Non-condensing Boiler Pumping with 3-way Valve

[ME27272727]

Gents,

I have come across a boiler piping arrangements last week where I can only identify one set of pumps, and what seems to be a primary secondary system decoupled by a 3-way valve. I'm used to seeing this arrangement with two separate sets of pumps, one set of the primary loop, and one set of the secondary. The latest work was done in the 70's. I am tasked with removing these older boilers, and replacing with low mass condensing boilers. Does anyone have experience with this type of piping arrangement? Thanks.

Picture attached

 

[LittleInch]

I realise you know what you're looking at in that maze of pipes, but how on earth does anyone else??

A diagram would be a lot better.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[HVACMech]

I do not have experience with that piping arrangement. Sounds like it isn't a primary secondary system if there is only one set of pumps... I just did a boiler replacement from the 70's on a primary secondary system, and make sure your other equipment will operate with a condensing boiler. I am sure you know, but you will need a higher delta T on your water to get the boiler to condense to maximize efficiency, this means lower flows on your other equipment. You might create issues in other parts of the building, or waste money paying a premium for a condensing boiler.

 

[ME27272727]

It's a Honeywell actuator. I found this diagram on their website, and I think it describes what's going on here.

I'm aware of the relationship between return temp and condensing boiler efficiency. I just want to make sure I understand the system as it currently stands. I've only seen primary secondary in older boiler systems, not this three-way arrangement.

 

[EnergyProfessional]

the way i see it the only function of this valve is to maintain a minimum return temp to the boiler to PREVENT condensation. those old boilers would corrode with condensation.

Your new CONDENSING boiler has a stainless steel or aluminum heat exchanger and benefits from condensation (hence the name condensing boiler). condensation is what you want, you want return temp as low as possible. So this valve would defeat he purpose of a condensing boiler.

 

[HVACMech]

So they are using it as a outside air reset for the supply water temperature. Older boilers could handle a maximum of 40 degrees F across their heat exchanger before there were issues, and typically when you go into the older plants those boilers get operated at 20 degrees F or lower. Since older boilers had low turn down ratios the best way to save energy was to bypass them in this manner with a modulating three way valve. Though this can't be the only story to the piping system at the boiler. There would have to be another pump for the boiler to maintain minimum flow through the boiler.

I wasn't pointing out return temp as being a function of condensing, I was pointing it out as the affect it will have on the downstream items, coils, unit heaters, etc. If they are designed around a 20 degree delta at 200 EWT, but you need them to be at a 60 degree delta, they wont function properly. To maximize your condensing boiler you need return water temps in the 120s, which is quite a ways from the 160 you are at.

 

[ME27272727]

Found this among the original documents:

 

[Ziggypump]

Per this schematic it looks just like a bypass mixing situation. If the boiler is sized for a 20 delta T and the return is 40 lower, then this is needed. It also is needed for non-condensing boilers to keep from condensing.

I've seen many copper finned tube boilers on much smaller applications need this.

 

[Drazen]

The piping arrangement is related to the existing boiler. What you need to do is to determine temperature regime and flow of secondary system, than to design new schematics.

You design your new boiler to serve existing consumers on secondary, not to serve piping