series or parallel boilers

Louie O.

Can you please elaborate on hot water heating boilers when they should be put in series or parellel? What are the advantages and disadvantages?

Steve Ebels

Done both

Typically when you're doing multiple boilers, you'll be using some kind of primary secondary situation. The advantage of a parallel arrangement is that the primary loop pump is also you're boiler circulating pump. It's creating your flow through the boilers as well as your primary circuit because the boilers are integral to it. This works well when the flow required through the boilers is the same or nearly the same as your primary loop. When the two flow rates are different however, it's often better to install the boilers with their own circulators, as in a series off the primary loop. This allows you to isolate (keep cold) the boiler that is not running. At least in a staged scenario. Many times the primary loop circ and the boiler circs can be made smaller in size also.
If you use a parallel setup, it's best to pipe the boilers reverse return so that they get equal flow through them. If you use a series where each boiler has it's own circ, that concern is eliminated. Controls and wiring are usually simpler in a parallel arrangement.

These are just a few thoughts and I'm sure others here can add more.

kevin coppinger_4

an additonal question...

is it possible to oversize a boiler and or primary loop pump? I did a job a few months ago and used a 007 for both. it seemed that the water was moving so fast in theprimary that the air scoop could not remove it...what do you think? kpc

hot_rod

The main advantage

of multiple boilers is the ability to stage them. Bring onn the second or additional boiler as the load increases. A common practice in large commercial application.

With series piping you don't want to circulate hot water from fired boiler(s) through the unfired boiler(s), as the unfired boilers become "cooling towers.

There are a number of ways to correctly pipe multiples. Depends on the application, loads, temperatures required at the emitters, etc.

With any non condensing boiler you really need to be concerned with return water temperatures. You MUST keep the temperature above the condensing (dewpoint) temperature. Extended run times in a cold return mode will create some ugly problems. Make sure you have a method to watch and react to this.

Here is a simple and effective multi piping. Be sure to use check valves or check pumps. As you can see, with this staged boiler arrangement, only the fired boiler sees flow. A tekmar stage control rotates, exercises and watches return temperatures. This was a mini tube injection system. The boiler loop runs a wide 40F delta tee allowing smaller diameter pipe (1-1/4") to handle 370,000 BTU.

Grab a handful of Dan and Siggy's books for some piping ideas and tips!

hot rod

hot_rod

Size the pump to the load

The BTU to be moved, the delta t used, and the pressure drop through the circuit will define the pump criteria.

Try the new multi speed Grundfos circs, for some fine tuning ability. Keep your velocities below 4 fps. If you pump away from a tank/air scoop combo air removal should not be a problem.

I have found with primary secondary piping, it can take a long time for trapped air in the secondaries to make it's way to the air purger. A good purge valve and power purge helps.

Injection mixing is a common place to see this problem, seems sometimes the air bubbles are reluctant to cross over that "bridge" A second air purger in the secondary loop, even a plain Jane iron scoop type, can be an inexpensive way to handle this quickly.

hot rod

Louie O._2

First I'd like to thank all you men on your replies and for you time.
Hot Rod, in the situation I have is a customer is adding aditions to both sides of his home, the boiler he has now is too small, so the engineer is going with two boilers since one large one wont fit into basement. He said they should be paralled. I'm doing the air cond. and never did any hot water design. Is this the right way to go? They are standard Hydrotherms non condensables. Should Tekmar sensor be in common supply and not the return??????

Boilerpro_3

Here are a few ways I do multiples..

On any space heating loads larger than 200,000 input, they are the only way I bid a job. IMHO, large single boilers in space heating applications are just too inefficient and rack up way too much cycling wear on the controls and stress on the heat exchanger.

The first two utilize two standing pilot boilers with thermix mixing valves on the returns with check valves on the supplies. Both are piped parellel to each other and then primary / secondary to the system.

In the first picture boiler one is always lead boiler and its pump is cycled by the tekmar 262 boiler control on a call for heat. This helps prevent short cycling by ensuring the stored heat in the boiler is used up before firing. Often only the pump cycles when loads are low. Then the control fires the boiler as needed. Boiler two and its pump are called on together during heavy loads. The use of standing pilots along with stack dampers keep the boilers up to temp during the summer keeping the heat exchangers completely free of rust during off periods. Also, during the heating season, and along with the Esbe thermix thermostatic return temp. (140 F)valves the standing pilots protect the boilers from low operating temps, allowing full reset of system temps and, usually overlooked, the potential for "reverse" thermal shock when an off boiler is brought on line with a hot system. This occurs when the pump for a boiler at room temp is turned on bringing hot system water into the cold boiler. Typical delta tees can be up to 140F under these conditions.... I'm sure not good for boiler life.

