must be non ferrous. Use an expansion tank on that side that's rated for potable water. You'll need a relief valve and air elimination on both sides, too.

Zman

Bob has you more than covered on this on but I can't help but point out you cannot lose BTU's in a heat exchanger (anymore than you would lose to the air in a similar piece of pipe). You can however missize the exchanger and end up not being able to transfer the energy you need at the temperature you desire. If you download the calculator from these guys it will give all that plus the resistance the circulators will face.
If you don't want the headache, I am certain that Bob's ball park and humming some Black Sabbath will also work just fine.

http://www.gea-heatexchangers.com/products/plate-heat-exchangers/

Zman

It is a bit clunky because you have to go back and forth until you get the program to give you the results you are after.
It was obviously written by engineers for engineers.
I do wish there was a better one.
At the end of the day it just comes down to the surface area and the flow on either side...

Zman

Hatterasguy said:

The results you typically desire are the two outlet water temperatures once you provide the two flow rates and the two inlet water temperatures.

No further data should be required.

However, you must also input the BTU transfer rate to get those two outlet water temperatures. Note that you have no control over that transfer rate. You can set it at whatever you wish and get whatever result you wish..............none of it relevant.

I agree completely.
You would make a crappy engineer. When you explain it like that, people can understand it. You would be out of a job in no time.

toasty-toes

hmmm.
You guys amaze me. I pop in to see whats what and there is so much info I need to ingest it and let it digest.
I will get back to you. I need to do some reading now and get up to speed.

toasty-toes

Ok, I read a bunch and I think I have a grasp on most of the whats and hows. (in a rudimentary way)

So, is there a diagram somewhere of this sort of set-up?
I have scoured the web thinking I would find a near boiler piping diagram, that matched this, but have come up blank. The CI boiler, hx, mixing valve, seems to be an odd duck maybe?

Also, I have seen wild variations on HX prices. Not looking to skimp on things, but also don't want to over do it if it can be avoided.

Zman

The I valve would be a good product for your system.
If you use the piping shown for "3 way valve" and replace the closely spaced tees in the middle with your heat exchanger you would be pretty close.

Zman

This one should work for your application.
http://www.supplyhouse.com/FlatPlate-FP5X12L-10-10-plate1-MPT-Low-Pressure-Drop-Heat-Exchanger-5-x-12

toasty-toes

Ok, I like the price way better than the 1389.00 dollar one I looked at but, are 10 plates enough?

Doodled this up. Sorry it is not done to normal Plumbing drawing specs, but I don't know what those are.
Is this more or less correct?
If yes, I would imagine I need expansion tanks for both sides. Various filter/clean outs, gauges, drains, etc.

Ironman

That looks good.
Did you get my P/M?

Zman

With the smart valve, you don't need the mixer on the boiler side. The smart mixing valve on the radiant side will measure the temp on the boiler side and adjust accordingly.
That exchanger will work fine. I will post the calcs if I have time.

toasty-toes

Awesome, thanks Zman.

Zman

You really could go smaller. I like the price and availability of this one. Plus it has very little head loss, perfect for radiant..

toasty-toes

That is super nice of you...appreciated.
This plumbing/heating stuff sure involves a lot of reading!