DPV Undersized?

Brad_19

The problem is the DPV will not close since the upper limit is 7.5 psi and I’m at 15 in all operating conditions. The valve is always wide open. I have no way to tell what will happen unless I replace the DPV with a valve.

Design flow for both circuits open is 8.6.

Not sure if I need the DPV or not. What I do know is I need more flow in the circuits.

Brad_19

DPV Undersized?

I have a simple 2 zone system controlled by zone valves and a single circ pump. The system stand still (and operating) pressure is 15psi. I have a Taco DPV (0-7.5 psi) installed and when the system is flowing I believe I’m losing a majority of the flow through the DPV, starving the heating zones.

There is a 30 degree temperature drop in both zones, it was designed for a 10 degree drop.

Will installing a Honeywell DPV that is adjustable up to 20 psi solve this? Other options?

Brad White_9

Sounds like

The DP valve is oversized and is not resisting the bypass pressure well enough.

For a "simple system", I would ask, do you really need a DPV? I suspect that you do not.

If you know your flow and head per zone and apply this to the circulator curve (pump curve), what is the net effect?

I am suggesting that it may very well ride the curve without detriment. Try cranking down the DPV to closed and see what happens. I submit that if your unused zone chatters on such a simple system, you have "too much pump".

For illustration, my own house has two distinct branches, east and west and reverse-return. No control valves for the house is one large zone, but I do have manual valves isolating each side.

I have a common B&G visual flow meter on my circulator. When both branches are flowing as is the normal condition, I flow 5.5 GPM. When I close one manual valve, isolating half the house, the flow drops to 4.5 GPM. No discernable difference in noise or any other effect.

ALH_4

differential pressure

The static pressure of 15 psi does not affect the differential pressure valve. It is the differential pressure across the zones at design flow. Unless you have relatively high head loss in your zones, turning the dP valve all the way to 7.5psi should force all, or nearly all, of the flow through the zones. 7.5 psi is more differential presure than a Taco 007 can produce.

What type of system is this? Radiant floors or baseboard/radiators? Does the system heat ok?

Brad White_9

I skipped over that part

and Andrew is absolutely correct, 15 PSI static means bupkes as far as your DPV is concerned. It is all about differential pressure. I paid that no mind and focused on the larger picture.

Do you know your desired head loss? I suspect strongly that a properly selected circulator can serve both reasonably and ought not generate significant differential pressure in the process. Most heating system distribution head requirements are lower than most people think.

EDIT: As you just reported (duh) you obviously do have a need for that kind of head pressure. Have you considered constant flow instead with variable temperature injection? Your use of control valves which shut off may be part of the reason you are getting cold spots. Constant circulation has a way of leveling out bumps in the temperature road.

Brad_19

This is a radiant floor (concrete) system. 2 zones, 1400sf and 1000sf. There are 2 large zones with high head. The larger zone is designed for 3.2 gpm and 18’ of head. The smaller zone 2 gpm and 10’. The system does not heat as it should. The water returns slightly above set point from the circuits. It loses all the heat before it returns leading to cold spots on the floor. The boiler has no problem supplying hot water. I believe I just need to get the flow up.
IA Taco pump was selected but the plumber installed a Grundfos pump and I do not have the model number with me.

Thanks for the replies!

ALH_4

design

What pump do you have? 7.5psi is 17.3ft, so it would be close to max for the dP valve. Real world temperature drop is only loosely correlated with design temperature drop. Shorter loops are the best plan of attack to keep the temperature drop small in a radiant system. Higher flow rates can also decrease the temperature drop, but at the cost of parasitic energy loss in the electricity to run the larger pump and possible problems with high fluid velocity in the pipes. Tube layout also affects the uniformity of the floor surface temperature. Spiral loops or embedded loops are most effective at prividing an even floor temp.

How long are the pex loops? The 1400sf zone should have (5) 280ft loops of 1/2" pex at 12" on center with a 0.7 gpm flow rate through each loop for a total head loss of 5 ft not including distribution.

Also at 10°F temperature drop, 3.2 gpm only delivers 16,000 btu/h or 11.5 btu/sf, which is a relatively low design load.

Brad_19

You guys are absolutely correct on the head loss/system pressure. I was thinking about it wrong. I’ll have to pull the pump model number when I get home tonight.
The pressure drop includes only the PEX, does not include the manifolds, H/E, valves...

1400sf has 7 loops at 280’. The 1000sf zone has 4 @ 290’. This is a new house that is very well insulated. The water is heated off a H/E on the boiler and I have no easy (cheap) way to modulate the water temperature.

Maybe the best thing to do would be to pull the DPV and replace with a valve ‘closed’ to see what happens with the return temp?

ALH_4

pump

Ideally, you would design for a higher temp drop (20°F) and provide better control of the system by using constant circulation on outdoor reset. That way the floor temp comes to equilibrium instead of hot-cold-hot-cold with on-off operation. This would be a significant investment in piping and controls, but is really the only way to provide a more even floor temperature.

Brad White_9

What he said.

Yup, Andrew and I agree and we do not pay each other. We never even met (yet!)

hot_rod

I don't see a reason for that valve

I use them on nmulti zone valve systems, typically 5 ZVs or more.

Seems you should be able to size a circ close enough to eliminate any need for the parasitic device.

If it were me I'd wait for the smart circs to come out. install a new Wilo Stratos in there and heat with a 10W circ

Sheeeding head through a DPV still costs you that 80W.

hot rod

Brad_19

modulating temp.

I agree to but that wasn't in the construction budget and there isn't anything I can do about it now.

I just need to find a way to limit the temp drop to 20 degrees so the return temp is above set point. I would be satisfied with warm and warmer spots, not so happy with the current warm and ambient spots.

I should add that the 1000sf zone is the garage so its not a real big issue.

Thanks again for the help.

Brad_19

Valve

The pump is a Grundfos UP26-64F. If I'm calculating correct it doesn't look like the DPV is needed and hot rod is correct. It was very hard to graph but the bottom line appears that in all cases there is a high head and the flow will be under 5gpm which keeps the system in the flat part of the pump curve.

Both open - Head=23, Flow=4.3
1000sf open - H=23.5, F=3
1400sf open - H=23.8, F=3.4

Brad White_9

Exactly

Amazing how little difference there is sometimes, especially when both zones are similar.