P/S question

ScottMP

With some contractors and a supplier about a job that had been piped primary,secondary and the problmes they had lst week in this cold spell.

The problem, if I can remember correctly, was that the primary loop had 48 GPM, and the three boilers had lower GPMS that could not overcome the flow to raise the temp of the primary loop ?

The primary loop had two circulators piped next to each other. When one was shut off the boilers , which where all firing, shut off one by one and the temp. in the primary loop started to rise.

The supplier was saying because the boiler pumps where not sized to match the primary loop they could never raise the temp.

My thought was if the boilers were hydraulically seperated from the primary loop with an injection pump, this would not be an issuie.

I don't have the capabilites to draw on the wall but see if someone else can. This would be a good discussion.

Scott

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ScottMP

drawing

Just for the fun of it I tried drawing this. I have ME's symbles and I will work on installing them this weekend.

I did this and will see if it comes out.

The really crude one is my first attempt and shows the system as it is now.

The second one is how I feel the problem could be corrected.

Scott

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Boonierat

P/S Question

Given the drawing, EACH boilers circulator need be sized to pump just the amount of BTU's IT's boiler can produce. If each boiler was capable of 200,000, each circulator would need to pump 20 gpm (not the primary loop total)based on a 20 degree drop. With both boiler's crankin' the primary loop circulator needs to be able to move ALL of the BTU's.from each circulator.
I'm a bit fuzzy on the piping arrangement at the top of the drawing. The primary pump is located BETWEEN the supply and return to a secondary zone ????? Is it really installed that way ???

ScottMP

I believe so

The problem as I remember it is that the GPM of the two primary pumps is larger than the GPM of the individual boiler pumps. There are actually three boilers each with two stage.

As the second primary pump was shut off the primary loop temp started to climb and each boiler ( all where running ) started to shut down untill only one was running and the primary loop temp was met.

Scott

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hot_rod

All the answers

can be found here, I pump away from the primary loop through the boilers. Although I have seen it piped either way and it seems to work. The twin pumps confuse me a bit?

hot rod

Boilerpro

Several problems

First is that the heat output of one boiler is preheating the water to the next boiler as sysetm water continues to move down the system loop. If the boilers are pumped away, then they are preheating their own return water. Both these piping arrangements mean that the boilers must run much hotter than the system supply due to all the preheating going on. Next, the boilers can be put through major hot shock with this piping arrangement(not cold shock) if thier circulators are not running continuously to keep the boilers at system temp or they are not kept at temperature on a low limit or by using stack dampers and a pilot light. If an off boiler is called on it can be at room temp and then suddenly it gets full flow of hot water from system...70F to 180F in a minute....110F delta tee at full flow. This piping arrangement also is very susceptible to cold shock if there are large zones kept at setback temps that occasionally kick on in colder weather. The cold slug of water comes back into the bioler room and the first boiler in line can get 100 % flow of cold water....depending on the flow rates through the system and boilers. Depending on whether each boiler is preheating its return, I think you need twice as much flow through each boiler as the system flow if you are going to keep boiler temps down. These high of flows may not be good for the boilers.
My solution would be to pipe the boilers in parellel to each other and have them connect to the system loop at 1 pair of tees (like Weil's mulitple boiler setups). EAch boiler gets it own pump flowing 1/2 the system flow, a check valve ij the outlet to prevent back flow through each boiler when the other boiler is in operation, and if the boilers can drop to ambient temp, drill a small hole in each check valve to allow some flow from the hot boiler to keep the other boiler up to temp.
If cold return shock is a problem, or you would like to running the system at low temps (below 140 with noncondensing boilers), you could install mixing vavles on the return to each boiler, or pipe the bioler in thier own loop and use an injectin pump to provide low temp boiler protection and if, desired outdoor reset. If you wait until this evening , I can post pictures.... still got to pick them up.

Boilerpro

hot_rod

Multi boiler piping

here are some other piping arrangements that handle the issues boilerpro brings up.

http://www.pmengineer.com/CDA/ArticleInformation/coverstory/BNPCoverStoryItem/0,2730,17213,00.html

hot rod

ScottMP

Please

Remember that the first drawing is a crude attempt to show a drawing on the wall. Also this is not my job nor have I seen it. We were in a discussion with some other contractors and this came up.

I believe the two boilers are piped as Wiel Mclain recommends. The tees are properly spaced as for P/S.

Scott

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Bill Wright_2

Bill Wright_2

Bill Wright_2

Boilerpro

Picture of one solution

Twin pumps with check valves on system supply for back up (on right), each boiler has its own pump and thermostatic mixing valve (just above grundfos pumps on boilers) on return. Check valve on outlet of boiler to prevent back flow from an on boiler back into an off boiler. Boilers piped together P/S in parellel to single pair of tees in system loop. If I was doing this install today each boiler would have its own IAS air separator along with the pump on the boiler supply. Yes you can sell staged boilers for homes! BTW, the boilers are staged by a two stage Honeywell Chronotherm T-stat.

Boilerpro

Boilerpro

Picture of one solution

Twin pumps with check valves on system supply for back up (on right), each boiler has its own pump and thermostatic mixing valve (just above grundfos pumps on boilers) on return. Check valve on outlet of boiler to prevent back flow from an on boiler back into an off boiler. Boilers piped together P/S in parellel to single pair of tees in system loop. If I was doing this install today each boiler would have its own IAS air separator along with the pump on the boiler supply. Yes you can sell staged boilers for homes! BTW, the boilers are staged by a two stage Honeywell Chronotherm T-stat. I should have another picture tommorrow using a variable speed for boiler protection.

Boilerpro