complicated residential system

icheat

Two years ago I moved into a large townhouse with a slightly complex heating system (my attempt at a simple diagram attached). Recently a (new) plumber suggested that I have been operating the system incorrectly, and I'm looking for advice.



The system has 2 identical Burnham series 2 boilers, 7 or so zones with a variety of emission (hydro-air, radiant floor, fin tube baseboard) and an indirect for DHW. As shown on the diagram there are two ball valves that allow the 2 boilers to work in isolation: one for the DHW and the other for all the heating zones. This is how I found the system when I moved in, and how it has operated for two years. In November my plumber pointed out that the two fan coils for the two hydro-air zones together have rated output of about 140kBTU/h, which is larger than the 118 kBTU/h of a single boiler. So he opened these two isolation valves and we have been operating the system this way for the past two months.



The circulator on the "inner" boiler (in the diagram) is a Taco 1610C, and comes on when any zone is calling for heat. The circulator on the "outer" boiler is a Taco 0012, and this boiler seems to simply cycle between its lo and hi limits. The circulator is connected through a honeywell T775 whose sensor is an aquastat in the indirect tank.



But I can't quite understand how all this sould work. Won't I always get reverse flow through the "outer" boiler, even when its not fired? Isn't this a bad thing? And the two boilers don't appear to be staged in any way.



I suspect that I will need to have a professional come and look at it (and if anyone has a good recommendation for the Boston area that would be great), but I would really like to understand it. And I feel funny calling a professional since it seems to heat the house OK in either set up (although my heating bills are kind of high:)) And finally I have worked on (simpler) systems before, and don't want to be defeated in my attempts at understanding this setup.



Thanks in advance for any advice, and just wanted to say that I have been reading the wall for ages and love it.

Gordan

I don't think your plumber took the time to understand the system

Output from fan coils is rated at given water supply/flow rate and air temperature/airflow values. Did the plumber calculate or measure any of those four factors to arrive at the conclusion that the "inner" boiler is undersized? Or did he do a heat loss calculation? Probably not.





The fact (if it is, in fact, a fact) that the "outer" boiler gets its call for heat directly and solely from the DHW aquastat is a pretty clear indication of what load it was meant to satisfy. The real mystery, then, is not which boiler was meant to heat what, but what is the purpose of those isolation valves... It's also a bit puzzling that they installed a whole other boiler for DHW, rather than just running DHW as a priority zone off of the "inner" boiler. The original tenants must have been taking a lot of very long showers...

icheat

re: complications

Thanks for the comments. The plumber did indeed check the two fan coils (together they are 146 kBTU/h with 5 GPM @ 160 degree water). This is what led him to suspect that a single boiler (with I=B=R of 118 kBTU/h) was inadequate.



But you are right, my real question is the purpose of those isolation valves. I am suspecting that I should put them back to the closed position, but I am unsure.



Thanks again for the advice.

SpeyFitter

Some work needed

Assuming your boilers are still in decent shape, I think it's time for a major overhaul of the mechanical room hydronic piping to better work with these boilers and the cluster-you-know-what of heat emitters.

You have some low temp heat emitters and some higher temp heat emitters. You also have some heat emitters that could probably work with either depending on flow rates you choose to utilize. There is also a bit of a balancing act to avoid having too much complication with the various heat emitters.

The first and most important issue is working witht the existing heat source (boilers) so that you can limit short cycling, promote longer burn times, put even wear on each boiler, promote true redundancy should a boiler go down, and ensure they are putting the heat where you need it when you need it. This calls in my opinion for a controller or controllers that has the capability to stage or rotate the boilers firing, and have a domestic (indirect) priority built in, such as something offered by Tekmar. To work with this controller, some repiping in the Mechanical room will need to be done. A good controller will save you money in the long run and protect equipment but adjusting the design will be required.

Then you need to decide how many different temperature circuits you want. For example, the fan coils (Hydro-air) could be put on a high temp circuit with the baseboards/fin tube emitters. The panel rad would be on a low temp circuit typically and have it's own zone with a 4 way mixing valve more than likely. Then you need to divise a balancing act with proper balance devices to adjust/throttle flow rates to the various zones for some moderation of flow when the system is working.  The indirect would have it's own zone with priority control more than likely. Those types of things.

Gordan

A few clarifications needed

You show a zone valve on the indirect DHW tank circuit. What operates the zone valve?



What's connected to the thermostat terminals of the outer boiler?



A wiring diagram would complete the picture.



