Pump size and flow issues for an old converted gravity system


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Old 03-23-11, 05:41 AM
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Pump size and flow issues for an old converted gravity system

I am trying to determine the pump size and set up for an old gravity piped system. The system has an oil fired Burnham 125 btu steel boiler. The near boiler piping comes off 1 1/4" and reduces to 1", has a Taco 007 F5, and then there are three runs on two zones. One run goes to first and second floor on the front of the house and is 3" horizontal (pitched) supply and return with 3/4" runs to the radiators; the second run is on the same zone and piped the same as the first run but goes to the rear of the house first and second floors. Both runs are piped from 1" from the near boiler piping to 3". There is an added third run that is not gravity; it is 3/4" looped running to old base ray base board. The first two runs are on one 1" zone valve and the base ray zone is on a 3/4" zone valve (both zone valves are wired together so they are all working off of one zone/thermostat right now). Hot water is made separately in a gas hot water heater. I didn't measure the radiators to determine their btus, but the house is about 2000 sq feet and I am estimating that the front and rear runs have about 50k btu of heat emitters each. There was no light in the boiler room so the pictures did not come out at all.

Problems: The system is not balanced at all, and with all the cold water in the large pipe and radiators, the call for heat takes almost an hour just to get the water in the system up to temperature. The circulator short cycles while the water is heated.

A boiler install company did the Burnham install seven years ago and the customer lived with it until now. She is on a fixed income and looking to corrrect it without spending much money. She will consider spending a little more if there is clear fuel savings in the short term to be achieved with the corrections. She can't even afford small piping changes, so we have to pick the solution very carefully.

Looking at it, we have determined that that the the size of the pump (one 007) and having a single pump for the three runs of different sizes is not right, and the 1" near boiler piping is wrong for the btus needed.

If I had my way, I'd take out the 3", split the runs by floors and put them on separate zones, an outdoor reset, a mixing valve and other neat things, but the budget is not there.

The plan right now is to re-pipe the near boiler piping 1 1/4" to a pump manifold and then set it up with a separate pump for each run to solve the flow issues. We were going to keep the 1" to 3" piping for front and rear runs. We figured a 007 for the 3/4 base ray zone and the front and rear runs each get a 010 pump. We were also thinking of just running it cold start to eliminate the circulator cycling issue.

Questions:
1. What size pumps should we use on the 3" runs to get the water moving around properly? The water is choked down to 1" from the pump for about two feet to the 3' pipe.

2. Should we convert it to primary/secondary to deal with the volume of water and the delta T of the returns (and can we even do that within budget and have customer heat savings to recover the cost)?

3. Should we run a boiler bypass to fix the problem of the delta T of the returns, maybe even with a mixing valve?

3. Should we set it up with an oudoor reset on a budget (Tekmar is not budget friendly)?

4. Should we run it cold start (It has a 7124 with low setting disable), which will let the circulator run on call for heat regardless of kettle temperature, which will resolve the pump short cycling from the cold water return/ Delta T issue, almost simulating outdoor reset to an extent by pumping colder water through the system and heating all system water evenly rather than heating the kettle and having Delta T problems on the returns?

Long thread! Thanks in advance for your input!
 
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Old 03-23-11, 08:01 AM
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Quick thoughts.

I would not mess with replacing/resizing piping. Shortest distance to better performance is likely going to be a simple boiler bypass, like here:

Bypass_Piping_Explaination

With the money saved from not repiping, you're more than covered for an outdoor reset like tekmar 256, about $120-150. Or go with a Taco ZVC-EXP to control the zones (and fix that circulator problem -- as you note it should run as long as there's a call for heat...) and add the PC700 outdoor reset plugin (it's actually tekmar guts).

All that big piping has very low resistance to flow, so there is probably not a problem with flow rate (it will be high if anything). If you are in an area where electricity is in the range of 15-20 cents/kWh, then I would strongly consider a Grundfos Alpha for this system. It will have a significant run fraction. It's somewhat pricier than a standard circulator, but has two major virtues for your application:

1) multiple modes to choose from to establish proper flow rate
2) very low electrical consumption

The extra cost will be paid back in maybe 3-5 yr, tops. Plus you have huge amount of settability to get the flow rate right.
 
