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Gravity conversion

jcz
jcz Member Posts: 11
I am a homeowner with a gravity hot water system, and need to

replace the boiler.  I plan to go with a primary/secondary

setup.  After reviewing the recommendations on your website, I

have a few questions.



1. My current system has three sets of feed-return loops.  I

   plan to connect them in series to make one long system

   loop.  Is this appropriate?



2. The current feed and return pipes are 3".  I plan to use

   1 1/4" piping for the near boiler work.  Can I connect the

   3" pipe together to make the system loop with 1 1/4", or

   does that have to be 3" except for the connection that

   includes the secondary loop?

   

3. I understand that it is bad practice to mix copper and iron

   in a steam system, but would it be appropriate to use

   copper for the 1 1/4" work in this hot water system?

  

4. Are there any particular restrictions on the design of the

   secondary loop, other than that the shared section with the

   system loop should be less than 12"?



5. Your suggested secondary loop piping shows two valves, one

   in the flow to the system loop, and one in a return loop

   back to the return side of the boiler.  What are the

   functions of these valves, and how should they be adjusted?

  

6. How should I determine the size of the primary and

   secondary pumps?

  

7. If the answer to question 3 is "use iron/steel", does that

   also apply to the valves? or is bronze appropriate?

  

8. I want to use outdoor reset control.  How should the three

   controlled items (pri pump, sec pump, boiler) be connected?

   Should the reset control only control the primary pump?

   Should there be a separate high limit control for the

   boiler and secondary pump?  What does the house thermostat

   control?

  

9. If I decide to use two half sized boilers, how should they

   be controlled?

  

10.My house dates from the mid 19th century, but has been

   extensively updated.  In many areas I have no idea of the

   R factors of the construction as it currently stands.  Thus

   I am having a hard time with the heat loss calculation.

   I do, however, have the K factor from my winter oil bills.

   Would this be a more accurate way to size the new boiler?

  

If the answers to any of these questions are already found on

the website, please direct me there, I don't mind doing my

homework, but I have not yet found these answers. 



Thank you for your help.

 

 

Comments

  • Paul48
    Paul48 Member Posts: 4,469
    edited August 2012
    Questions

    Most of your questions are dependant on the type and size of boiler being used. Have you put some pressure to the system to test it yet? You have the potential for 3 zones, why change that? As for primary pumping requirements and configuration of primary/secondary piping, many manufacturers have very specific requirements that MUST be adhered to. You have to go through the process methodically....1. Heat Loss.....2. Address any issues with a pressurized system...3. Add radiation to any cold areas....4. Boiler selection.....5. Determine the best way to marry the new boiler to the existing system, while adhering to the requirements of the boiler manufacturer
  • Zman
    Zman Member Posts: 7,330
    Questions

    I am a homeowner with a gravity hot water system, and need to



    replace the boiler.  I plan to go with a primary/secondary



    setup.  After reviewing the recommendations on your website, I



    have a few questions.







    1. My current system has three sets of feed-return loops.  I



       plan to connect them in series to make one long system



       loop.  Is this appropriate?

    How do you plan to control the temp? Pipe them together and use TRV's or Make each one a zone with a circulator and a t-stat?







    2. The current feed and return pipes are 3".  I plan to use



       1 1/4" piping for the near boiler work.  Can I connect the



       3" pipe together to make the system loop with 1 1/4", or



       does that have to be 3" except for the connection that



       includes the secondary loop?

    The size of the boiler loop is dictated by the boiler size. 1 1/4" is correct for converting from gravity to the circulator on the secondary.



       



    3. I understand that it is bad practice to mix copper and iron



       in a steam system, but would it be appropriate to use



       copper for the 1 1/4" work in this hot water system?

    Copper is OK for hot water.



      



    4. Are there any particular restrictions on the design of the



       secondary loop, other than that the shared section with the



       system loop should be less than 12"?A drawing would be helpful.







    5. Your suggested secondary loop piping shows two valves, one



       in the flow to the system loop, and one in a return loop



       back to the return side of the boiler.  What are the



       functions of these valves, and how should they be adjusted?

