Mixing Tank vs. Valves

bruce pirger

Greetings All:

Thinking with earnest about my heating system for my new home...it is getting cool. I have radiant floor (in slab) and also staple up. Indirect hot water as well.

I have read much of Seiganthaler's textbook (sorry for the spelling John) and have an engineering background.

I have been planning to use a mixing tank (an indirect water heater) dedicated for my radiant system, as well as an indirect tank for DHW. It seems to me that the 4 way mixing valves would lead to short cycling of my oil boiler...where as the mixing tank would prevent this.

While I have radiant in the concrete slab, much of this will be carpeted with thicker carpet, you know, the R-3 stuff I should NOT use. The staple up is upstairs between the I-joist and also will be carpeted with the thicker stuff. I have Wirsbo manifolds with the balancing valves, so presumably I can reduce the flow to the areas which are tile and engineered hardwood.

I like the idea of separate loops (boiler vs. radiant flooring). My oil boiler will be outside the house, in an insulated shed with the oil tank. Water will be brought inside through insulated PEX lines, in conduit, the conduit itslef insulated in a deep (4') trench. I plan to sometime in the future install an outdoor wood boiler.

I guess I am looking for general comments/affirmation that the mixing tank is not a bad way to go. Seems to me it removes all concerns about short cyling (even though heating 50 gallons in the mixing tank 10 or 20F doesn't take all that long either) and also removes the need for a 4 way mechanized valve. Furthermore, outdoor rest control in conjunction with the tank aquastat should make for a stable, responsive system.

Looking into using a smaller Crown oil boiler. Heat load is low, about 50,000 BTU/HR...so the smaller 3-section boiler seems to be the way to go. MegaStor indirects....

All comments/attacks are welcome.

Thanks!

Paul Rohrs_2

Buffer Tank\ Boiler

I have attached a JPEG file I hope you can access. We powered up this system last night and the buffer tank works as designed. We put in a Danfoss balancing valve in to ensure a correct mixing temperature. Based on a primary-secondary flow, you can run high temp and low temp and design around the future install of the wood boiler. This contractor is using his wood boiler as primary heat source and the peerless CI is his back up. Hope to hear more from you.

Paul

hr

That Heat Source

wood furnace is an open (non pressurized) system,isn't it? How are you keeping the snowmelt loop protected from freezing? Am I missing some HXs somewhere in the loop? What happens to your loop and buffer temperature when the 32 degree return from the snow melt kicks in?

With the 163 gallon capacity of the HS-1000, buffer tank, and other equipment in the loop you will need 100 gallons of glycol or more?

Also glycol open to the atmosphere will go bad rather quickly. If this is an open system really no need for an expansion tank.

I'd be interested in seeing how you wired all these various components together.

hot rod

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bruce pirger

Thanks for the picture. I don't understand everything though...please help. How is the temperature of the radiant floor loops mixed down? It looks like it would be the primary loop temperature, somewhere around 160. But I must be missing something...as you only have 3 lines in/out of the mixing tank. I would also think you wouldn't want to mix an open loop (wood boiler open to atmospheric pressure) with your closed loop system (boiler and all) to prevent oxygen from being brought in to the boiler and all.

I quickly tried to "sketch" out the basic thoughts I am having...and I will attach a picture here. There are many things I need to add, like valves, expansion tanks, etc. to the picture...

Any comments/attacks would be most appreciated.

Thanks!

hr

Some thoughts, Bruce

I'd rather see your indirect tanks in a primary secondary piping. In your drawing a cold start could keep the boiler in condensing mode until both tanks warmed up. Primary secondary with V/S injection to the buffer would control the primary loop temperature and allow return protection visa the injection control.

Depending on the amount of radiant slab a cold start would also draw down your DHW temperature until the boiler could catch up the load. Primary secondary with some smart control work would make a nice package. Tekmar has a 369 I believe that would allow two seperate mix control V/S or motorizied mix valves.

hot rod

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Cosmo

Hey Hot Rod

Where can I get info on this wood fired boiler, is there a website?

Thanks'
Casmo

Paul Rohrs

Wood Boiler

I have a BP412-20 Brazed plate exchanger at the point where the wood boiler enters the primary loop. We also put (as you suggest) isolation valves and 3-way bypass valves prior to and after the CI boiler. HO knows that system must be bypass the CI boiler in snowmelt mode. CI was not sized for that load and would die a quick and painful death if not bypassed. Http://www.Heatsource1.com was a website of a similar wood boiler manufacturer. You are correct HR in that there was alot of Glycol for this system. The system belched a few times after completing the fill.


Paul

PJO

Second Indirect Tank...

I see that you are using the second tank to indirectly heat the radiant. Besides the need for a buffer tank, do you also have to have an O2 barrier or something?

