Modcon / home run system ?

SV9_9

I am currently in the process of replacing the supply piping to my second floor CI radiators. I am using an Uponor TF-j manifold with 4 loops, the longest being about 80'. I did this work on the first floor a few years ago and it has performed really well. A bit of history, I have a Burham FCM120, with outdoor rest, that was (pro) installed in 08. The house was built in 1892, and had an oil fired gravity hot water system when I purchased it in 2000. When the FCM was first installed it was tied into the old system as one heating zone, plus a 40g SuperStor indirect for DWH. I had a zone header added to the system a few years ago with the intention of separating out the 1st and 2nd floors as well as the kitchen. The kitchen is a 13'x18' single level addition exposed along 3 walls with 4 large windows, and an entry door. The contractor that built the zone header separated out the kitchen at that time. He used a Taco 00R circulator and 5/8" hePex to supply an 8500 Btu CI radiator. I have since removed all the old black iron supply piping to both floors, including the mains. I ran new supply tubing to the 1st floor radiators based mainly on JS's MHH, home runs back to an Uponor TF manifold. I have been through 3 seasons since doing this work with no apparent problems. I do have some questions however, and would like to get some general advise. I have done a room by room heat loss calc. The total heat load for the house using a 70 deg. delta-t, ranges from 47k Btu to 56k Btu depending on the infiltration factor. The house is about 1600 sq. ft. and is not particularly tight ( I am doing what I can to improve this ) so 50k is probably about the right #. My main concern is that the boiler is oversized. I think it was sized to meet the recovery rate of the SS indirect. At full turn-down the minimum firing rate is 40k Btu. I am guessing that it is short cycling much of the time. Particularly when just the kitchen is calling, or at low demand months.
So my questions are,

What is a reasonable minimum firing duration?
What can be done to mitigate short cycling?
Would a buffer tank be an appropriate solution?

I also have a question regarding tube sizing for the radiator loops. I really only need 1 gpm or less to meet the individual room Btu load using 1/2" pex. Is there any advantage to using larger tubing, 5/8" or 3/4" and higher flow rates with a mod-con / home-run system?

Thanks In advance, J

Ironman

By removing the large pipes, you're taking away the mass (buffer) that they provided. Now you will need to replace that mass by adding a buffer tank.

If you zone the system, that will compound the problem.

I see some things in your piping that look wrong. Why do you have a circ on both the supply and return of the boiler loop?

SV9_9

Ironman, thanks for your response. The gray Taco in the boiler loop is supplying the SS indirect. Both circs. pump away from the supply side of the boiler, and never run simultaneously.

Zman

After looking at this for a bit, I think it is generally piped correctly. The indirect piping and the return header create some optical illusions.

I do believe you are over pumping all of it significantly. The 00R is designed for high head radiant applications not home run with sizable tubing. I am not sure which circ is on the primary but it appears to be way bigger than needed to overcome the ~8ft/8gpm requirements of the boiler.

Aside from wasting electricity, this also gives you very tight return delta t to the boiler, increasing return temps and costing efficiency.

The boiler should not have been sized for the indirect. This is a common mistake. The indirect simply transfers heat from the boiler. If the boiler is smaller than it's maximum transfer rate, it just does not recover as quickly. Unless you have very large DHW needs this is not an issue. The correct way to size DHW is to look at the needs and size your storage and demand capacity accordingly. Using some random chart provided by the indirect manufacture is just going to get you an oversized boiler.

As for the short cycling, how long are your typical cycles?
the cast iron rads may be helping you out by adding mass.
6 cycles per hour max is a standard which is tossed around quite a bit.

You should be able to use the smaller tubing you suggest. You will have balancing issues if you combine the larger and smaller tubing on the same distribution circ.

EBEBRATT-Ed

Buffer tank will help smooth out operation. I like to see 20min on time probably unachievable.

NY_Rob

An unrelated question to the OP...

Is that rather large expansion tank supported somehow other than hanging by it connecting pipe?

hot_rod

10 minute run times is a number many agree is the minimum.

Here is a link to a Idronics journal explaining the buffer tank options. Page 28- has a formula for sizing the buffer. Read the whole issue it gives you some piping options.

https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_17_na.pdf

The HDS Software at www.hydronicpros has a buffer sizing module. You enter all your specifics and you can toggle all the loads on and off to see ho the time changes,

A demo at that site, PC based program.

SV9_9

Thanks Zman. Just to be clear, the only work I have done to this system is the addition of a Spirovent, and the Uponor manifold and supply tubing to the first floor radiators. I used 5/8" and 3/4" hePex. My thinking at the time lead me to using the larger tubing. I am not sure now if that decision actually provides any benefit. If I use the same arrangement for the 2nd. floor zone, it will have a separate circ. and manifold. My question was, is there any benefit to using tubing larger than 1/2"?

I agree that the circs. appear oversized. I have no idea why the 00R was used for that zone. The 007 was the original secondary circ. The primary circ. is an 0014. This is what is specified in the FCM manual, and what came packed with the boiler. The same size circ. was used for the DWH loop. Is there some reason that Burnham would supply an oversized circ. for the boiler loop?

To your point about over pumping? Would't a close delta-t in the boiler loop result in higher return temps? I am not arguing here, I am just trying to understand the principles involved as best I can.

I have never counted the cycle timing. I am really just speculating about short cycling, given the overall heat load / boiler size. And that I took all that large black piping out of the system.

Thanks J

SV9_9

hot rod, Thanks for the link. I will check it out. J

Ironman

You've still got 2 circs in the boiler loop. The way it's piped, only one circ is needed for both space heating and domestic, since your domestic has a circulator in the primary (moose antler). The way it is now, whichever circ is not running in the boiler loop is a restriction to the one that is.

1/2" pex can carry about 1.5 gpm which is good for about 15k btus; however, when connected to a manifold system, .75 gpm is about the best you'll normally get.

SV9_9

NY_Rob, Yes, the expansion tank is just hung off the pipe. It is a hold over from the original high water volume system. It is significantly oversized for the current set up. however it takes on very little water, and consequently does not weigh very much. I probably should support it more securely anyway, or replace it with a properly sized tank.

Thanks for your interest. J

Zman

Bob,
The supply on that comes out the top. The circ that looks like it is redundant is going through the back wall to the indirect.

SV9_9, I did mean increase return temp and have corrected my post, thanks.

Boiler manufactures are famous or shipping oversized circs. I suspect they can't resist the bulk price and figure it doesn't hurt (they don't pay your electricity bill).

Pipe sizing is kind of a delicate balance. You start with the Btu's required and design delta T which is converted to GPM (Google universal hydronic formula). This gives you a starting point. From there decisions are made based on a variety of factors. In general a larger pipe is desirable as it requires less circulator energy. My comment about the 00R had more to do with the wrong circ not the wrong tubing size.

If you post the distribution layout you are considering with BTU requirements and distances, a bunch of folks on here have the software and know how to help you size the tubing and circ.

JUGHNE

Part of the high volume water system is the CI rads which I believe you are keeping.
Hanging off the 1/2" 90 and short nipple could put a load on that piping if the tank gets water logged, which will happen to all of them someday.
I have supported them with a floor rod. Assuming you have a 3/8" female thread in the bottom of the tank......I used a floor plate and 2 lengths of 3/8 rod with a LH-RH coupler in between. Snug the rod up to put a little lift on the tank and have jam nuts against all fittings to keep things in place.