Lennox RT Roof Top Unit - Coversion to Heat Pump???? [Archive]

aztech03

10-01-2012, 03:39 PM

I have a requirement to convert a number of Lennox RT Roof top units.

The machine was originally designed with a Integral DX Cooling Only and Remote LPHW fed for Heating via a heating coil.

Lennox made the same machine as a heat pump called the RTPC. I am unable to get parts list for either machine to see if the coil are the same.

Now, I know there are many issues to overcome but if you have ever been involved with a conversion of this kind before, even if it failed or failed to get of the ground, I'd like to talk to you.

What I'd like is a Technical/Practical partner on this as a viable alternative to the complete replacment to the plant. The client just does not want to spend £250k+ on new plant when all these machines have is a catostrophic heating failure.

Please contact me if you can help.

Greengrocer

10-01-2012, 04:18 PM

How old is the existing plant & what refrigerant are they running on?

aztech03

10-01-2012, 06:30 PM

Circa 1998 and currently on R22. I dont want all the 'Oh it's on R22, Blaa Blaa Blaa' debate.

I'd like to explore a solution to convert the equipment to an air cooled heat pump and ommit the current LPHW circuit and controls.

I appreciate it will be fairly involved. Ther is lots of space in the Condensing unit area to carry out the installaton of the new componants.

It may even involve replacing the current Evaporators. I susspect they are too small to be used in heating mode. Unfortunatly, the abscense of parts lists for the Cooling only and Heat pump machien isn't helping.

monkey spanners

10-01-2012, 06:56 PM

Worked on some Daikin 80kw jobbies that had been converted to heatpump from cooling only.
Had accumilator, reversing valve, some check valves, and a couple of txvs for the condenser coil which had the header cut and split into two. And a suitable change to the controls. Seemed to work ok.

Don't see why you couldn't try a conversion on one and see how well it works, might pay to put some monitoring equipment on it to see how efficient it is.

If the customer is happy for you to try and knows the risks that it might not work, why not give it a go. its jobs like these that stop me getting bored!

Jon :)

aztech03

10-01-2012, 07:05 PM

Funny you say that Jon,

I was going to suggest to the customer that we try it on one machine first and test it.

I can D&B the fridge side but the controls have been raped on most of the machines and in any case wouldn't work as a heat pump.

Can anybody suggest a wholesaller or Build/Sell me a stand alone control Panel to operate the roof top unit as an air cooled heat pump.

I can provide more info if anybody needs it.

Greengrocer

10-01-2012, 10:38 PM

The fundamental differences between a cooling only & Heat Pump rooftop system in your situation are:-

1. 1. New Outdoor & Indoor heat exchanger coils will be required as you cannot use the existing ones. Heat Pump system coils will need to be larger or have more rows / internal volume and different circuiting to operate as both an evaporator & as a condendenser. This will be dictated by the physical space within the existing chassis & the air volumes – see 7 below.

2. 2. Additional check valves in pipe-work &/or in coil blocks. The coil circuiting in cooling & heating mode for each coil needs to be different for each mode of operation. The internal volume of the new coils will dictate the size of the accumulator - see 5 below.

3. 3. 1 Reversing valve for each frig circuit – I assume there is more than one frig circuit in each rooftop.

4. 4. Mechanical or electronic Expansion devices (two way type required for each coil or, 2 off one way TXV’s with check valves for each coil).

5. 5. 1 new accumulator for each frig circuit to protect the compressor from liquid flood back & to hold the excess refrigerant volume when the system operates in heating mode.

6. 6. New Compressors. If you are going to consider this amount of work then new compressors and a change of refrigerant should be included to maximise performance. R410a would be my choice.

7. 7. New interconnecting refrigerant pipe-work. R410a runs at higher pressures than R22 so there's no gaurantee that any of the old pipe-work is up to the higher pressures. Also because the mineral oil used in the old R22 system is not compatible with an R410a system and the new compressor(s).

8. 8 . Airflow – HP units have a higher air volume on the indoor & outdoor coils for the same duty as a cooling only system. If you plan to keep the same indoor fan / motor then it's capacity / volume will limit the overall capacity of the modified machine. Note that you will need to know what the existing fan air volumes are when designing the rest of the system components.

9. 9. Add 3 phase electric heaters in the indoor air stream to top up heat pump capacity &/or as back up to heat pump capacity. Electric heater capacity should be the same capacity as one of the frig circuits & controlled as 3/4th stage heating or in an emergency if one of the frig circuits fails.

1010. Controls. Complete replacement system required including any BMS interfaces that may be required. New safeties will be required and calibrated for the new refrigerant and heat pump operation. Sorry don’t know of any panel manufacturers to reccomend.

In effect you need to completely re-design and replace the entire frig / airside & control system because very little of what you have will be re-usable.

Also, do these existing Roof top units introduce any fresh air & do they have “free cooling” economisers fitted? If so, you need to know how much fresh air is needed (to meet regs) or is being introduced via the rooftop units. Note. Heat pump systems cannot operate with more than 10-15% of outside / fresh air in heating mode (10-15% of fresh air in comparrison to total air volume). Depending on the system air volume the TD across the indoor coil in heating mode will be around 12-15°c. So, if due to the temperature &/or quantity of fresh air the air-on temperature to the indoor coil drops below 15°c the system will not work. It will struggle to produce a leaving air temperature much higher than 27°c which is not enough for space heating purposes and would take an age to warm up the building from a standing start or a Monday morning in mid winter. The minimum design leaving air temp in heating mode should be around 35°c. Electric pre-heaters could be added to temper the incoming fresh air temp in colder weather. The other alternative is to perhaps consider configuring some sort of airside heat recovery between the exhaust air and fresh air supply to pre-heat the incoming fresh air. This won’t be easy given the constraints of the existing unit chassis and ductwork. Some current Lennox units offer airside heat recovery as an option.

Also, check what the building heating requirement is, including that required for any fresh air. The old LPHW coils would have probably operated on 82-71°c hot water and would have had considerably more heating capacity than the heat pump is capable of. This is why additional electric heaters will probably be required to top up the heat pump capacity and act as a backup heating.

If you haven’t already realised, what you are proposing constitutes a huge amount of work with numerous potential pitfalls and no guarantee of success. Do not under estimate the amount of new component design work that will be required if you go down this route. I’m not sure if I was the customer I would be prepared to fund what amounts to an R&D project which has no guarantee of success or long term system reliability.

Apart from replacement equipment (which you or your customer have discounted) have you thought about talking to a specialist equipment re-build company? There are a few of these companies around now that specialise in this type of work. Whether they would take on the re-design as well is another question. One such company that springs to mind is VES http://www.ves.co.uk/Services/Specialist-Site-Services.aspx
There are other companies & I don't work for VES (in case you are wondering).

Trust this helps in some way or another.

install monkey

10-01-2012, 11:07 PM

getting info,parts off lennox-good luck, they wont answer the phone, u have to email them and if they reply within 2 weeks then ur lucky,also as its r22 there will be limited parts,is the unit an l box (lga210shim) then theyre 20yr old,also coils on lennox coils are thin and if u fart then they fracture on the ends,best either lphw coil as the gas burner that u can fit in are crap,they set on fire with condensation on the pcb,burners lock out on strong winds,85kw heater batterys will be expensive to run,scrap em and start again

Gerrard

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