How do you connect two boilers?

I said I would help – but I only can if you let me. That said I understand the mind set of “testing” and I think it’s a good thing. – Sometimes. But -- Well, what ever makes you comfortable and that’s what a free forum and free advice is all about. Free advice is only as good as what you paid for it anyway and a figment of the public Internet is free!

Making the replacement connection as ez as possible would be duplicating what is existing. My rub is you described that connection as not functioning for part of the dwelling.

The other part of your “test” that bothers me is the pex connection – aside from the possible drama of over temping the capability of the pex – to get enough hot water or “energy” transferred to the old system --- Pex of the diameter size required to give you a fault tolerant flow of energy will not be cheep either. DO NOT expect that ¾ inch pex is going to move near enough liquid to make this work. Your going to need 1-1/4 at least and I would use 2 inch (maybe even 3 inch) black iron pipe. Remember this is a gravity flow system – there was a reason the the Old Dead Plumbers hung all that HUGE pipe in your basement and it wasn’t to build a circus trapeze --- All that pipe is about slow, gentle, fault tolerant flow.

If you build with pex --- what you will get would be like a “straw’ between your boilers – it would be similar to filling your swimming pool with a garden hose. Yes it can be done if you put a big enough pump on the garden hose or your “straw” - but you will not receive a fault tolerant flow of water with your effort – you get a Hurricane – a tempest in a tea pot – a mix of hi pressure, extremely fast flow, and LOW GPM water causing a heck of a turbulence in what should be a very calm pool.

Think like the water

This link leads to a pipe and pressure drop sizer, free download.

http://www.bellgossett.com/BG-SystemSyzer.asp

There are some rules of thumb which have appeared in various threads. For heat transfer, maximum gpm: 1/2” - 1.5 gpm, 3/4” - 4 gpm, 1” - 8 gpm, 1-1/4” - 14-17 gpm, 1-1/2” - 22-25 gpm, 2” - 45-50 gpm.

One gallon per minute of flow (GPM) will move NO MORE THAN 10,000 BTU's of energy in a fault tolerant manor. My lunch lady used to say "bigger is better" so stay on the large size with your pipe to get a gentle flow of teh amount of energy you will need to heat the old system correctly.

For btuh calculation: gpm x delta-T x 500. For line head, total pipe run length (round trip + equivalent for fittings, etc.) and then use the calculator at Pipe Friction Loss. http://www.freecalc.com/fricdia.htm For calc of additional pipe length to add for fittings: see attachment. Once you’ve calculated pump head at target gpm, then look at the pump curves from Taco, Grundfos, B&G;etc., and find a pump that provides the target gpm at the calculated pump head, and you want your target gpm/pump head to fall about in the middle of the curve.

If your pump head is too high, you need to go to larger pipe size or shorten your run. As a practical matter, pump head 5’-30’ is about the operable range, I think, for most home applications, and the lower the pump head the more flexibility you have for system design

That's about all I can do unless you want to do it right the first time!

Kind Regards
Sting

Sting,

I think my last post must have seemed a bit like asking for advice in one post then telling you how I was going to do it in the next, so sorry for that. I do appreciate the advice and information that people are willing to share for free on the internet.

I agree that the best answer is large pipe. 2” runs from boiler to boiler would make the most sense. Gravity might even work to move the heat between the two boilers at that point, but I think a pump would probably help and/or be required.

I did find a pump sizing calculator at http://www.heatinghelp.com/article/11/H ... ng-Systems - it has a number of recommended pumps at the bottom. I think, based on reading the article, that I would need somewhere in the neighborhood of 45 GPM flow. This would be based on the rating of the old boiler as I haven’t gone around and measured all the radiators or counted their sections to get a good EDR rating.

Two of the things you noted in your last post are:

“There are some rules of thumb which have appeared in various threads. For heat transfer, maximum gpm: 1/2” - 1.5 gpm, 3/4” - 4 gpm, 1” - 8 gpm, 1-1/4” - 14-17 gpm, 1-1/2” - 22-25 gpm, 2” - 45-50 gpm.

One gallon per minute of flow (GPM) will move NO MORE THAN 10,000 BTU's of energy in a fault tolerant manor.”

