DHW heat exchanger

[rhett1948]

I have a Tarm 402 boiler (30+ y.o.) in which the domestic hot water coil is badly calcified. So far, I haven't had any luck finding a new one and am considering acidizing what I have. I'd really rather not do that, would rather replace it.

A flat plate heat exchanger was suggested to me and my only experience with them is in an industrial setting (big ones). As far as I know, they require a powered flow of hot water from the boiler to provide the exchanged heat for the domestic hot water. If someone can explain a proper setup, I'd appreciate it.

What I like about the immersed coil is that I don't have to run a pump of any sort to have domestic hot water during the summer when I'm not heating the house, only build enough fire to heat the boiler contents (about 45 gallons), which provides me with a day's worth of hot water via the coil. My system is quite simple--boiler/one zone valve for the house baseboard convectors/baseboards/no heat storage tank.

I'm open to all ideas and suggestions and thanks to all who respond.

 

[Frozen Canuck]

You would need 2 pumps albeit small ones for a flat plate, 1 for the boiler water & 1 for the domestic water. That is if your considering a setup like an indirect water heater.

 

[rhett1948]

You would need 2 pumps albeit small ones for a flat plate, 1 for the boiler water & 1 for the domestic water. That is if your considering a setup like an indirect water heater.

The domestic water would be "powered" by well/holding tank pressure, but that would still leave me needing a circulator to run boiler water through the heat exchanger. How would one set that up for turning the circulator on when hot water is drawn?

 

[Frozen Canuck]

If I understand you correctly you would need a flow switch mounted in dhw piping that would turn on the boiler pump. I am not sure how well that will satisfy temp though as I doubt anything but an exterely large (see also very costly) exchanger can get close to a 1:1 exchange rate. Most systems use an auastat to control a pump for DHW circ & run that DHW & boiler pump until aquastat is satisfied.

 

[rhett1948]

If I understand you correctly you would need a flow switch mounted in dhw piping that would turn on the boiler pump. I am not sure how well that will satisfy temp though as I doubt anything but an exterely large (see also very costly) exchanger can get close to a 1:1 exchange rate. Most systems use an auastat to control a pump for DHW circ & run that DHW & boiler pump until aquastat is satisfied.

You're right, doesn't look like it'd work in my situation. Webbie suggested a Superstor indirect heater and that would be more workable for me.

Thanks for your input.

 

[Bob Rohr]

You're right, doesn't look like it'd work in my situation. Webbie suggested a Superstor indirect heater and that would be more workable for me.

Thanks for your input.

If you have a buffer tank of 100 gallons or more, consider a plate style HX. I used a 30 plate 5X12, around $200 some bucks. It will supply about 3 gpm continous with 55F incoming, 115 out. The "A" side pump is turned on with a flow switch on the cold water inlet to the HX. I am thinking about a aquastat instead of a flow switch as mine has been problematic. It turns on fine, but not always off. The temperature control would respond to the incoming well temperature and turn off at 80F or something like that.

The advantage to a plate HX for instant DHW is you do not need to store ant DHW, if you already have a boiler buffer tank, of course.

Any HX installed in hard water systems should have a means to de-lime. With a plate HX, install it with unions. If performance drops take it to a radiator shop and let them cook it clean, or use a de-lime chemical available from any HVAC supplier.