Pump VSD control without a pressure sensor

[tabarouette]

Hello,
I have a building with hydronic radiant floor heating. Each floor loop is balanced for equal pressure drop and has a control valve and goes back to a manifold which wich serves 3 to 6 floor loops (depending on the manifold). Each manifold has a pump with a VSD but there are no pressure sensors with which to control the VSDs.

The control valves for the floor loops are two position valves and open and close based on a call for heating from zone thermostats. Currently, the speeds for the VSD are set directly proportional to the amount of control valves that are open. So if the manifold has 4 loops and only two of them have control valves that are open, the VSD will run at 50%. This doesn't seem right to me since the head produced by the pump will drop off significantly and the floor loops which are calling for heat will get choked off, correct?

Is there any way to figure out the proper VSD speeds to set at part load conditions without a pressure sensor? Would the best strategy be to just set it to run at 100% speed when any floor loop control valve is open?

Thanks,
JR

 

[DRWeig]

Hi JR,

Welcome to eng-tips.

From reading your post, it sounds like this is an existing occupied building?

Why can't you get a pressure sensor installed?

Remember that when valves close, the system curve changes. Your pump has to slow down to keep the required differential pressure. The settings that are in place now could very well be just fine. You'll have no way to verify without a pressure sensor.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[Drazen]

if all radiant loops are balanced for equal pressure drop, that is mistake.

loop valves serve to precisely adjust flow for each loop, not to balance loops for equal pressure.

as regards to vsd pumps, many pumps now sense current and adjust speed, so you can say pressure sensing is integrated with pump body. external sensor is an option which is applied for long circles where you want to hold pressure at distant spots.

 

[EnergyProfessional]

Try the Grundfos Magna or Alpha. they have sensor and algorithm and speed control built in.

those seem to be good when you don't have a sophisticated Building automation system.

 

[Compositepro]

It is foolish to have any VSD at all. What is the purpose of a VSD? To save energy? On an heating system designed to heat your house? Any small drop in energy efficiency in your pump simply adds a little heat to your house, so you save nothing. You will save a little weight in your wallet.

 

[EnergyProfessional]

Compositepro: are you for real? Are you saying every HVAC design of the past few years was designed by idiots?

Did you ever hear that electricity you need to run for pumps is more expensive than the natural gas you burn in boilers?
It goes beyond energy, just look at less wear and noise.

With variable speed pumps you also can match the boiler burner rate and reduce return water temp, increasing boiler efficiency. I use small VSD pumps on small boiler primary loops (new boiers have built in control to regualte the pump). On solar systems I use them to maintain a constand dT for better efficiency and less pump-cycling. When you use VFDs, you may be able to keep fewer pumps in stock for maintenance.

VSD arepretty much standard and don't really add that much more cost whne you figure in savings for balancing valves, cheaper plumbing with 2-way valves etc. The VSD also can monitor current and detect motor failures, provide softstart.

You may think there is areason why they are used everywhere.

 

[tabarouette]

Thanks for the help everyone.

This is an existing system in a school. I'm assuming they put in the VSDs to save energy since this is a LEED gold building.

I don't want to go through the expense of replacing the pumps with new ones with speed control built in or adding pressure sensors to all the manifolds. I just want to improve the control strategy of the VSDs because the current strategy doesn't seem to be working. Under part load conditions certain loops that are open don't heat up properly.

 

[EnergyProfessional]

Tabarouette: since thsi is a school they likley have a Building automation system (BAS). So with the VSDs you will need a differential-pressure sensor (you only need one in the system ...either at the largest coil, or somehwere 2/3 in the system as rule of thumb).

Without the dP sensor, what are the VSDs doing? did they jsut get used to balance (instead of balancing valve at pump) and run at a set speed (i.e. 51 Hz)?
I recently came across a badly designed so-called LEED building with the same setup where they installed VSDs without any dP sensor or control sequence in the specs. Apparently designed by a fool, but a green design fool in this case . Don't get me started on LEED...

 

[tabarouette]

The building does have a Delta controls BAS system which was poorly programmed. I have been going through the issues and that is how my question came up.

The VSD speeds are currently set based on the amount of control valves that are open (ex: if a manifold has 4 loops and 3 of the associated control valves are open, the VSD will run at 75%). I understand adding dP sensors at each manifold would be the best solution, but I don't have the funds right now (plus this isn't a critical issue just an annoyance).

 

[ChasBean1]

Tabarouette, correct.

Flow is assumingly proportional to speed based on pump laws, but in your application you want to maintain a relatively constant differential pressure regardless of how many zones call.

If you operate at half speed, there is about one-eighth the DP. If you operate at a quarter speed, there is about a sixteenth the DP.

For your application, assuming each load is equal and there are 4 zones, I would operate the zones as: 100% pump speed for 4 zones (balance the pump circuit setter for the design flow value); 87% for 3 zones; 71% for 2 zones; and 50% for one zone.

 

[ozmosis]

There are companies who manufacture VFDs that can run "sensorless"(with patents I might add). These are typically supplied to pump companies who incorporate their specific algorithms designed around their own pump curves. One such company is Armstrong pumps:

http://m.youtube.com/watch?v=IFMrsy4zmv0&desktop_uri=/watch?v=IFMrsy4zmv0

Sensorless control does not work in every situation, but when applied in a retrofit situation and the cost of sensors and installation starts bumping the overall system cost up, it can be an interesting alternative.
I have often found that even when a (pressure) sensor is available for a VFD, it is often placed wrongly (usually too close to the pump) and does not actually bring any benefit at all. These could be described as "senseless"

 

[MintJulep]

You could do this with the temperature information from the zones. You must already have this as an input to the BAS to make the open or close a valve decision.

