Gravity to circulator conversion with new boiler - circulator and pipe sizing

sovavto

Converting a gravity hot water system to a circulator driven high efficiency boiler Burnham Alpine.

Burnham alpine manual (attached to this post) <a href="http://pro.usboiler.net/pdf/Alpine%20I&O%20w%20Sage%202.1%209-10.pdf">http://pro.usboiler.net/pdf/Alpine%20I&O%20w%20Sage%202.1%209-10.pdf</a>

See page 47 diagram for heat and indirect DHW piping.



I have read on this site (<a href="http://www.heatinghelp.com/article/332/Gravity-Hot-Water-Heating/72/Gravity-Hot-Water-Heating-FAQ">http://www.heatinghelp.com/article/332/Gravity-Hot-Water-Heating/72/Gravity-Hot-Water-Heating-FAQ</a>) that a bypass (loop) is typically used when converting from gravity high mass system to a modern boiler (where the near boiler pipe diameters need to be decreased).

The old (current) gravity boiler has a single 3 inch supply and a a single 3 inch return inlet. Each of these 3" pipes split out into two 2 inch pipes right near the old boiler, so effectively there are 2 two inch supplies for two sides of the house and 2 two inch returns correspondingly. Everything is a single zone, there are rads on 1st, 2nd, and 3rd floors; most of them cast iron original, several steel baseboard rads have been added recently (and worked with gravity).



The new boiler I believe should be piped (near boiler piping) with 1" pipes.



I wanted to validate my understanding on the following two questions:



1. Does the new near boiler pipe size of 1" sound like the best fit for this conversion?



2. Since I don't see the Alpine manual mention the bypass piping near the boiler, should the bypass piping still be done given the high mass water volume in the system, or would it be better to follow the instructions in the manual directly?



3. Finally, according to <a href="http://www.heatinghelp.com/article/343/Circulators/238/Sizing-Circulators-for-Hot-Water-Heating-Systems">http://www.heatinghelp.com/article/343/Circulators/238/Sizing-Circulators-for-Hot-Water-Heating-Systems</a>, the circulator that I would need (for this gravity conversion job) is about 15 GPM (given 93 MBTU output). Am I correct in understanding that this circulator (refering to the burnham alpine manual page 47) should be the "heating circulator" on the heating loop? Should the boiler loop circulator be the same size, or different, or is it even needed?

Zman

On track

You are on the right track with this one. The near boiler piping from the manual should work fine,1" is appropriate. The boiler loop circulator should be sized per the boiler manufactures recommendation.



The diagram from the manual shows a primary/secondary configuration, this is not a  "bipass loop" to protect the boiler against condensing. This insures proper flow through the boiler.Yes you do need it. Pay close attention to "dimension A" on that drawing. The closely spaced tees are very important.



Steamhead's chart on gpm and edr or btu is referring to the size of the emitters (radiators)  not the BTU capacity of the boilers.You need to look up the values of the radiators and do it that way.



Have you considered going to 2 zones? 2 heating circs instead of one. It sounds like you have a good opportunity.



Dan's rule on downsizing the gravity piping is 1/2 the size and the next size down.Your 3" would go to 1 1/4" for one zone. Your 2" would go to 3/4" if you did two zones.



Someone who does gravity conversions will likely chime in. I do not. I am just trying to point you the right way.

sovavto

introducing zones

thanks for the reply.



i would like to clarify:

- 1" recommendation for near boiler piping is good

- 3" pipe would drop down to 1-1/4" and the 2" pipe would drop down to 3/4".



I understand the 1/2 size and one size down formula. What I'm not sure about is how do these new figures (1-1/4 and 3/4 diameters given by this formula), relate to the near boiler recommendation of 1". Are the "1/2 size and one size down" for some locations other than "near boiler piping"?



