primary/secondary piping on an atmospheric boiler

jonny88

hi,i just finished one of dans books an primary/secondary piping.if i have a regular atmospheric boiler is it necessary to pipe it this way or should i just come of the boiler with a header and pipe in seperate zones.i have installed many boilers with my company but thru this site i rtealize the heating experts in my company arent as good as they thgink.eg pumps on return etc,basically we do everything opposite to what dan says in his books.

Alan (California Radiant) Forbes

Primary-Secondary

If the entire house is one zone as is so often the case in older homes, you don't need to pipe primary-secondary since the flow through the boiler will satisfactorily pick up all the heat from the heat exchanger.  But if you have a multi-zone building with one or more smallish zones, I always pipe primary-secondary to guaranty adequate flow across the heat exchanger.

jonny88

primary/secondary piping on an atmospheric boiler

thanks alan,there will be multiple zones with two of them being radiant.thanks for your help.

Charlie from wmass

I know you just got done with primary secondary

piping but hydraulic separators can offer a lot to a system and may even be a more effective solution. Less blending of water temps and a more steam lined piping arrangement.

Mark Eatherton

The reason for using P/S piping....

is to protect a high temperature heat source from continuous long term exposure to low operating temperatures that should not condense, and to allow pumps of different sizes to work in unison. If your distribution system is low mass high temperature (hot water base board convectors), then you really don't need to utilize P/S piping, unless there is a potential conflict in pump sizing.



If you are dealing with a high mass, low temp distribution system (radiant tubing in cementitious materials) then it is a necessity if you are using a non condensing heat source. But, without proper control, simply doing the piping in a P/S method does not guarantee that the heat source will not condense. The ONLY device that can guarantee that without the use of an electronic control is a ESBE Thermic valve, or equal from other manufacturers,



Got pictures and drawings?



ME

NRT_Rob

I still don't agree with this assessment

water in equals water out, whether it's a hydraulic separator or close tees.



If stream 1 is faster than stream 2, stream 1 will blend with its own return water.



I think both will blend exactly the same amount.

Greg Maxwell

Over Kill

You absolutely do not need to install a P/S header on a "regular" boiler, atmospheric or otherwise. P/S on a boiler loop is there to regulate pressure drop and prevent short cycling on very limited volume systems, such as the wall hung gas boilers and such. On a system such as you have, you have virtually no pressure drop through the boiler, and even on a multiple zone system, you would not receive any benefit from it. All it would do is cost you money on piping, and another pump. Save the primary seconday boiler piping for the wall hungs, or multiple temp systems. If you are worried about bringing cool water back to the boiler, dont. Unless your system has an incredible volume of water, then install a simple bypass between supply and return, and throttle with a ball valve.

Jean-David Beyer

P-S piping and low loss headers...

I am not a professional, but I study these systems because I have one and because they are interesting to me.



It seems to me that doing primary-secondary piping using closely-spaced Ts or a low-loss-header are conceptually the same. There are practical differences, however, if you use a commercial low-loss header. One difference is that the piping may be easier, saving on labor cost. The other is that these units can come with a dirt separator and an air removal and venting mechanism, saving on the cost of the extra parts and the labor and space to install them.



Now as to whether to use primary-secondary piping at all or not depends on the complexity of the system being built. If there are multiple zones independently controlled, and there is no isolation, than changing the flow through any zone will affect the flow in the other zones. If this were a static thing, then the changes could be worked out once and for all. But in the case of zone valves or circulator zonign, the flow through a zone is affected by the circulation through the other zones as zone valves open and close, or as zone circulators switch on or off. Primary-secondary piping allows the flow in each circuit to be independent of the flows in the other circuits.



Now with boilers like my mod-con that holds only 3 quarts of water, it is important to maintain adequate flow through one particular circuit -- the one with the boiler it it -- irrespective of what the loads are doing. But this is just a special case of the previous paragraph.



Am I overlooking something?

NRT_Rob

I don't think you are

and that's why siggy shows 4 way mixing valves on atmospheric boilers with NO boiler side pump. because traditional cast iron is fairly flow insensitive, so one pump on a 4 way can handle all the flow requirements if the piping is laid out carefully.

Plumdog_2

Atmospheric is too broad of a term

Some atmospheric boilers, like the copper or steel fin-tube design (particularly in the larger sizes) will require primary-secondary or an equivalent design to prevent rapid overheating with the resultant squealing, howling, and internal scaling. The installation manual is a good resource to follow. Some modern cast-iron boilers require at least a by-pass, some are equipped with a thermal by-pass. So the term "atmospheric" refers to the design of the burner/flue/draft and NOT the design of the heat exchanger or the pumping strategy!