Need some advice about working with a buffer tank and an indirect

epmiller · March 27

I have an existing install that the customer wants me to add an indirect-fired water heater to a mod-con system that is used for QuikTrak floor radiant heat. In other words, pretty low water temperatures. Because this system is zoned by rooms it requires a buffer tank to reduce short cycling. The buffer tank is the hydraulic separator between the boiler circulator and the central heat circulator.

If I just put a pair of closely spaced tees into the boiler loop as shown in the recommended installation drawing from the manual, whenever the DHW calls it will be heating the buffer tank, unnecessarily from a fuel usage standpoint and unacceptably from a floor temperature standpoint. Aside from an expen$$ive 3-way valve with a Cv of 10 or higher and possibly another hydraulic separator for the DHW circuit, does anyone have some ideas that I am missing?

I've learned a lot from this forum, thanks everyone.

HydronicMike · March 27

just pipe it correctly, per the manual using the diagram showing the indirect.

You’re correct in thinking. The DHW call will reset the SWT to a higher temp, and that temp should only run to the indirect, not the buffer.
Otherwise, you get no efficiency gains from modulating or condensing.

Caleffi Idronics has a number of publications showing best practices for piping.

pedmec · March 27

What boiler model do you have? it's easy enough to just put tee's on the boiler side of the buffer tank, on the supply and return. You would have a separate circulator just for the indirect tank and would tie into the return to the boiler after the boiler circulator. This allows the indirect circulator to run without heating up the buffer tank when the boiler is in warm weather shutdown. The boiler circulator stays off until the outdoor temp drops below the set point. page 32

https://www.htproducts.com/literature/lp-185.pdf

hot_rod · March 27

the indirect pipes as a parallel loop to the boiler, as a priority load, the boiler circ is held off during a DHW call

The boiler or system sensor is in the buffer tank

The buffer could be piped as a 2 or 4 pipe arrangement also, to keep boiler efficiency up

epmiller · March 28

@pedmec The boiler is a Burnham Alta. Your tee idea might be able to be made to work there. Interestingly, that idea will not work with the Burnham K2 which has the boiler circ inside the unit. I haven't reread the Alta manual yet, possibly that is in there. I know it isn't in the K2 manual. I guess I was hoping that I had missed something obvious but it doesn't appear so. Thanks.

@hot_rod I don't quite follow your hand drawn diagram. It would appear that the DHW circ would just short circuit thru the buffer tank without a check valve somewhere in that loop. On a central heat call water would circulate thru both tanks, no way to apply a check valve there. Also if the boiler circ is shut down on a DHW call the DWH circ would would have to pump through it. I have no idea how to calculate that added head. What am I missing? Thanks.

I probably should explain myself a bit here, I am not a dedicated heating tech, I do home repair and maintenance and seem to install about one boiler a year. Retrofit floor heating is where I get that work. I enjoy installing those. I use Burnham because I like their controls and they are local here. Cheers all.

Kaos · March 28

An indirect on a modcon if properly set up is about 80% efficient. That is not much better than a power vented unit which is much cheaper install. This is a very common combination around me.

If you don't want to add an extra fuel burner, a heat pump water heater is also a better value than an indirect and lower operating cost.

The nice about either of the above options is if the boiler goes down, you still have hot water.

If you must have indirect, I would look at air to water heat pump setups as buffer tanks are very common there. With AWHP they always use a 3 way valve as you want to ensure full isolation between the heat and indirect. This also lets you do the setup without adding an extra pump.

GGross · March 28

@epmiller

does your current circulator have an internal flow check or can you add one? if you are getting into the piping to add an indirect then you can add check valves.. If you are not a licensed hvac contractor you really shouldn't be doing paid work on heating systems in most of the US but I can't tell you what to do just some advice..

