Viessmann short-cycling on call for DHW.

Alan (California Radiant) Forbes

I just looked at this job where there are two, 119 gal. Superstor indirects. Owner says that the boiler is always making DHW and the house is cold. The DHW pump is a Grundfos 26-64 and the Superstor HX's have an 11' pressure drop at 10 gpm and the two tanks are piped in series, so that's 22' of head plus another 10' of head for the boiler.

Not even a Grundfos 26-99 can flow at 30' of head. Is my thinking right and does anyone have any pump suggestions?

Larry Weingarten

Hi, Would putting the tanks in parallel help enough?

Yours, Larry

GW

Maybe pump each system by itself, with a bit of a header in between the pumps. Keep an eye on the amp draw. Might need a relay

Alan (California Radiant) Forbes

Would putting the tanks in parallel help enough?

Definitely! I'm thinking about doing that; isolating each tank and even turning one of them off and draining it. He has seven or eight bathrooms, but it's just the two of them and I don't think they need that much DHW.

EBEBRATT-Ed

That would fix it as long as the 26-64 will pump the gpm at the head you need. Your flow is cut in half of what it needed in series so it probably will.

How long has the poor boiler been sitting there tripping limit and burning fuel?

Seems like it would have heated the house it it was hitting limit and not moving enough through the indirects

Alan (California Radiant) Forbes

How long has the poor boiler been sitting there tripping limit and burning fuel?

They've just been in the house for 1 year. Who knows how long it's been going on.

Seems like it would have heated the house it it was hitting limit and not moving enough through the indirects.

The slope of the ODR was set at 0.2 which I think is too low. I bumped it up a bit.

sunlight33

Two 119 gal indirect? How big is the house?

Paul Pollets

I've installed a 120g tank in several 8+ bathroom homes and never ran out of hot water. The control can be set for priority DHW recovery and I'd isolate one tank and try to see if it keeps up with the demand. The heating curve is also low. I've found that a .7 curve is about as low as it can work for a well insulated structure.

Derheatmeister

No Problem...
I would repipe the >>domestic side<<in series while heating the second tank only...
Of course I would raise the tank temp to 140 F and have a ASV and flow tru thermal expansion tank involved..
Allthough i do not think that it may be needed but this size house probably also has a Domestic recirc line that could be interfaced with the first tank..this setup will recirc between the two tanks..
Hope this helps.
Happy Holidays

Alan (California Radiant) Forbes

The house is over 12,000 square feet; two people.

The DHW setup is overkill. No way they need that much hot water, especially piping it the way the installer did, creating that much flow resistance to require such a large pump.

.......I'd isolate one tank and try to see if it keeps up with the demand.

Paul: No can do without extensive re-piping since both heating and DHW are piped in series.