Is a pool heat exchanger suitable for a communal heating system?

[siricosm]

Indeed, I guess I assumed that when a professional is doing the install, they would automatically go for a sealed system, it did not occur to me that they might want to leave it vented. Along with inhibitor, sealing the system should stop the corrosion that is generating the problem in the first place, which is always going to be worse on a vented system. Without going too far off topic, I agree with the whole system approach, I am bit surprised you can run 12 flats with 35kw for hot water. I'd be tempted to go with a larger buffer and size the boiler for both, although separate systems also have their advantages.

 

[LordWest]

It would be ideal to seal the secondary heating network with the radiators, but the pipework is more than 50 years old, and thus this part would preferably remain vented.

 

[siricosm]

I noticed you mentioned a 23m flue, may I ask how tall is the building? Or more importantly, what is the height difference from the lowest pipework in the basement to the top of the highest radiator? 10m is around 1 bar, and you can run a sealed system at that pressure, or maybe a bit above, so it could be your system is already operating at the pressure you would need for a sealed system, since the boiler is in the basement. You would probably need to go a wee bit higher to make sure the rads on the top floors fill, but not much more.

 

[John.g]

Re Original query,

A Nordic B3600 Swimming Pool Heater, 1060kw, 6.4M2 might do the job for you, I'm assuming the heat flux at 2850 w/m2.degC, Nordic will advise.

 

[LordWest]

@John.g

In reply to my pool heat exchnzger query, Nordic suggested the pool heat exchanger B2400 at 704kw, as it has a surface of 4.47 m2, not too far from the required 3m2 for the 100kW boiler . However, Nordic's stronger preference is a brazed heat exchanger Ba-95-40 which is 1000kw and has a surface of 3.8m2, which is cheaper but more efficient. As discussed before, my concern is of a brazed HE clogging as our system is old and had been poorly maintained. Keep in mind that one of our requirements is to have 2'' connections, as this is what our single pipe system has.

@siricosm

The height difference from the lowest pipework in the basement to the top of the highest radiator (on 5th floor attic) is about 22 meters.

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[siricosm]

@siricosm

The height difference from the lowest pipework in the basement to the top of the highest radiator (on 5th floor attic) is about 22 meters.

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This means you have over 2 bar of pressure in the lower parts of your unvented system already. So sealing it would not require a dramatic increase in pressure, but perhaps a discussion for a different thread.

 

[John.g]

The required 3M2 to give 100kw output is based on a PHE, the required surface area of a S&T to give the same output (100kw) with the same flowrates and flow&return temps and LMTD is far higher than using a PHE because the more turbulent flow in a PHE gives a far greater (IMO) heat flux, so to compensate, the surface area of the S&T must be far higher, this is why they say its more efficient, all this means is that you have a smaller Hx, they both give exactly the same input/output., ie 100% efficient. If you have the space to install a S&T if you think it's easier to clean (and it is) then I would install it.
I am a little surprised that Nordic sized the S&T at 4.47M2 (704kw) but they have the requirements?. I would be far happier with a 6.4M2 (1060kw).

 

[John.g]

This means you have over 2 bar of pressure in the lower parts of your unvented system already. So sealing it would not require a dramatic increase in pressure, but perhaps a discussion for a different thread.

Shouldn't increase the pressure too much, don't know what the system contents are, say 400/500 litres, a large 100 litre EV would ensure the pressure kept to 2.25/2.3 bar on heat up, boiler would require a PRV if not already fitted.

 

[LordWest]

I appreciate the benefits of converting the vented heating system into a sealed one, but I do not want to take the risk of leaks in the old pipework and radiators. If this were for my house only, then I would take the risk. But the heating system serves 12 flats (including my flat) and I do not want take the risk nor for the increased policy policy premiums that may result from damage from the leaks.

Going back to the heat exchanger, I will discuss again with Nordic the right size for my circumstances. John you are correct that plate HEs are indeed mroe efficient because of turbulent flow conditions.

Assuming (i) we recently installed a 2'' Magnaclean filter, and (ii) our system has not been well maintained over the years and clogging the HE is a possibility, which of the three options below woudl go for?

1. Install a brazed heat exchanger, which in principle is ideal and the most efficient. However, they usually come with waterways of 1.8-2.2 mm depth and can be clogged, despite having a magnetic filter. Magnaclean’s CMX commercial MIDI and MAXI filters can work as inline filters and if suitable then I could possibly use them with a brazed HE. The CMX come with 50 micron replaceable filters, which will certainly limit any clogging. But I am not sure if this will impact the flow into the return of the heat exchanger.
2. Install a shell-and-tubes heat exchanger (sold for pools by Nordic), which will not be as efficient as a brazed HE but we will get a larger size and will not clog easily. Our installer prefers this method for our circumstances.
3. Install a gasketed HE, which can be cleaned easily. Danfoss Sondex makes a gasketed HE that has 4.3mm depth in the waterways and thus would clog less and it is cleanable. However, they are also more expensive, and in theory they can leak.

 

[John.g]

I would stay with the Vented system if it has served well with no air ingress problems over the past 50 years. (like my own)

I would also go for option 2, like your installer, have a good look at the attachments again as I would strongly suggest the 6.4M2, 1060kw Hx as your rads, if based on the normal "50deg" will still give 72% output and if oversized originally (by a factor of 1.39) then you will still get 100kw. If you opt for the 704kw Hx then you will only get ~ 70kw.

Finally, a Happy and Healthy 2022 to you and yours and everyone.

John

 

[LordWest]

John,

Happy New Year and many thanks for your analysis. I will enquire about the B3600.

Do you have any views on options 1 and 3 I listed above, compared to B3600 (Option 2)?

 

[John.g]

Option 1, depending on how clean you can get the system is probably a better choice than option 3 despite option 3 having bigger waterways and the ability to split it for cleaning, my experiences of this type (3) of PHE was on a number of ships I sailed on, they needed very careful re assembly with the correct torque settings for the tie bolts etc after cleaning, the rubber gaskets frequently were torn on splitting them and any new gaskets had to be glued on, if you go down this route then who will disassemble, clean and re assemble them properly?.
Option 1, if you are prepared to treat the Hx as a throwaway item should it get plugged is far less trouble IMO but of course may prove very expensive in the long run.
My preference is still option 2.

Of course, as suggested above, by others, you could go for a LLH, with suitable filtration, both magnetic&particle, of the boiler return, you might find that the boiler Hx might last for years and its replacement cost may not exceed and may even be far less than the maintenance cost of either of the PHEs.

 

[LordWest]

ok, many thanks for your reply.