    Probably isolation valves, where is this drawing?



      



    6. How should I determine the size of the primary and



       secondary pumps?

    The primary is dictated by the required gpm of the boiler. The secondary will be quite small and low head.



      



    7. If the answer to question 3 is "use iron/steel", does that



       also apply to the valves? or is bronze appropriate?



      



    8. I want to use outdoor reset control.  How should the three



       controlled items (pri pump, sec pump, boiler) be connected?



       Should the reset control only control the primary pump?



       Should there be a separate high limit control for the



       boiler and secondary pump?  What does the house thermostat



       control?

    What type of boiler? Many new mod/con boilers will do all of this.Oil burners may requires additional controls



      



    9. If I decide to use two half sized boilers, how should they



       be controlled?

    Depends on the boiler, some will control themselves, others require separate controller i would base this decision on your minimum load vs maximum load.



      



    10.My house dates from the mid 19th century, but has been



       extensively updated.  In many areas I have no idea of the



       R factors of the construction as it currently stands.  Thus



       I am having a hard time with the heat loss calculation.



       I do, however, have the K factor from my winter oil bills.



       Would this be a more accurate way to size the new boiler?

    Do your best at a heat loss. Also look at the K factor and radiation.



      



    Start with the heat loss calc and the type of fuel (are you stuck with oil?) Think about zoning and control then post some photos and sketches.

    Carl
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • JStar
    JStar Member Posts: 2,752
    Somebody has to ask.

    Where are you located?
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,221
    My opinion.

    First of all you have to do a heat los calc if you are replacing the boiler. You will have to figure all the r-values and everything. It is the only way to do it right.



    Typically when you are merely switching gravity to circulation rule of thumb for the existing boiler piping is to divide it by 1/2 and add a 1/4". That being said your situation is different. You are planning to replace the boiler and go with primary secondary piping. You will gauge your near boiler pipe size dependant on the btu load and the delta t you plan to run. I would use a mod-con boiler with a low head loss. Your boiler should have a built-in odr function. Use it. It should also have the capabilities of zone control and dhw. I'm thinking Triangle Tube. On your primary loop put a circ capable of handling max flow at the head created by the boiler and then over size it to work with the delta-t circs. On your secondary loops (3) i would use delta-t circs. This should allow your boiler to condense more of the time and improve overall system efficiency and comfort. I would not do a cascade system. Do parallel piping off the primary loop. Arrange the primary loop and secondary loops in such a way to promote gravity circulation. Use a directional swing check on each secondary loop. Not a flow check! Make sure to install it in a horizontal position. This will prevent reverse circulation through dormant zones while there is only one zone in operation. It will still allow gravity circulation while all zones are dormant. Reason for all this is, you have a lot of water in the system and we want it to circulate after all zones shut off. This will allow you to get the maximum btu's out of a cycle and reduce boiler short cycling.



    Check your distribution piping. If it is supply and return piped in reverse return you should be ok. If it is a single loop with monoflow tees you will most likely have to install at least 2 90's between the supply tee and return tee. This will establish resistance and promote flow through the radiator.



    Your other option is to hire a professional.



    Cheers,

    Harvey
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    New formula Harvey?

    I've always used Dans formula of 1/2 of existing pipe size and one size less than that. I've never heard of the 1/2 + 1/4.



    To the original poster,



    1. My current system has three sets of feed-return loops. I

    plan to connect them in series to make one long system

    loop. Is this appropriate? NO, ABSOLUTELY NOT. Series piping will not work, and you WILL be changing it back to parallel.



    2. The current feed and return pipes are 3". I plan to use

    1 1/4" piping for the near boiler work. Can I connect the

    3" pipe together to make the system loop with 1 1/4", or

    does that have to be 3" except for the connection that

    includes the secondary loop? THE RULE OF THUMB, as shown by others, dictates 1-1/4" pipe for THAT circuit, but the combined flow from all circuits will dictate pipe size. Go to the enginerringtoolbox.com for information regarding allowable flow in a given pipe.