The second is very similiar, except a two stage Honeywell chronotherm thermostat stages the boilers. Boiler one is again always lead and the boiler, the boiler pump, and the system pump all come on with the thermostats first stage. Boiler two and its pump are only cycled on when stage two calls for heat at the thermostat.
Both boilers are standing pilot with Esbe thermix valves, allowing the system is operate at any temp and protecting the boilers.

The next picture is a set of copper tube boilers piped in parrallel and all pumped together. Injection mixing pumps feed various parts of the facility providing full reset system supply water temps and boiler protection. One portion of the system is supplied with water directly off the boiler loop with primary/ secondary piping. A tekmar boiler control stages the boilers and provides reset down to thier miniumum operting temp of about 130F.

The boilers are equipped with stack dampers to prevent the "cooling tower" effect when heat travels up the chimney during off cycles. Also, due to the unique design of these boilers where the draft hood opening is below the heat exchanger level, little heat is lost to the boiler room through the relief opening when the damper is closed because a hot air trap is created.

The last pictures are of boilers piped similiar to the first set, but a single tekmar injection pump control provides boiler temp protection. It is used in the setpoint mode, with the setpoint set at the max required supply temp and the sensor on the return to provide protection. The other controls, a tekmar zone control and boiler staging control interact with each other to provide indoor feedback for full reset and offsetting of the outdoor reset curve for optimum efficiency. The zone and staging control operate completely independently of the mixing control. On the chimney vent installation, standing pilot boilers with stack dampers are again used so the boilers are always at or near operating temp. even when off to prevent summer condensation and more importantly "reverse" thermal shock (hot water shock).

On the installation with the power vent boilers, where standing pilot is not available, a small 1/8 inch hole was drilled in the outlet check valves to allow some hot boiler water from the on boiler to keep the off boiler warm to prevent "reverse" thermal shock (hot water shock).

Hope this gives you some ideas.

Boilerpro_3

Here are a few ways I do multiples..

On any space heating loads larger than 200,000 input, they are the only way I bid a job. IMHO, large single boilers in space heating applications are just too inefficient and rack up way too much cycling wear on the controls and stress on the heat exchanger.

The first two utilize two standing pilot boilers with thermix mixing valves on the returns with check valves on the supplies. Both are piped parellel to each other and then primary / secondary to the system.

In the first picture boiler one is always lead boiler and its pump is cycled by the tekmar 262 boiler control on a call for heat. This helps prevent short cycling by ensuring the stored heat in the boiler is used up before firing. Often only the pump cycles when loads are low. Then the control fires the boiler as needed. Boiler two and its pump are called on together during heavy loads. The use of standing pilots along with stack dampers keep the boilers up to temp during the summer keeping the heat exchangers completely free of rust during off periods. Also, during the heating season, and along with the Esbe thermix thermostatic return temp. (140 F)valves the standing pilots protect the boilers from low operating temps, allowing full reset of system temps and, usually overlooked, the potential for "reverse" thermal shock when an off boiler is brought on line with a hot system. This occurs when the pump for a boiler at room temp is turned on bringing hot system water into the cold boiler. Typical delta tees can be up to 140F under these conditions.... I'm sure not good for boiler life.

The second is very similiar, except a two stage Honeywell chronotherm thermostat stages the boilers. Boiler one is again always lead and the boiler, the boiler pump, and the system pump all come on with the thermostats first stage. Boiler two and its pump are only cycled on when stage two calls for heat at the thermostat.
Both boilers are standing pilot with Esbe thermix valves, allowing the system is operate at any temp and protecting the boilers.

The next picture is a set of copper tube boilers piped in parrallel and all pumped together. Injection mixing pumps feed various parts of the facility providing full reset system supply water temps and boiler protection. One portion of the system is supplied with water directly off the boiler loop with primary/ secondary piping. A tekmar boiler control stages the boilers and provides reset down to thier miniumum operting temp of about 130F.

The boilers are equipped with stack dampers to prevent the "cooling tower" effect when heat travels up the chimney during off cycles. Also, due to the unique design of these boilers where the draft hood opening is below the heat exchanger level, little heat is lost to the boiler room through the relief opening when the damper is closed because a hot air trap is created.