As for the zones - I though about it. It certainly would seem like there's potential to add zones, however, the physical setup is something like this:

Imagine the boiler being in the middle of the house. One set of supply/return runs to the front of the house and supplies heat for 2nd and 3rd floor of the front rooms. Another set of supply/return runs to the rear of the hour and supplies heat for the 2nd and 3rd floor of the rear rooms. Given this, would there be any benefit to introducing 2 separate zones? I can't think of any benefit, but if from heating or ciculation point of view it may be better, I'd like to hear your thoughts.

Eric_32

with the system being gravity...

I bet it is a 2-pipe system... 1-pipe going to every supply on every radiator, and the 2nd pipe going to every return. the only way water flows into the 2nd pipe is thru the radiators, so the radiators will be warm before the boiler gets warm. It will be a lot of water and btu's going out before it comes back.



This will wreck havoc with a standard boiler unless it has a bypass piped in. The Mod/Cond boilers will love this. You will be able to set your ODR curve nice and low with cast iron as they put out heat at a much lower temps than fin baseboard.

The bad thing about this system is it probably is not a reverse/return, as the first on the supply is the last on the return and so on. With that set up, every radiator will heat at the same time. Otherwise the closest radiators to the boiler will heat faster than the ones at the far end of the house. 

How many radiators are on this system? Can you take some pics of the boiler and some tees that run off to the radiators in the basement.

Paul48

Radiators

If the radiators are sized properly for the rooms,I'd leave it as a single zone and let the mod/con do it's thing. If there are bedrooms that people like to keep colder,I'd install TRVs on those few radiators.One of the biggest problems we see on this site, is with zoned mod/con boilers. Just my 2 cents.

sovavto

piping for gravity system

pressed the post button too early...

sovavto

piping for gravity system

Please see attache pictures. One highlights the 2 supply lines (start as 3" right near boiler, then drop down to 2" copper and the convert to 2" cast iron). Second picture show same boiler from the back where you can see the same diameters for 2 returns (2 x 2" return merge into a single 3" return right near the bottom of the boiler).



So, I believe this is a 2 pipe system as you mentioned. The supply pipe supplies radiators and the return pipe comes down from the radiators. In other words, all the radiators run in "parallel". We have lived in the house for 2 winters so far, and I can't say that there has been a noticeable difference in terms of top (3rd) floor radiators getting hot before 2nd floor radiators.



While I like the fact that this a gravity with millivolt gas valve (not dependent on the power grid), one of the problems has been very long heating cycle. I.e. the thermostat drops and sends a heat call, but by the time the system actually warms up it is 2 hours later and the temperature on the thermostat has dropped even further. Conversely, once it is fired up, and the thermostat no longer calls, the boiler and radiators continue to give off heat for another hour or two. The end result is 4-5 degree temperature swing between the lows and the highs.



My expectation is that with a circulator in the system, there won't be as much of a delay between the heat call and the actual temperature increase since the warm water will reach the radiators quicker.

sovavto

single zone and boiler efficiency

Yes, the original cast iron radiators seem to be sized proportionally to the room size. I think they were possibly slightly undersized before new window and insulation, but now (with windows and insulation) they are probably slightly oversized. But, like I said, the size of rads is proportional to the size of the rooms they are in.



We have also added 3 new steel base board radiators (not the fin type, but runtal baseboards), and after some tweaking they have been working with the gravity system last winter.



All together, there are about 12 radiators (9 cast iron and 3 steel); and 2 more steel ones will be added later this year, for a total of 14.



In terms of boiler efficiency, my understanding is that when there's only one heating zone and the boiler runs at more heating capacity than it would have had there been 2 or 3 smaller heating zones, it will actually be more efficient. On other words, if the heat lost for the dwelling is for example 80 KBTU, a single zone for 80KBTU will be better than 2 smaller zones (e.g. 30 + 50 KBTU), because the boiler will not run in condensing mode with the smaller zones since it will require less energy to heat each zone individually. Is that correct? If there had been multiple zones, I would not expect (or calculate my heat loss) them both for call for heat at the same time.

Paul48

Tank

Have a tank in the attic?

sovavto

expansion tank

yes, the old gravity system I have has a steel expansion tank which is located immediately to the left of the boiler (it would have been in the 1st photo above, in the ceiling between the floor joists). It just didn't fit in the picture.

icesailor

Gravity Vs. Pumped:

Your problems will increase tenfold when you put the circulator in. Your heating lead/lag is due to the thermostat heat anticipator not being set properly.