    3. I understand that it is bad practice to mix copper and iron

    in a steam system, but would it be appropriate to use

    copper for the 1 1/4" work in this hot water system? THERE ARE THOUSANDS and thousands of "closed loop" systems out there with dissimilar metal mixes, that have had NO issues. The key is keeping the oxygen content as low as possible, and in a true closed system, it is not a problem.



    4. Are there any particular restrictions on the design of the

    secondary loop, other than that the shared section with the

    system loop should be less than 12"? GOOGLE JOHN SIEGENTHALER and read some of his writings on P/S piping. There are some other requirements in the way of straight pipe lengths before and after take off tees that need to be maintained.



    5. Your suggested secondary loop piping shows two valves, one

    in the flow to the system loop, and one in a return loop

    back to the return side of the boiler. What are the

    functions of these valves, and how should they be adjusted? TYPICALLY SPEAKING, as others have noted, those are service isolation valves, and should not need to be used for balancing. Balancing should be done at the radiators, if needed at all.



    6. How should I determine the size of the primary and

    secondary pumps? Do a site search on this web site for an article written by Steamhead or Frank WIlsey. You don't want high velocity.



    7. If the answer to question 3 is "use iron/steel", does that

    also apply to the valves? or is bronze appropriate?BRONZE IS FINE.



    8. I want to use outdoor reset control. How should the three

    controlled items (pri pump, sec pump, boiler) be connected? WITH THE USE OF PUMP ZONE CONTROL RELAYS. Google Taco controls. They make some nice ones.

    Should the reset control only control the primary pump? THE SENSOR, in most cases, can monitor the secondary loop.

    Should there be a separate high limit control for the

    boiler and secondary pump? NO LIMIT ON PUMP, limits for boiler operating temperature only.



    What does the house thermostat

    control? THE THERMOSTAT sends a call for heat to the zone pump control relay box. The relay box sends a call for heat to the boiler. The boiler turns on its primary pump.



    9. If I decide to use two half sized boilers, how should they

    be controlled? IF YOU USE A MODCON gas boiler, most boilers have a 2 wire interface to allow one control to control numerous boilers. If not, there are off shelf controls (tekmar) that can do the job for a non intelligent boiler.



    10.My house dates from the mid 19th century, but has been

    extensively updated. In many areas I have no idea of the

    R factors of the construction as it currently stands. Thus

    I am having a hard time with the heat loss calculation.

    I do, however, have the K factor from my winter oil bills.

    Would this be a more accurate way to size the new boiler? IN MY OPINION, nothing trumps a loss calc, and there are many on line freebies to help you get the job done. K factor takes into consideration the combustion/operating efficiency of your boiler, which may or may not be correct.



    Unless you have a lot of experience working with all of the trades (electrical, plumbing and mechanical) you may want to consider hiring a pro to do this work for you, because it can get pretty harry...



    HTH



    ME
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Primary-Secondary

    On all the large gravity systems I've seen, zoning is not an option. They are all one big zone and the multiple supply and return pipes in the boiler room are not dedicated with zoning in mind. If this is your situation, primary-secondary piping is not necessary and would be a waste of labor and materials since you are getting full system flow on a call for heat.



    You also want to install a good wye strainer on the system return to keep any concretions from the old piping out of the boiler.
    Often wrong, never in doubt.
  • jcz
    jcz Member Posts: 11
    location

    Gardiner Maine
  • jcz
    jcz Member Posts: 11
    location

    Gardiner Maine
  • jcz
    jcz Member Posts: 11
    reply

    I still need to decide what size of boiler to use...see question2 9 and 10.

    No pressure test, why should I do that?

    The various loops in some cases service different sides of the same room...not suitable for zones.

    1.Heat loss calc is an issue..see Q10.

    2.No issues I know of, system is already closed.

    3.No cold areas.

    4.See 1st line above.

    5.Trying to do primary/secondary per recommendations on this website in the gravity heating section.
  • jcz
    jcz Member Posts: 11
    reply

    1. Outdoor reset. Can't do multiple zones.  Gravity heating article in

    this website seems to recommend connecting the loops together to make a

    single long loop.