The last pictures are of boilers piped similiar to the first set, but a single tekmar injection pump control provides boiler temp protection. It is used in the setpoint mode, with the setpoint set at the max required supply temp and the sensor on the return to provide protection. The other controls, a tekmar zone control and boiler staging control interact with each other to provide indoor feedback for full reset and offsetting of the outdoor reset curve for optimum efficiency. The zone and staging control operate completely independently of the mixing control. On the chimney vent installation, standing pilot boilers with stack dampers are again used so the boilers are always at or near operating temp. even when off to prevent summer condensation and more importantly "reverse" thermal shock (hot water shock).
The injection pumps are the two Grundfos purple pumps in the midst of the sea of B&G red.

On an installation with the power vent boilers, where standing pilot is not available, a small 1/8 inch hole was drilled in the outlet check valves to allow some hot boiler water from the on boiler to keep the off boiler warm to prevent "reverse" thermal shock (hot water shock). (Sorry couldn't find the pictures for this one)

Hope this gives you some ideas.

Boilerpro

hot_rod

nice assortment, BP

A couple thoughts, if I may When I talked about "cooling tower" effect of hot water flowing through an un fired boiler, yes the damper will surely stop a lot of heat up the flue. The other concern, which can be almost as much loss, is the jacket loss to the room. Most boilers have a very minimun 1/2- 1" insulation around only parts of them. The loss of 180F cast block to a 65- 70F ambient air temperature room can be very large also.

Putting a boiler in a small mechinical room will show this loss! I have to hand it to Viessmann on this point. Their thick insulation package does in fact cut this loss down. Putting your hand on the sheet metal jacketing confirms this. Seems like more cast iron boiler manufactures should consider this inexpensive upgrade!

I'm still of the opinion, best to not flow through unfired boilers, especially high mass cast iron. I suppose the losses through the moden low mass, low water content, condensing boiler, especially running radiant floor temperatures (below 120F) could be fairly small, however, if piped in series.

Also the reverse thermal shock concept? If I am following your thoughts of hitting a lag boiler, again at ambient room temperature, maybe 65-70F with 160-180F or higher temperatures? Wouldn't this lag boiler see almost the exact thermal range when it fires from a cold start? Seems the lead boiler, if identically sized to the lag boiler, BTU wise, could only dump it's identical output into the colder boiler. Just as the cold boiler firing it's own burner. Seems the lead boiler sees this thermal range at it's start up, unless it its idled at a minimun temperature? I'm not sure the fuel cost for the loss through the unfired would offset any thermal shock potential?

I agree the less cycling of a single, or multiple boiler arrangement, the better. This is why the modulating burner concept intrigues me so. Even the older step firing conceptm used on larger boiler is a big help.

hot rod

Boilerpro_3

HR .....Insulation and Lag boiler shock

I agree with you on the insulation issue....Most manufacturers do a very poor job. I believe the insulation only serves to protect personal from burns and not to reduce losses. I was especially disappointed with the Dunkirk condensors.....there is no insultion whatsoever. I guess that since AFUE considers this loss as part of the usable heat ( I think that's right), they don't really care much about it. I have been told it doesn't amount to much, but its pure waste it many applications, especially with an indirect in the summer.

As to the Lag boiler hot shock, I didn't really give a complete description. The other factor besides temp change is how fast it changes, which I think you were also referring too. As you said, with modern low water content cast iron boilers, it can be a problem. Even with those big, heavy Burnham 807 B's pictured it seems to be a problem. They hold about 16 gals each and weigh about 850lbs with an output of about 320,000 btu/hr. If you figure they are pumped at 20F delta Tee that gives you 32 gpm.... so the water changes twice a minute when the pump kicks on. 16 gallons is about 133 lbs of water... Cast iron holds about 1/10 the heat as the same weight of water so altogether you have about133 + 85 = 218 lbs of equivalent water. 1 btu raises 1 lb of water 1 F so to take 218 lbs of equivalent water from 70 to 180 (110F) .... 218lbs x 110F = 24,000 btu's. At 32 gpm at an average of 65F delta tee (the delta tee starts at 110 and ends about 20 when boiler is warmed up), it works out to take about 1.4 minutes for the boiler to go from 70 F to 180F. This is the source for my concern. Large water content boilers probably handle this better, but you still have alot of stress, I would expect, as the hot water enters the boiler return connection. This point in the boiler would be alot colder than the rest of the boiler. I suppose its like allowing an unthrottled condensate pump to dump cold condensate back into a steamer.

Boilerpro