GroundUp · March 29

Just pipe the indirect WH direct to the boiler with the properly sized circulator and be done with it. Basically cut a tee into both the S&R going to the buffer and add a circ, so that the indirect piping is effectively mirroring the buffer piping. Easy peasy, no wasted BTU.

epmiller · March 30

@GGross around here there are few licensing requirements for installers and I have worked on installs by companies that have the necessary licenses for a local municipality and they were not necessarily done correctly or even well other than the looks. I remember a lead installer from a large local company on one job that did NOT understand closely spaced tees. He was not new at the job by any stretch but the only thing he "knew" was that the tees should not be farther apart than 11". No idea where he got that but hey, the mod-con boiler worked as far as everyone could tell when the job was finished and the install was neat. I was the one that had to go back to them and ask for and install the outdoor reset thermistor, they said it was completely unnecessary. The original boiler was a hugely oversized 50 year old beast that kept the basement area at 90°+ so they sized the new one at about 2/3 the BTU output. Much more efficient and still oversized but hey, they had the licenses. One of the reasons I have gotten into HVAC is because of some of the crap I have seen done by local high-name-recognition companies. They want to replace parts, and sell "upgrades", it does make money, look at the trucks. My mod-con installs don't short cycle and run in condensing as much as I can get them to given the radiation in the house. I asked the starting question to see if I could learn something. Yes, I haven't done this all my professional life but I have read and studied far more than most techs and I'm still trying to learn. I am just slower on the job than many techs because I don't do anything before I've thought it through.

@GroundUp the install instructions for the particular boiler show only a closely spaced tee hookup on the supply arm of the boiler loop for an indirect. I had not thought of just teeing in above the boiler circulator if I could and then using check valves. The buffer tank/hydraulic separator short circuited my thinking because closely spaced tees wouldn't work there.

@Kaos I understand what you are saying about efficiency although with transmission losses and all, the existing electric WH really is less than 35% efficient. (Now, someone explain to me why all the hype about instantaneous water heaters if an indirect isn't a great choice, they can't be any more efficient other than standby losses.) Correct me if I am wrong, but a forced draft direct fired water heater is also only somewhere in the 80's% efficiency area too, the units I'm familiar with don't condense. For me, an indirect tank really is about the same price as a direct combustion water heater and the gas and exhaust work is more than ProPressing in and insulating some copper supply lines to the mod-con.

Another reason for an indirect in this particular situation is because reasonably locating a direct combustion WH intake and exhaust is not feasible. It's at a very restrictive location in a partially finished basement and the main floor HP ODU would take the brunt of the exhaust. I won't go there. The best alternative is a HPWH and this basement is chilly during the winter and cool during the summer. They are wanting heat down there.

Thanks all, I have had my thinking jogged.

hot_rod · March 30

Gosh, if you have a boiler and no good way to add another appliance, power for electric tank, gas, venting for another fueled device, an indirect is a no brainer.

The large surface area tanks can provide adequate DHW at condensing temperatures. Check the Heat-Flo website, they show their high, and extra high output tanks down to 120 boiler temperature! So you could generate DHW above 80% efficiencies. The reverse indirects like TurboMax can work at low boiler temperatures also. It always comes down to how much and how quickly you want DHW provided.

HPWH can make sense in the right application. They need access to heat to PUMP.

Next to a steam boiler (surplus heat) or any cast boiler running high jacket loss, for example. But in the off heating season, that room needs to have enough volume to supply the heat energy.

I think some of the HPWH manufacturers are rethinking that "room volume" calculation" If the HPWH constantly kicks in a resistance element to get the job done??? Although generous rebates are enough to drive some people to try one on.

No harm in snooping around

https://www.dsireusa.org/

to see if some $$ is available for energy efficient DHW and circulator options.

Kaos · March 30

A HPWH when supplying typical DHW use takes on average around 2000BTU over the course of a day from the space. Unless you go nuts with insulation, even a low temp hydronic install will put that much heat into a basement. That is not to say that a HPWH won't cool the basement a bit, but the amount of cooling is basically squat. Having a steam boiler next to it definitely makes a HPWH more efficient but it works in any basement including one with a regular furnace.

Cold basements are always caused by air leaks. This also costs your significant heating costs, so the best way to keep your basement warm and comfortable is to air seal.

Need some advice about working with a buffer tank and an indirect

Comments

Categories