    4. See drawing in the gravity heating article on this website.











    5. See drawing in the gravity heating article on this website.











    8. Oil burner.
  • jcz
    jcz Member Posts: 11
    series piping

    Series piping seems to be what is described by Dan in the gravity heating article, although that diagram only shows two loops, whereas I have three.  Something seems missing here, what don't I see?
  • jcz
    jcz Member Posts: 11
    reply

    >>First of all you have to do a heat los calc if you are replacing the

    boiler. You will have to >>figure all the r-values and everything. It is

    the only way to do it right.



    Seems a little impractical...this house was built over 150 years ago, has been extensively remodelled several times since then, has been updated in several ways, has been reinsulated in several ways...unless I open up walls and ceilings in a number of places, how to guesstimate R values?



    I'm thinking a more reasonable approach will be to pipe for multiple boilers, start with one sized based on the K factor, and add more if and only if it proves to be needed.



    Thoughts?
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,221
    Thanks for the

    correction on the formula Mark.



    To the original poster,

    Take a couple pictures of your boiler room. Draw a schematic of your existing distribution system, including pipe sizes. Post it on this wall. There are plenty of good folk on here that will be able to guide you through the process step by step. I'm not trying to imply that you won't be able to do it yourself but what you are attempting is rather difficult what with all the variables.



    Cheers

    Harvey
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Seems a little impractical...

    Look at it this way. All heat loss calculations involve some error because of all the imponderables. But the better job you do, the more accurate your calculations will be. And the more accurate, the more likely you will have the right size boiler. Say you take two days doing your heat loss, and it is 20% off. That will still be way better than putting in another boiler the same size, or slightly larger, than the one already in there. They tend to be oversized, sometimes more than 100%. So even a heat loss that is 50% off can be an improvement.



    And the savings will pay off not just once (lower boiler cost), but year after year, in fuel savings.
  • jcz
    jcz Member Posts: 11
    reply

    Counter arguments:

      1. Heat loss calc in this case will be full of estimates, calcs based on estimates differ by

          as much as 40-50%.  How is this better than an estimate based on actual K factor?

      2. K factor estimate says current boiler is about 200-300% oversized, why not go with a

          small boiler and leave provision for a second one if ultimately needed?

      3. I don't feel at all qualified to do a reasonable heat loss calc, and after consulting with

          several local HVAC contractors/consultants, the ones who seem to know what a heat

         loss calc is agree that it would be very difficult and unreliable for this particular house.



    Thoughts?
  • Zman
    Zman Member Posts: 7,330
    Details

    How many square feet are you talking about?

    What type of fuel will you be using?

    What is the existing boiler rated?

    Do you have pictures of the 3 return loops? Are there really 3 separate 3" supply and returns (6 pipes) coming into your boiler room?

    Carl
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • jcz
    jcz Member Posts: 11
    reply

    >>How many square feet are you talking about?

    around 3000





    >>What type of fuel will you be using?

    oil





    >>What is the existing boiler rated?

    max 360,000 btu

    2GPH nozzle





    >>Do you have pictures of the 3 return loops?

    no pictures right now



    >>Are there really 3 separate

    3" supply and returns (6 pipes) coming into your boiler room?

    yes
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    How is this better than an estimate based on actual K factor?

    How is the "K factor" different from a heat loss calculation? In other words, what went into you calculation of the K factor? How is your K factor more accurate that a heat loss done the best you can? Or find a qualified professional to calculate the heat loss for you.



    My installing contractor paced two sides of my house to see the square footage. But my house is a Cape Cod, and I do not know if he doubled his calculation for the upstairs or not. But he came up with a boiler size of 105,000 BTU/hour. This house is about 60 years old, so it may be a little easier to estimate. I did a heat loss and came up with about 30,000 BTU/hour when it is 0F outside. Since design temperature around here is 14F that says it never get that cold here. In any case, I did not get the size boiler the contractor recommended, but the smallest one in the product line, 80,000 BTU/hour (input). It is about twice the size it needs to be. So much for square feet and K factor.
  • jcz
    jcz Member Posts: 11
    reply

    K factor is empirical rather than theoretical.

    i.e K factor is based on actual usage history, with the only factor to guesstimate being the efficiency of the boiler.  Heat loss calc would involve guessing about multiple R factors, infiltration rates, etc. all of which can vary widely.