So the second part first. My initial impression is that with 130k boiler I only need to move 13 GPM? I don’t really know, boilers are rated in BTU per hour right? I am guessing this could be used as a lower GPM limit for a pump?

The first part I am unclear about. Is this talking about moving hot water or transferring the heat from the hot water to the environment?

I do worry about the issue brought up of turbulence and head based on the calculator link provided. I do wonder what the best location to take off the cold and return the hot to the old system would be. Certainly the Stelrad setup is less than ideal.

Perhaps I should pipe it across the basement using 2” steel and then connect it to where the current Stelrad goes into the system. I will have to call around for some prices. It would allow for the potential that I would get it done before this heating season begins and allow me to get it completed without having to drain the system.

So what would you recommend if I “wanted to do it right the first time”?

Jay

Oh and I just plain didn't understand this:

For btuh calculation: gpm x delta-T x 500

What is delta-t and is 500 just a constant? The gpm referrs to the gpm of the pump?

OK thats all for tonight!

Jay

ok -- I went too far --- again -- skip past all the delta T crap


For instance, if the net heating load is 40,000, the proper flow rate is 4 gpm. So I am a little scared with your conclusion "I would need somewhere in the neighborhood of 45 GPM flow" You can get trouble free 4 gpm flow in 3/4 pipe

by these rules you can transfer 130,000 Btuh with 1-1/4 pipe because 1 inch will do no more than 80,000 -- 130,000 works even better in 2 inch pipe. But modern systems do not need oversize pipe -- HINT: YOUR NOT WORKING ON A MODERN SYSTEM

You will need to pump the coal boiler to your old system - it is unlikely you will get any gravity lift out of a remote boiler that was designed originally into the system

In the past I have had the "opportunity" (under the din of the nay-sayers) to try to convince folks to choke the flow of their systems botched up by modern Hi pressure pumps, and small pipe.

YOU have the opportunity to build correctly with pipe sized like the old dead guys did (pipes too big by modern standards) ; with a pump(s) that give the correct trouble free flow -- and get a dual boiler system that works.

Just look in your basement -- see many small diameter pipes in your old heating system?

READ THE FIRST TWO PARAGRAPHS of this article SLOWLY and let that thought sink in

stelradCoal wrote:

I did find a pump sizing calculator at http://www.heatinghelp.com/article/11/H ... ng-Systems -

Pictures?

Got a few photos and some details from the basement. I have added them as well as some notes with each.

What I am getting out of the heating help link is that an efficent gravity system can be maintained with the addition of a pump at times ( I had read elsewhere on the hh site that often old gravity systems need a pump to overcome frictional losses introduced into the system due to corrosion etc, or they are hooked up to a modern boiler that can not accomidate the heat load under gravity). So as I will need to add a pump to circulate between the two boilers it made sense to me to use one that was sized to my system per the article on hh.

At this time my only way to evaluate the system size is based on the btu's of the main boiler ( I have yet to inventory my radiators and calculate the ERD) I was going from memory on the 250k btu figure which is how I got a 45GPM pump. Now, after looking at the boiler again its either a 270k btu or 360k btu. I understand I don't need that much to move the total heat, but in the hh article it notes that although the boiler size may not need to be as large the circulator should be sized to the old system due to water volume.

Am I heading down the right track?

So,
I added a few pictures and am hoping for any additional advice.

As far as pipe sizing is concerned, is the general consensue that I can use 1 1/4" steel pipe to move the water from the coal boiler to the main boiler?

General consensus is that 1" PEX wont work for the same application?

I'm starting to think that going with a smaller circulator will be more semsable to start with as the reading I have done indicates over pumping the gravity system could cause problems. I really don't want to affect the flow on the gravity side, only move the hot H20 from the coal boiler to the gas boiler.

stelradCoal wrote:So,
I added a few pictures and am hoping for any additional advice.

More pictures will not help. YOU need to do some design analysis. All the equations, charts and rules of thumb are on this forum. Use advanced search to find them.

stelradCoal wrote:As far as pipe sizing is concerned, is the general consensue that I can use 1 1/4" steel pipe to move the water from the coal boiler to the main boiler?

You can also use a fire hose but that's not an idea solution. Do some calculations. Calculations with numbers that fits YOUR installation needs.

stelradCoal wrote:General consensus is that 1" PEX wont work for the same application?