Look, it's possible that something other than the simple operational mode (with the two systems isolated) is what was intended but, like you, I have a difficult time imagining how it would ever work. As for the coils, they can't pull more BTUs out of the water than the boiler puts in on a sustained basis; the coils and the boiler can just reach equilibrium at a lower return temperature, at which point the output of the coils will have dropped enough to match the input of the boiler. If the house is heating well, then there's no comfort-based reason to "upsize" your heating plant by including the other boiler. There would be only a potential concern with the boiler condensing for prolonged periods of time, leading eventually to the rusting out of the heat exchanger. The way to address that, if it's at all an issue, would be with boiler return temp protection (a bypass with a thermostatic mixing valve.)

icheat

wiring

The zone valve on the indirect is controlled from a honeywell T775. The honeywell has a sensor input which comes from an aquastat in the superstor tank and two relay outputs, one of which controls the zone valve on the indirect and the other which turns on the circulator.



The boiler itself is just cycling between its lo and hi limits.



thanks again for taking the time to look at this.

icheat

re: work needed

Scott, thanks for looking at this.



The whole system is not very old (less than 10 years). The boilers seem to be in excellent condition.



The controls in place seem to offer decent capability for more sophisticated control, but aren't wired that way. I think the honeywell t775 (there are two of them) could be used to stage the boilers, and along with the two taco ZVC406s I think setting up a priority zone for the DHW would be easy. Just as you say, the real issues are limiting short cycling, even wear on the boilers, proper redundancy, etc. and most important piping for the different supply temperatures appropriate for the various emitter. I wanted to understand as best I can what the thinking behind the current design is so that I can make the best decision as to how to redo it.



Thanks again.

SpeyFitter

Don't worry about the current design

I wouldn't worry too much about the current design, but if you're willing to spend the money on a better design that is where I would focus my efforts. If you go onto the Tekmar website for example, there are application brochures for everyone of their different controllers. You can get an idea of how each controller can be applied to various different design schematics. It lists the sensors and their respective positions as well. 

I can't believe the sheer breadth of different heating emitters you have in your house. I would have thought you bought your place off of Siggy for a while until I read his most recent article LOL.

Gordan

Boiler cycling between high and low limits

I can't imagine that this would be particularly great for the boiler but it's definitely bad for your gas bills. Which indirect do you have?



I agree with what Scott says about more sophisticated controls (and piping) being necessary IF the inner boiler is undersized for your space heating load, but according to what you said (it has heated your house well for a number of seasons) there's no evidence that it is. And it might be beneficial to alternate the boilers, but these things tend to last a very long time when piped and wired correctly, so it is my opinion that the benefit would be minimal and perhaps would not warrant the added complexity and expense. If I were in your shoes I would simply isolate the two boilers/systems from each other and make some minor adjustments to make them perform better, such as protecting the boilers from low return temperatures, convert the DHW boiler to cold start, and perhaps set the DHW aquastat to a higher temperature if necessary (and install a thermostatic mixing valve on the outlet if one is not already there.) Maintaining that boiler at operating temperature has to be wasteful of gas, especially in Summer.

icheat

re: boiler cycling

The indirect(s) are two superstor 80s (I think this capacity is insane, and while I'm making changes to the system I will probably remove one).



I might be wrong about the only control on the circulator being the aquatstat in the indirects: the taco 0012 circulator is very quiet and produces minimal vibration, so when the taco 1610 is running everything vibrates somewhat and I can't be sure whether or not the 0012 is running. And the wiring is kind of a rats nest so it isn't obvious how everything is wired. I'll have to get out a meter and check.



Our hot water usage right now is pretty low (even one 80gal tank is huge overkill), and this means that in the summer the boiler doesn't run very long when it needs to heat the indirect. Maybe this is why its set up for warm start. But I think you are absolutely right that converting this to cold start makes more sense.



Although I think I can rather easily use the existing controls to stage the boilers, I think that I am going to follow your advice and re-isolate them. The isolation valves were put there for a reason, but I think it must be one of two things: redundancy in case one of the two boilers fails; or for exceptionally cold weather when one boiler can't keep up. But so far our experience is that one boiler is capable of heating the house. The only downside is the short cycling of the hot water boiler, but I guess there are things we can do to help with this.



The "inner" boiler has some protection from cold return (there is a loop that has no zone valve on it that goes to the kitchen fan convector which is usually off; this acts as a kind of bypass) and I haven't noticed the temp dipping below 140 except occasionally after very long periods with no call for heat, and even then it gets up to 140 rather quickly.



Thanks again for all the help and advice.

Gordan

Short-cycling of the outer boiler

The way you might address this is by setting the DHW aquastat differential wider, and bumping up the setpoint accordingly to make sure that you won't run out of hot water while the (high mass) boiler comes up to temperature. Then, once the boiler kicks on, it should have to run awhile before the DHW call is satisfied.



Anytime you raise the DHW setpoint beyond 125 F, however, you really need to have a thermostatic mixing valve at the outlet. (It's a good thing to have in any case.)