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Old 03-23-11, 09:20 AM
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Your pipes are plenty big. You need to figure the heat loss of the house, not the btu output of the emitters to know what size pipe you need. Figuring the output of the emitters will aid you when it comes to setting your reset curve if you install an outdoor reset.

Does the boiler have a tankless coil that provides domestic hot water?
 
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Old 03-23-11, 11:05 AM
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Originally Posted by seaworthy41
Questions:
1. What size pumps should we use on the 3" runs to get the water moving around properly? The water is choked down to 1" from the pump for about two feet to the 3' pipe.

2. Should we convert it to primary/secondary to deal with the volume of water and the delta T of the returns (and can we even do that within budget and have customer heat savings to recover the cost)?

3. Should we run a boiler bypass to fix the problem of the delta T of the returns, maybe even with a mixing valve?

3. Should we set it up with an oudoor reset on a budget (Tekmar is not budget friendly)?

4. Should we run it cold start (It has a 7124 with low setting disable), which will let the circulator run on call for heat regardless of kettle temperature, which will resolve the pump short cycling from the cold water return/ Delta T issue, almost simulating outdoor reset to an extent by pumping colder water through the system and heating all system water evenly rather than heating the kettle and having Delta T problems on the returns?
To be clear and further explicate...

1) Don't use three pumps for this. Among other things you are committing someone on a fixed income to huge, unnecessary parasitic electrical use, the cost of which will only go up. Use one circ. There is plenty of oomph in something like an Alpha to do all this big piping plus the baseray zone. Consider that a 0010 draws 126 watts. Now double that for the other one. Now add another 80 watts for the 007. When all three zones are calling (and with only one thermostat that's going to be a lot of the time), that is 333 watts. i.e., one third of a kilowatt PER HOUR. Now multiply that out by runtime hours over a heating season and that's just ridiculous. A conservative estimate is around 1000 kWh per heating season just to run the circs. At $0.2/kWh, that's $200 per heating season. You wanna pay that electric bill? Every year?

2) As noted above, a simple bypass should address this problem.

3) I would. All that cast iron mass and water volume is great for maximizing the benefit of outdoor reset. The space temperature will even out and provide better comfort. It will pay back in pretty short order.

4) Sure. But pay attention to your wiring so that the circ runs continuously on a call for heat, like it's supposed to.
 
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Old 03-23-11, 12:56 PM
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Xiphias, thanks for the responses. A few more questions:

If I run one pump, what size pump should I be running (If I don't run the alpha)?

Have you tried the Wilo pumps? Opinion?

If I run one pump, won't I have a problem with the 3/4" baseboard run moving much faster (more friction but much less volume)? So would I be choking back a balancing valve on the 3/4 run to even up flow?

So the near boiler piping set up would be one pump (supply side) 1 1/4" flanges and piping which reduces to 1" zone valves for each of the 3" former gravity runs and a 3/4" zone valve for the baseboard loop?

If she opts to not have the outdoor reset, should I run it cold start and what should the high limit be set at? Do I have a flue gas condensation issue?

On the boiler bypass, should I just valve it or should I use a mixing valve?

Drooplug, I don't think I am concerned with the heat loss as I am not resizing/replacing the boiler. I needed to know the BTUs of the heat emitters because with 1" supply, I can only get about 75k btu of heat, which by my estimation is leaving a lot of btus in the boiler and not running it in the system. I am fairly convinced that I need to pipe it 1 1/4" up to where it splits off for each run, after that, assuming that the emmiters in the bigger runs are about 50k btus for each run, the 1" should be fine. Maybe I am missing something though. Domestic water is a separate gas hot water heater.
 