But PEX-AL-PEX will likely work. Do YOU know the difference?

stelradCoal wrote:I'm starting to think that going with a smaller circulator will be more semsable to start with as the reading I have done indicates over pumping the gravity system could cause problems. I really don't want to affect the flow on the gravity side, only move the hot H20 from the coal boiler to the gas boiler.

An analysis using pump curves and the pipes resistance to flow will answer what size circulator is needed. The analysis needs to be tailored to your piping, so only YOU can do it. If your not comfortable with doing it, or have the time to learn how, hire a experience hydronic heating professional.

WTF ? the guy asked for help.

stelradCoal wrote:So,
I added a few pictures and am hoping for any additional advice.

nope - time to start working

stelradCoal wrote:As far as pipe sizing is concerned, is the general consensue that I can use 1 1/4" steel pipe to move the water from the coal boiler to the main boiler?

I would use 1-1/2

stelradCoal wrote:General consensus is that 1" PEX wont work for the same application?

yup

stelradCoal wrote:I'm starting to think that going with a smaller circulator will be more semsable to start with as the reading I have done indicates over pumping the gravity system could cause problems. I really don't want to affect the flow on the gravity side, only move the hot H20 from the coal boiler to the gas boiler.

use a pump that will move the capacity of your remote coal boiler -- I would use one that had a variable speed so you could make a flow based on how hard your firing the coal boiler. With out the hard data of a heat loss calculation we can only guess - but the "new" coal boiler is too small and your old boiler may need to be brought on line for some of those "colder" nights or weeks - but I do that in my little dwelling too.

cut Yanche some slack -- we all have our days

and his calculations are always dead on - in far more detail than I am willing to post.

Although I would note: his response is not the edifying advice I might expect from a site moderator -- its more something a hack like me would post.

Sting wrote:...At this point Ill suggest that you build a series system so the coal boiler heats the legacy boiler but we are going to have to be sure that we do it so as not to screw up the stratification of the gravity flow system.

I had the same thought. Here is my suggestion, and note that I am not a professional heating contractor...just someone offering their opinion. Hook the two boilers in series and use a variable speed circulator to keep things flowing properly. From the pictures it looks like he could cut into the return header and install two T's with a shutoff between them, then connect the supply & return of the coal boiler to these two T's.

The only large gravity system that I have seen in person was really messed up by connecting a modern boiler with a high-head pump...so I am going to agree with Sting on the 1.5" pipe. The 1.5" pipe should allow enough flow to harness the btu's produced by the coal boiler and avoid excessive turbulence/velocity in the main system. Lots of flow, nice and slow is the idea...

-Rob

Thanks for the input Yanche -

I'm not sure but I get the idea that my posts come off like I am asking for someone to tell me what to do and how to do it without the willingness on my part to do any of that research myself. I'm sorry if it seems that way. I have done quite a bit of reading and digging through this forum as well as the Heating Help site and the net in general for information, wisdom, ideas and so forth.

I have yet to run across a situation quite like mine. There are quite a few posters here on this forum who have dealt effectively with adding a boiler to their existing system. The primary difference is their existing system is not an old, non-pumped, gravity system. There are quite a few posters on the Heating Help site who deal with gravity systems in a number of ways, but finding ones that are adding on a boiler is not so common, and most are concerned with pumping the system as opposed to keeping the gravity system functioning.

Maybe I am not asking the right questions or don't know what to ask. I shall attempt to clarify. I wasn't planning on adding any more pictures, there had been a request for them a few posts back and I had thought there addition would bring additional commentary. And look it did

I haven't bothered to do a heat load calculation as I am not buying a new boiler, but have the boilers I already own so I can not size them to the HL of the house. Having said that, the Stelrad boiler rated at 110BTU was able to keep up with most heat demands except on the coldest of days, and this was run as an add-on gravity boiler. The coal-o-matic is rated at 130 so I am thinking I should be able to keep up with the heat demand on most days. There may still be a few cold ones that the gas needs to fire on, but it is clear I can use all 130k btu on the coldest day.

So - I need to move 130kBTU from coal-o-matic to main system.

Is that a sensible conclusion?

I think I can push this through a 1” pex pipe with an appropriately sized circulator. (And by pex, I do mean Pex-Al-Pex, I had mentioned it as PaP some posts ago, but then got lazy and referred to it as just pex.)

