Minimal flow requirement for a heat pump

[IcerCRO]

Greetings

I'm working on a air-water heat pump system for a residential building (both heating and cooling) using split system heat pumps.
I'm wondering what the 'minimal flow requirement' on the water side means exactly - is it just centrifugal pump protection - requiring only minimal 'FLOW' as stated, or is the requirement for 'FLOW at a delta-T' implying protection for other elements of the heat pump (eg. ensuring sufficient evaporation/condensation in the inner unit of the heat pump).

I'm asking this question because of space limitations - they are for small apartments and space is too scarce to fit even a small buffer tank - thus I have to be creative in solving the problem and ensuring the heat pump doesn't go into a blockade every 5 minutes.

The installed heat pumps would be Viessmann Vitocal 222-S units, but I imagine every split air-water unit on the market has a similar 'minimum-flow' requirement.
The water side consumers are a combination of floor heating, fan-coils and a bathroom radiator.

Thanks in advance, hope the question wasn't misplaced

 

[317069]

Can you explain your system a little more, you say you are using air water heat pump(I guess you mean water source heat pump) ,then you say you are using a split system heat pump? I couldn't get what did you mean by split system.
you are using heat pump for small apartment, then you say you are connecting your water side to fan coils, radiator. floor heating how come it is small apartment and contain all this systems and what buffer tank you want to install even if you have a space.

 

[Drazen]

it can be either of reason you mentioned, or both, but it cannot be ignored.

you don't ultimately need buffer tank, you can use hydronic decoupler instead.

 

[IcerCRO]

Thanks for the quick replies.

317069 the heat pump is air source and the heating/cooling medium is water (going through floor heating, radiators and fan-coils). By split-type heat pump I mean the 'outside' unit (containing the compressor and one freon heat exchanger) is separated from another 'inside' unit (which usually contains another freon heat exchanger, secondary water pump, electric heater and optionally a domestic hot water tank). The alternative type to the split-system heat pumps are so called 'monobloc' units which contain all elements in one compact structure.

Dražen - I know it's supposed to work in theory, I'm a bit hesitant due to systems without buffer tanks working unreliably.

 

[ChasBean1]

Air-water might mean you have some sort of condensing unit on the roof which could have a minimum flow requirement, then an air conditioning unit that might have another, then the pumps themselves which should move about a quarter of their design flow if they are CV.

To keep a minimum flow, have a loop bypass. The building automation system should look at all loads; as the average of the loads reduces from 50% to 0%, the bypass valve modulates from 0% to 100% open. This is a simple control that can ensure everything is set right and can be adjusted if necessary..

 

[EnergyProfessional]

Manufactuere can tell you flow and temp requirements. HP can be very finicky when they don't get enough flow or too cold/hot flow.

Regarding buffertank, that depends on your loop size (volume). Most manufacturers tell you how many minutes of flowloop there needs to be. If for example they say 4 minutes and your flowrate is 100 gpm, you need 400 gpm system volume and if your piping is too small, you need to add the buffer tank. Obvioulsy more is always better.

But really look into units with digital scroll compressor so you can modulate. Much better operation (and efficiency)

 

[IcerCRO]

The freon loop is user/designer independent, the minimum flow requirement I'm referring to here is only on the water side. Water side being inside the apartments, from freon-water heat exchanger (inside the heat pump hydro-box) to fan-coils and floor heating units. The circulation pump is integrated inside the unit offering some 45 kPa head at 1000 l/h flow.

The user manual states a minimum flow of 800 l/h at Δt = 10 K , implying a requirement of volume flow and heat loss(gain). I suspect this is only a volume flow requirement, since the buffer tank does not guarantee a Δt, and a sufficient volume flow can be achieved via piping and bypass valve solution (as Dražen and ChasBean1 suggested). Unfortunately there is no room to even dream of a buffer tank, I'll be happy if I'll be able to fit expansion vessels out of sight.

HerrKaLeun - the compressors are rotary piston type but are equipped with an inverter (modulator), at least the manual states as much.

The manufacturer has a disappointingly poor technical support network in Croatia regarding heat pumps, when a technical support official stated that 100 kW of electrical power would need to be leased for 50 kW of heating via heat pumps we kinda figured we were on our own on this one... The nominal power requirement for the worst case winter is about 30kW of electrical power (for the entire apartment complex, naturally).

 

[Drazen]

Buffer tanks had their time when compressors had much more limited means of modulation, they are mostly history now.

Decoupler is usual solution which is installed almost by default. You need additional pump for secondary circuit, though, but that should be minor addition to costs for small capacity range you described.

You can add to it by removing on-off valves from fc's completely, so that fc's are controlled only by fan speed.

 

[EnergyProfessional]

"The manufacturer has a disappointingly poor technical support network in"
After reading this sentence (and considering you didn't even go into installation and startup problems yet) I only can advise to look for a different manufacturer.

 

[IcerCRO]

I'm only an apprentice, just out of university in July '13, so I wouldn't want to get ahead of myself regarding hardware dealers. From what I hear, their hardware is top-line, but we are seriously disappointinted with them (support team in our country) on this last project. We might as well change to Daikin or Mitsubishi...

Thanks for the help, I'll revise the schematic and BOM so we can get to work with the installations.

Agree with Dražen again - modulation is a standard in air-source heat pumps but is rarely seen in water- or ground-source counterparts, so buffers there are a must I guess. I as well came to the conclusion of leaving the water flow through the FCs, I won't be installing on-off valves, since their primary use will be for cooling. Don't know about another secondary pump though, according to standard calculations this one should be more than enough (the lines are really short). I am considering a pump -mixing valve group, but since the leaving water temperature will never need to be above 40 °C I don't think it will really be necessary.

 

[EnergyProfessional]

what manufacturer do you have? If possible, change to a good one... unless you like trouble. In the end your client will remember your name when he has problems, not that manufacturer.

 

[IcerCRO]

That's true. We've got to make it work, and the retailers only have to sell. See first post for unit detalis HerrKaLeun

 

[EnergyProfessional]

I'd think Viessman typically is good. Not sure about their heatpumps, though.

But you will learn, even with very good equipment you will have problems and will need service for both design and problems. And if not for you, what is your client supposed to do n 5 years if he needs repair and Viessman doesn't provide good support?

Will your client then also have to come to this forum for help? Will he then read this thread and see how we warned you?

If possible stay with Mitsubishi. They are large, spare parts are plenty and at least here int he US they give good support. Daikin also is good, but here in the US is much more proprietary than Mitsu. In my town i can get Mitsu spare parts form multiple retailers, Daikin only from one (not sure if they are jsut too small, or have protective areas). If everything else is equal, I go with Mitsu for that reason.

How many service contractors in your area are licensed to service Mitsubishi vs. Viessman heatpumps? How quickly and cheaply can you get spare parts? I thought so...

 

[PZC]

You can visit the Kensa Heat Pump ( UK ) web site and find out the technical details.