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Old 03-23-11, 02:02 PM
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Originally Posted by seaworthy41
Drooplug, I don't think I am concerned with the heat loss as I am not resizing/replacing the boiler. I needed to know the BTUs of the heat emitters because with 1" supply, I can only get about 75k btu of heat, which by my estimation is leaving a lot of btus in the boiler and not running it in the system. I am fairly convinced that I need to pipe it 1 1/4" up to where it splits off for each run, after that, assuming that the emmiters in the bigger runs are about 50k btus for each run, the 1" should be fine. Maybe I am missing something though. Domestic water is a separate gas hot water heater.
That's not how it works. Regardless of size (unless too small), the boiler will only put the same amount of BTU's into the house that are lost. The output of the emitters doesn't matter as long as have you have at least the same amount as the heat loss. You only need to deliver enough BTU's through the pipe to match or exceed the heat loss.

My house has a loss of less than 50k BTU's. My radiators put out about 100k BTU's. My two heating zones are supplied with 3/4" pipe. I can only put a total of 80,000 BTU's into my radiators. That's more than I'll ever need.

Once you change that boiler over to cold start, install the bypass, and get that pump to run continuous no matter the water temp, you will see a dramatic difference in comfort. That boiler will probably never bring the entire system up to 180. The thermostat will satisfied far before that happens.
 
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Old 03-23-11, 06:19 PM
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Take a few minutes and figure the head and flow for these "zones".

Simple cookbook here: http://www.taco-hvac.com/uploads/Fil...irculators.pdf

But in terms of heat output, flow rate is much less influential than water temperature. You want to ensure that your temp drop isn't so high that the ends of the zones are underheated. The bypass should take care of the boiler issues.
 
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Old 03-25-11, 05:59 AM
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Xiphias, I am struggling to calculate the circulator.

For the large gravity runs I have TEL as follows 3"=240'; 2"=15'; 1 1/2"=15'; 1"= 50' . The baseboard zone has TEL of 3/4"=55'.

How do I round this to one number since the pipe sizes are not the same. When I do it all as one size, I come up with a negative number for head. Does the math change since it has a short run of 1" going to the 3" basically throttling it back? How do I calculate that into the total math?

I am still not sure about the 3/4" run. The GPM needs for the 3" seem to be over 45, yet the 3/4 seem to be about 5. It seems to need much less gpm and if it runs on the same circulator as the 3", how do I balance the flow? Choke it back with a balancing valve?

My needs seem to be off the chart from taco: over 45 gpm with no head loss on the 3" piping.

Other questions from before:

If she opts to not have the outdoor reset, should I run it cold start and what should the high limit be set at? Do I have a flue gas condensation issue?

Thanks!
 
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Old 03-25-11, 06:34 AM
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The velocities will change in the different pipe sizes but you need to be concerned about the flow required and resistance in the highest resistance loop since all are connected together. The reason for the heat loss is to determine flow required. If the heatloss is 100k (which I doubt) the flow required would be 10 gpm total for all loops.
The flow in the 3" pipe will be slower then the flow in the 3/4" pipe but the proper amount of gallons will be flowing.
You stated that the 3" = 45 gpm but that does not mean you will be flowing that much. If you need 6 gpm in the CI radiator loop than you will be moving 6 gpm in the 3" pipe, 6" gpm in the 1" pipe but at different velocities.
Boiler bypass (not system bypass) is the cheapest solution here. Then reduce the flow through the boiler so it stays running. Take the DOE output or gross output which ever rating you have and divide it by 10,000 and that is the maximum flow the boiler will handle and 1/2 of that is the minimum flow the boiler will handle.
The other way to know the flow through the boiler when adjusting the valve to determine boiler flow is to adjust the valve until you have a delta-T through the boiler of 30 - 40 degrees. The balance of the flow will go through the bypass. The pump will stay on during the heating season continuously and give a much more even heat throughout the system.
Again use a boiler bypass not a system bypass. No need for a mixing valve.
 
