WLHP w/ Condensing Boiler 3

[dewall]

I am trying to determine the energy use of a water loop heat pump system augmented by a consdensing (munchkin) high efficiency boiler. This is beyond my software packages' capabilities (treat and rem). Are there any resources for this type of calculation?

I'm the intern and the engineer who advised this option left before providing calculations of this sort.

Thanks
Eric W.

 

[MintJulep]

(Boiler energy in / boiler efficiency) + (heat pump energy in / COP) = total building heat load.

More or less.

 

[dewall]

Thanks. I'm not sure I adequately described the system. The boiler is used to keep the wlhp at the desired temp. The idea (as I understand it) is to be able lower the size of the boiler. Does this formula still apply? If so how would be able to determine what size boiler would be required for the retrofit?
Again thanks for the advice.

 

[Whoops]

I would have thought that the building's heat distribution system would be piped on a primary circuit, with the heat generators (WLHP and boiler) on a secondary circuits. The WLHP would probably be working as the lead boiler.

You should have the design heat load required by the heat distribution system from the heat loss calculations. The capacity of the existing boiler & WLHP should not be less than the design heat losses.

You have to make an informed decision on how many boilers/heat pumps you want to supply the heat required. You may want to provide some stand-by capacity, to allow for plant failure. You may want to have a small boiler or a modulating burner to deal with summer part-load conditions (HWS generation).

Is the existing boiler & HP being removed or have they proved to be inadequate? The Munchkin can't be very old, can it?

 

[MintJulep]

Whoops' post makes me think that we still don't have what we need to understand the system.

I've seen buildings that have numerous water-source heat pumps throughout the building, all sharing a common loop.

The concept is that core areas of the building need cooling year-round. These core areas work in cooling mode, rejecting heat to the loop, where the perimeter area heat pumps can draw it from the loop.

Is your building one of those?

 

[dewall]

We decided to scrap the recommendation that the former advisor made. The idea was to recommend the heat pump-boiler combo as a means to save money over high efficiency boiler or furnace but the pump loads do not justify the measures.

The term retrofit was incorrect as you can't go in and replace a system with heat pump easily. I was trying to make an equivalent model of the building that used heat pump boiler combo and compare energy usage to see actual savings. The building type in question is a common type that is being built across the state.

I'm going to try and garner more info on how these systems actually work, how they are sized and if the lower boiler size/ set-point warrants enough savings to offset the cost of the electrical load caused by the wlhp.

Thanks for the info.
If you know of any references Id welcome them.

 

[ChrisConley]

MintJulep,

I think the systems is as you described, but in heating climates there is also a boiler used to keep the loop temperature at the desired setpoint (72F usually). You can also hook a cooling tower into the loop to reject heat on cooling days.

The main advantage (as I see it) is that you can effciently move heat from the core to the perimeter, and you can circulate 'heating' and 'cooling' water without insulating the mains. And, by using 'cold' water in the loop as well you can easily ensure that your expensive condensing boiler actually condenses all the time.

 

[MintJulep]

Chris,

I've seen several systems like the one I described. It's an interesting concept, but none that I have seen have worked well.

One was installed in a large building that was essentially all perimeter space and no core. The boiler was doing all of the work, but the loop temperature was - as you note - low. Pay for the boiler to make a lot of heat. Move it at low temperature to occupied space. Pay to run heat pump to raise the quality of the heat to a useable level. Electric rates were not favorable.

Boiler was short-cycling, and running cold - very poor efficiencies. It was condensing in the flue, although it was not a condensing design.

We recommended abandoing the heat pumps in place, converting their air handling sections to conventional water coils and cranking up the loop temperature to more conventional levels. Savings were something like a quarter-million $US per year, with a three or four year payback.

Other systems I have seen used black pipe for the loop, with plain water and no treatment. Continual problems with water-side of HX in the heat pumps fouling. Eventually I suspect there will be loop leaks.

My view is that this type of system requires a very specific combination of building design, occupancy/use patterns, climate, and utility rate structure to be feasible, and the magic combination doesn't occur very frequently in the real world.

In climates where it might work you can usually get plenty of free cooling via economizer-type operation for the core building areas.

 

[walkes]

Dewall,

Another thought in assessing the application is the requirement for simultaneous heating and cooling throughout the building. The heat pump application sees most of the benefit in shoulder seasons where various parts require cooling (south facing windows, solar loads) compared to north side that may still need heat.

The boiler would be smaller than a building with fan coils or perimeter radiation because as Whoops said the cop of the heat pump adds to the heat (compressor heat rejection offsets the boiler load)

You can download heat pump design manuals from the McQuay website that may prove useful in understanding various types of heat pump applications.

http://www.mcquay.com/mcquaybiz/literature/lit_at_wshp/Catalogs/C_330-1.pdf