    BTW, K factor has no relation to square feet......
  • Ironman
    Ironman Member Posts: 6,868
    The Problem with Using K Factor Alone

    K factor is one of three steps that I use in sizing. But I only use it for comparison purposes. A manual J, or equivalent, based load calc. is the only way to assure accuracy. Determining the amount of insulation, R value, etc., is not difficult for a pro that knows what he's doing.



    Using K factor does not take into consideration the efficiency of your old boiler. It can only be estimated. And that estimate may be nothing more than a wild guess at best, especially with a very old oil boiler. Even if you knew the manufacturer's efficiency rating, that was under ideal lab conditions and doesn't take into account real world operation. For instance: an eight inch of soot in an oil boiler will reduce operating efficiency by about 18%. Too much excess air to the burner can very easily result in a 15% loss of efficiency even though it's burning cleanly. There are just too many variables.



    You'll do yourself, your pocketbook and your new equipment a huge favor by getting a real hydronics pro to calculate and design your system instead of trying to piecemeal it together from info and advice you receive here or elsewhere.



    The best minds in the business contribute here, but someone needs to see the job and know what your expectations, budget and total scope are to make the best design available.



    There are pro's in Maine that frequent this site. MEPlumber is on that comes to mind. I highly suggest you contact him.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    K factor is based on actual usage history

    Does that mean that you calculate your K-factor from the daily fuel consumption and the daily degree-day records for the previous couple of decades, along with wind speed data, and recordings of any setbacks used?



    I could not calculate my K-factor this way because I had only a single oil consumption data point per year, and only for the past three years or so. And while I could get daily degree-day information for quite a while, I could not get wind speed, and do not know, offhand, how to use it. And I use a little setback in one zone and none in the other. But the one zone probably has more heat loss after my boiler replacement because I put in extra baseboard up there, and so it finally got warm enough there, and probably increasing the heat loss thereby. And I recently replaced the cheap aluminum builder's grade windows of 1950 with 2006 thermopane type Marvin windows, added insulation, and a new roof, and outdoor reset, so the historical date would have been little use.



    At some point, it seems to me that it would be simpler to calculate the heat loss. Especially if you did not have sufficient environmental data.
  • Ironman
    Ironman Member Posts: 6,868
    Very familiar with the article

    As are most of us here. But it deals with several piping scenarios.



    Under most conditions, I would use p/s piping. But, as stated earlier, the manufacturer's guidelines need to take precedence.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Zman
    Zman Member Posts: 7,330
    Boiler

    I would have to agree that your heat loss will be a serious SWAG. It is really the best SWAG you have.



    A couple of thoughts:



    There is no way your heat loss is greater than 50 btu/ft. We are talking about a house not a circus tent,right?. So you are correct that you are way oversized.



    As long as you get fairly close on your boiler sizing I don't think 

    you will short cycle. With 3" branch piping you have more buffer than

    Moosehead lake.I see no reason for 2 boilers.



    Control your system with outdoor reset. I would break it into 3 zones

    using zone circulators. You could use wireless t-stat if you need to.Do not make one big loop.



    You don't really need primary/secondary as the cast iron boiler doesn't care about flow rates.



    You do need to think about boiler condensation on start up. Ease valve?

    I agree with Ironman. This a not a great DIY project.

    Hope that helps,

    Carl
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Paul48
    Paul48 Member Posts: 4,469
    Gravity

    I have an old gravity conversion, and would like to have the 3 way split you have. I will agree that the radiators are not specifically seperated as proper zones. They are seperated by side of the house though. I would pump it as 3 zones, using Delta-T circulators. I would give careful consideration of which rooms in each zone were best for placement of the thermostat and put TRV's on radiators in the other rooms, or just the smaller rooms.
This discussion has been closed.