I am going to do something similar with an Ergomax for my system....except put it directly in the primary loop and pipe the boiler on a secondary circuit. I was wondering if any Wallies have done this combination for radiant (instead of domestic hot water) and how it worked.

Are you sure 120F will be enough with thick carpet?

Just my thoughts. Take care, PJO

Jacob Myron_2

Short cycling

What is short cycling?

If your mixing tank is at proper temperature and you lower the water temperature 1 degree F won't you have replace the btus removed form the mixing tank?

If the 50 gallon tank dropped one degree yo will need to replace a little mire than 415 BTUs. If the tank dropped 10 degree's yo would have to replace a little more than 4159 BTUs.

Since you are burning oil you know the input from 1 gallon of oil is about 110,000 BTUh. If you had a 1 gallon nozzle in the burner the oil burner would be on line about 2 1/2 minutes, if you have a lager nozzle in the oil burner the burner will be on line a shorter period of time.

Sooo. What is short cycling?????

bruce pirger

Just what is short cyling...good question.

To me, running the oil boiler for less than say 10 minutes is not a good thing. Also, in this case, I intend to use a smaller boiler, closer to 80K than 110K. So that 4500 BTU's would still be made up quickly, less than 10 minutes.

When folks use tempering valves instead of mixing tanks with radiant, how do they prevent the boiler from short cyling? It seems that the boiler would heat up the internal water and mass very quickly...as well as the relatively small amount of water in the radiant loop.

What is a typical boiler burn time for a radiant system?

hr

These are good questions, Bruce

I'm not sure there is a definite answer to what constitutes a short cycle. To me anything less than a 10 minute burner on cycle would seem "short". In theory anything less than long enought to warm the hx and flue piping above dew point will lead to problems.

Of course on a design day a "perfectly" sized boiler would run non stop, as the building heatloss would perfectly match the boiler output!

Oh, I suppose a few short cycles now and then would not be to serious as long as at some point the boiler runs long enough to "dry out" the condensation.

You are right, a tempering valve will not "fix" this. What a tempering valve, properly installed" can do is protect the boiler from seeing low return temperatures. A tempering valve cannot change a small load on a large burner. Thermostatic tempering valves allow the boiler to get up to a preset temperature then slowly blend in the load. It will not help the short cycling, if the heat load is much smaller than the boiler output (micro loading).

A buffer tank, sized properly, will extend a burner on cycle. You will notice most wood boilers or outdoor furnaces have buffers or large water content HX's, to allow them to burn long and hot.

A burner that modulates to closely match the varying heat demand would be closer to ideal. You will notice a bunch of these on the market currently, and more on the way.

For low temperature, below 130 degree, continous operating (radiant floor heat) a modulating burner, condensing boiler would be perfect, in my opinion. These too are hitting the market from various manufactures.



hot rod

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bruce pirger

Thanks hot rod. Yes, that is why I am looking into the Viessman biferral boiler. While not condensing, I think, it has been designed to modulate to the temperature necessary and has been designed to accept low temp return water. I guess I still worry somewhat about long term corrosion if constantly running below the condensation point...but I have also come to respect and appreciate German engineering!

As an engineer myself building a house, from the footers up, I have been having a blast over the past year. My final engineering gig for the house is the heating system. I have the radiant slab (1400 sq. ft, 5" thick) and staple up upstairs (about 1100 sq. ft). My thermal load calculations show about 48K heat load. I have the Wirsbo hePEX in the floor...manifolds...etc. But it is time to decide on a boiler and control strategy. No gas, has to be oil. Indirect hot water.

Also have a wood stove I'll burn all winter inside until the outdoor wood boiler is brought into the plan. Not holding my breath though...they are NOT inexpensive. I'm not trying to skimp anywhere...but certainly can't drop the 5 digit price tag either. The building all started out as a "workshop" to live in until I could build the full scribe log home...LOL

I have purchased Seiganthaler's book and also a pile of books/tips for Dan...and am trying to decide how to put it all together. I am somewhat interested in having professional help, as the deadline soon approaches, yet I won't be happy just having something "thrust" upon me without my understanding and agreement. I suppose I could be the professional designers worst nightmare...a DIYer asking way too many questions and looking for the numbers to back up the decisions! LOL

Adding a tempering valve to protect the boiler makes lots of sense. What I don't understand is how a tempering valve is used to provide the radiant manifolds the low temperature water. I mean, I know how they work, but it seems to me there'd be massive amounts of 3 minute boiler burns as there is just not much water to heat. Hence I tend to a mixing tank (another indirect tank). But even that, unless it is very large, in excess of a couple hundred gallons, doesn't seem to prevent short cycles (less than 10 minutes) with a small boiler and working with a 20 degree temperature difference!

Thanks for your help guys! It sure is fun.