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Old 03-25-11, 06:47 AM
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The boiler bypass will protect the boiler from condensation problems with the deltaT set properly. A boiler bypass exists between the boiler and the pump. Make sure you have a temperature gauge installed on the supply and return so you can set your deltaT. Switching to cold start will save a lot of money in itself.
 
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Old 03-28-11, 12:34 PM
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Thanks rbeck, good information. It has a bypass already (although it is piped wrong (from the system side of the pump to the return)) and the plan is to properly bypass the boiler to keep the bulk of the system water cooler. Thanks for the info for setting the bypass valving. The boiler is a Burnham RSA 125, and Burnham actually provides near boiler diagrams, which is what I plan to follow. I am also going to set it up with a Tekmar 256 with outdoor reset.

I did not do a heat loss calculation as it is a lot easier just to upsize the pipe from 1" to 1 1/4" while I have it apart, and I can ballpark the number for the pump sizing. Let me ask you this, though; do you do a heat loss calculation for all jobs that just need a pump resized or do you ballpark it? I can't see the justification for all the hours spent on the calculation, and the cost to the owner, when a very good guess can be made, unless you are changing out the boiler. The home is 2400 sq feet (a little bigger than I mentioned earlier) Queen Anne with two floors on a corner lot with no house next door (three sides are exposed to wind). It was built around 1910 and has baloon construction and no insulation. My guess is somewhere between 75k and 100k of heat loss. Since the near boiler piping is 1", which is good to only 75k, and since the piping will be apart and all I need are new pump flanges and very little piping to upgrade to 1 1/4" and get up to 160k btu capacity, it seemed to be a no brainer to increase the pipe size. I was tempted to eliminate the 1" piping going from system side of the pump to the 3" run, but I don't think it needs to be done (It will be 1 1/4 to the manifold with two runs of 1" running for about a few feet to the 3" pipes.

Regarding a single pump vs. two or more pumps, since the 3/4 run can use a small pump, should I just add a second pump for the two 3" runs and size that circulator a little larger, or does the cost of running the second pump (as per Xiphias) make it not worth the effeciency of design?

Another question, if I am running it with the outdoor reset and with the boiler bypass, should I set it up cold start, or keep a minimum temperature on the low limit to protect from the flue gas condensation?

Thanks for all the help, the disucussion has been very helpful.
 
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Old 03-28-11, 03:42 PM
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Don't focus to much on your near boiler pipe sizing...
You will be running a 40 degree or so delta T across the boiler with a proper set bypass (rule of thumb...). At 40 degree delta T that 1" is good to over 150,000 btu. right.

Besides, even with higher velocities thru a 1" pipe flowing 125,000 btu, the length of the pipe would be so short to not really influence the over all head of the system.

Get your bypass set properly, put some money in controls. You should have a very happy customer. I can't count how many CI boilers I have put into homes with old gravity CI rad set-ups. As long as that bypass is right, your boiler will heat the space nicely and protect itself.
 
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Old 05-23-11, 08:01 AM
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Thanks for all the help. The system is all set up with the proper piping, a boiler bypass, one pump (thanks Xiphias), a tekmar 256 and a switching relay for the circulator.
I could not get more than 25 degrees delta t on the bypass which I figured was because of the warm weather. I will go back next winter to try and dial it in.
I have some tekmar set up issues which I posted in a new thread.
Thanks all. It worked out great!
 
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Old 05-23-11, 08:23 AM
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I believe your delta T should be set at your maximum operating temperature. It will be less when you are running lower temps.
 
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Old 05-23-11, 10:54 AM
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Delta-T can be set at almost any temperature. I would advise to adjust at higher temps as the water temperature is not changing as fast.
If the input and flow does not change the delta-T is not going to change.
 
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Old 05-23-11, 12:33 PM
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Originally Posted by rbeck
Delta-T can be set at almost any temperature. I would advise to adjust at higher temps as the water temperature is not changing as fast.
If the input and flow does not change the delta-T is not going to change.
rbeck, do you mean outdoor temp or system operating temp? Thanks.
 
 

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