If a valve is open but the temperature is not increasing then increase pump speed.

 

[EnergyProfessional]

MintJulep: if the zone unit is undersized, that would lead to a lot of pump waste without much outcome. For example if the VAV box air flow is limited, increasing coil flow would just lead to a bit higher DAT, which may stick to the ceiling, making the heating problem even worse.
this only would work if the VAV box has DAT sensor and the controls limit DAT to 110°F or whatever is chose.

What I do is set back the dP setpoint based on valve positions, but I eliminate 1 (or how many I feel like) valves as rogue zones. So if 2 (1+1) valves are 100% open, I increase pressure, if only one is 100% open, i decrease dP setpoint. I also set a pressure band. Like 4-5.5. psi. and static pressure will float in between depending on valve position. this really works well, I'm implementing this in all our existing buildings.

 

[MintJulep]

HerrKaLeun,

We've been told that the system is radiant floor heat.
We've been told that each zone has an open or closed control valve.
It appears that that the controls programmer and the heating engineer didn't talk, because the control logic is wrong for the way the piping is installed and balanced.
We've been told that the problem that needs to be solved is lack of heat.
We've been given a constraint that we can't add pressure sensors.

My suggestion works within those bounds.

One or more zones call for heat and open their valves.

Of those zones, one or more do not get sufficient heat. So the logic calls for the pump to speed up - delivering more heat.
Some zones may reach the upper limit of their setpoint deadband. The will close their valves.

Any remaining zones will either still not get enough heat, causing the pump to speed up again, or will warm up and close their valves.

Eventually all zones will be satisfied and close their valves and the pump speed gets reset, and we start again.

Or another zone calls for heat and joins the party.

Doesn't matter, it will work just fine.

 

[Drazen]

mint, surface temperature of radiant floor is limited by norm for health reasons, so increase of pump speed should have to be controlled by surface sensor for each loop.

that on-off valves would have little sense as well, with non-linear output-input characteristic of floor and high inertia, you could easily get into frequent pump jumping.

the only purpose i can see with vsd pump is to maintain constant pressure on supply, other solution renders many things - control valves, flow adjustment valves, even sheer radiant thermal calculations - useless.

 

[GMcD]

I also wonder what type of radiant floor heating this is, considering it's a school. If it is tubing in a concrete topping or slab, then the whole control strategy needs to be reviewed due to the very slow response time of heated concrete. Too many times I see concrete radiant floors trying to be operated like a piece of wall-fin, and it just won't work. You MUST have a floor or slab temperature sensor so you don't overshoot, and trying to use room temperature setback is also asking for trouble.

Even if the radiant tubing is in a 2" thick topping, that can be a 2 to 3 hour response time to change the temperature of the slab.

 

[EnergyProfessional]

GMcD: good points. Do you pour the slab sensor in the concrete? Adn if so, how do yourepalce it? Or do you pour some counduit in, or basically have it like a well-sensor? and what height should the sensor be, surface?
The reason I'm asking is that I'm right now commissioning such system (designed by others) and the slab is 5"and 6" thick and the tubes are at the bottom. So I have the choice to drill a hole to add such sensor.

I was kind of concerned about reaxtion time as well. From cold to warm you easily have 24 hours.

 

[GMcD]

HerrKaLeun: With that much concrete you MUST have a slab temperature sensor to be able to control it properly. Some folks have been controlling slabs "sort-of" by using the slab loop return water temperature to use as a proxy for the slab temperature, and this could work with some tuning. Generally I've used a standard stainless steel immersion sensor attached to a standard junction box cast into the slab and then run the wires in a conduit so you can service the sensor in the future. The sensor should be placed near the "controlled surface" so you can know what the slab surface temperature is going to be.

Question on your particular system - with the tubes in the bottom of the slab - are you trying to create a heated floor surface, or a radiant ceiling surface for the space below the slab? Generally the idea is to get tubing as close to the surface of your intended "radiant surface" for best control and heat transfer.

 

[EnergyProfessional]

GMcD: thsi system is a heated floor. This is a slab-on-grade installation with 4"of insulation under the slab. the plans called for the tubes to be 2"below the surface. However, during construction it turned out there wasn't a way to elevate the tubes when the concrete gets poured. teh designers felt it woudl work to just leave it on the insulation and stable the tubes so they don't move around when the concrete is poured.

I'd be curius to hear if there is apractial way to install tubes 2"below sirface (3-4" elevated.

 

[GMcD]

HerrKaLeun: Elevated radiant tubing is easy if it's done properly - depending on how the reinforcing is installed in the slab, one can use a 6"x6" (150mm x 150mm) steel mesh elevated by rebar chairs to keep the tubing up near the surface. If there is a top layer of rebar then fix the tubing to that, or use the mesh and tie that to the top layer of rebar to keep the tubing within a couple inches of the surface. If the concern was telegraphing shrinkage cracks from the tubes being too close to the surface, use concrete with a fibre mix that resists shrinkage cracks. Or is the fear of hitting tubes from saw-cutting control joints in the slab? Either way, it can be done if the designers are on the same page with what the end goals are supposed to be for the slab.

That being said, your tubes down near the bottom of the concrete (and great that you've got really good insulation below it!) will still work, you'll just have to tune the fluid temperature to work with the amount of concrete between the tube and the concrete surface to get the floor surface temperature you want. There are some good CFD diagrams and time to temperature charts at

http://www.healthyheating.com

- try here:

http://www.healthyheating.com/Effec...nd-heating-pipes-in-concrete.htm#.Uol4XKwWLmQ

and scroll down through the slides and search around the site for more detailed information.