If the thermostat was replaced with a modern one, it probably isn't set properly for a milivolt gas system. Your real problem is that you need ODR to control the system temperature. The boiler input is no longer matched to the load because you made improvements. Make sure that you use the ODR feature on your new boiler so that you eliminate the problem. You don't want the system water that is circulated by the pump to be any hotter than it needs to be.

IMO

IMO.

sovavto

gravity vs pumped

The current setup is OLD mercury thermostat connected to millivolt on OLD gravity boiler.

Once I have the boiler replaced with the new mod/con model, the thermostat will also be replaced with a new one. I believe alpine boiler comes with SAGE controls and includes the ODR module.



Does any one have any insight or experience with the burnham's SAGE control and its ODR module?

nicholas bonham-carter

Setback

Are you setting back the temperature at night? For opinions on the sometimes dubious savings from setback, do a search here.

If you were to keep a constant temperature setting, is the house evenly heated?

I presume there are restrictor disks in the valves of the radiators to balance the gravity flow to various levels.--NBC

icesailor

Old thermostats:

Does your "old" thermostat have three wires connected to it and where it goes into the control on the boiler, is there signs that a wire is left off and only two connected? Does the three wire have a red, white and blue set of wires which are connected to the thermostat and the boiler control has the red and white connected and the blue left off? If the thermostat is a Honeywell series 10, the blue wire is the wire that will run it.

With gravity systems that really are old ODR systems, you can't have deep setbacks without raising the water temperature in the system. The system is working normally. You just don't understand the intricacies of the system.

In spite of all the cool things that you can do with a Mod/Con, a 4-way mixer, will do a better job of controlling your pumped gravity system and the Mod Con will work better.

But that's my minority opinion.

sovavto

setbacks

No. The problem with temperature swings I am referring to was not caused by setbacks. The problem manifests itself without touching the thermostat at all. There is no outside sensor either. It's an old mercury thermostat with two wires going to the boiler/gas valve.



The issue (and tell me if this is just the way that gravity systems are supposed to work) is this:

1. imagine it is 0F outside (cold)

2. thermostat calls for heat when air in the room drops to 70F

3. it takes 2 hours for the system to get the radiators in the room hot

4. within those 2 hours, the temperature drops further to 67F

5. during the next (3rd hour) the room heats up from original 67 to desired 70 degrees

6. at the end of 3rd hour thermostat stops calling for heat

7. during the next (4th hour) the room continues to heat up from desired 70 degrees to 72 degrees because the radiators and water are still hot

8. by the end of 4th hour, the room is at 72 degrees.



So this is the temperature swing that I am talking about - about 5 degrees on cold days when the house looses heat rapidly. Is this expected? Besides insulating hte house (which was have already done), can this be further remedied by the mod con boiler? Is the outdoor reset going to address this (by anticipating) and starting to heat earlier?

Paul48

Yes

The boiler is sized for the requirements of the whole structure.If the emitters are sized correctly for the heat loss of each room,a single zone is the best option(opinion).Chopping the structure up into many zones was a comfort improvement based on old school technology. Fuel was cheap, and no one cared if the boiler was over-sized, or double the size actually needed.We conquered cold areas in the home by zoning, and no one cared that we ran that 250k btu boiler to heat a 5x10 bathroom.Unfortunately,we're having a hard time getting away from those ideas. We still see folks here describing a zone attached to their mod/con as 10 feet of fin-tube, and wanting to know why they're having problems.Again, these are my opinions and some may disagree. You make your own decision based on common sense and logic.

SWEI

outdoor reset

should fix this.



In a properly tuned ODR system, the thermostat serves basically as a high limit controller - set it a few degrees over your desired space temp and it will prevent the boiler from continuing to run if the room gets warm from another source (a wood stove, solar gain, cooking heat, or a bunch of dancing partygoers.)