Will I be able to push all 130k btu’s through the 1” pipe?

The post from Sting a few posts back indicates that this is beyond the limits of the 1” pipe. In the posting he is referring to heat transfer.
Am I misunderstanding something here?

I have looked at the free calc site and what I get from that is that I gain about 9’ of head if I try and push 13gpm (using this figure based on the 130k boiler and a “rule of thumb” posted here). This is through 1” PVC (site doesn’t offer PEX as an option).

So, based on that number and the chart referenced http://www.builditsolar.com/References/ ... Curves.htm below I could reasonably expect good performance from a Taco 0010.

Does this make sense?

Now I get confused though as I read things like http://www.heatinghelp.com/article/11/H ... ng-Systems which indicates I should plan on flowing much more gpm to account for all the water in the system. Based on my look at the data plate on the gas boiler (which say 360kbtu) I need something in the neighborhood of 55gpm flow.

So, do I need to size the circulator for 55gpm or 13gpm?

What am I not thinking about?

Sting mentions the turbulence factor. Once that high preasure water coming from the circulator reaches those larger pipes it will create turbulence. Is this something that will cause me lots of trouble? Can I alleviate that by upsizing (1 ½” - 2”) the near-boiler piping on either end?

I don’t feel uncomfortable doing the analysis, but was hoping the expertise of the board could help me in figuring out what I need to know. I don’t have an infinite amount of time, but I really don’t think this is impossible. I would very much like to have this installed before the heating season gets really going. I find it very difficult to find a hydronic heating professional with experience in operational gravity systems, let alone adding a coal boiler to them, particularly as I am a fairly rural part of CT.

Thanks again for any and all help provided.

And look I got two more posts while I typed my own

If I understand what you want to do is place your coal boiler some distance away (other side of basement) and pump heat from it to your main boiler. The radiation from the main boiler is by gravity feed with perhaps some issues due to aging pipes. My solution would be to pump the coal boiler's heat to the main boiler and let the gravity system do it's job. Since you know rating of the coal-o-matic is 130K BTU let's use it as the design for heat flow. As other have pointed heat transfer is determined by temperature difference and flow rate. For water BTU = 500 x delta T x flow rate. Delta T is the temperature difference. So for you it's 130,000/500 = delta T x flow rate, simplified it's 260 = delta T x flow rate. Delta T is the coal boiler supply temperature minus it's return temperature in deg F. Flow rate is the water flow through the coal boiler (GPM). So you can see there are several ways to get the desired BTU transfer, low flow high temperature difference or high flow rate low temperature difference.

Now you need to make some educated guesses. Let's say your gravity system was designed to supply 180 deg water and on the coldest day of the year the return water temperature is 140 deg. Delta T = 40 deg. Let's also assume the load is the full boiler load 130K BTU. Your circulator pump and piping between the boilers needs to flow 260/40 = 6.5 GPM.

The circulator size and piping size is also a trade off. Small pump & large pipe vs. large pump & small pipe. But you know you must have a combination that will flow 6.5 GPM. Using analysis I've posted previously look at the following graph.



It shows for a particular flow rate, say 6.5 GPM, different circulators will have different flow heads. Head is a measure of flow resistance in the pipe. My graph illustrates 1 inch PEX-Al-PEX vs. copper. Other pipe types and sizes will be similar but different in the details. Bottom line would be you could have almost 500 feet of 1 inch copper pipe between your boilers and still meet design conditions. The 500 feet is the total length supply plus return plus the equivalent resistance of the elbows and other fittings.

As with any design the devil is in the details. My design temperature guesses for your gravity system could be right or wrong. But the example will show you the trade offs, between design guesses, circulator flow rate and pipe size. Lots of design examples for the pipe size details have been posted, just search for the analysis techniques.

Note: I edited my typo on the return water temp 9/14, was 160. In retrospect 140 is likely too low a return water temperature for a gravity system, even on the coldest day of the year.

Yanche,

Thank you for the information you posted. I did not get an opportunity to look at this in depth this weekend as I had army, but I will do that today. I am sure to have some additional questions.

Jay

Keep pluggin' and asking those questions brother. I can tell you that I personally learn a little something every time an install like this pops up and I thank you and the other posters for it.