HVAC / MHRC - connection to more than one circuit = RCD problems?

[Rockingit]

I've just been told about the intentions for how a mechanical heat recovery unit is wanted to work in a new build house and I can forsee a problem:

One unit doing all the various suck and blow things that they do, but with a 'boost' function that is triggered by the lighting switching. I've got bathrooms / WC's on both floors so hence two lighting circuits, and the unit on a different circuit again. So that means that I've got two different L's switching another circuit, loss of the relevant neutrals, so it's going to trip the RCD's everytime (by my reckoning).

Anyone got any experience with this?

I could always put all the bathrooms on the same circuit as the unit I suppose, but it's a massive amount of extra wiring to consider in a house where every last inch counts on the long runs.

 

[spark 68]

Hi,
Is it not possible to use a contactor/s or relay/s to Isolate the switching side, and then use the existing LF for the heat recovery bit ?

 

[Rockingit]

Hi,
Is it not possible to use a contactor/s or relay/s to Isolate the switching side, and then use the existing LF for the heat recovery bit ?

I've wondered the same thing. I'd need a contactor with two separate coils independantly operating the same set of N/O contacts. Suggestions anyone?

 

[spark 68]

I'd need a contactor with two separate coils independantly operating the same set of N/O contacts. Suggestions anyone?

No you won't, what you would need is two contactors with the contacts paralleled and the coils seperate

 

[Rockingit]

In effect that's doing the same job by same method just in two units not one, but thanks for that, it's helped set my thoughts.

 

[spark 68]

In effect that's doing the same job by same method

In effect yes, I have not come across Twin coil contactors yet, do they even exist ?, but you may be able to get them ???

It would probably cheaper and easier to use two off the shelf items though

 

[Knobhead]

They do exist but not for this job. Dual coil relays are usually latching, so of no use here.

Two contactors will do the job and you can pick them up off the shelf. Remember to label the panel with the origin of each supply.

 

[hasel5]

Still do two lighting circuits one to all the wet rooms and kitchen and one to others then make the last point on the circuit the mvhr unit simples

They only draw a small load so not a problem on the lighting circuit.

Just double pole switch the rooms using one side for the rooms light and the other side for the boost to the mvhr

So basically wire a three core round to all sws and up to the mvhr

 

[Rockingit]

Still do two lighting circuits one to all the wet rooms and kitchen and one to others then make the last point on the circuit the mvhr unit simples

They only draw a small load so not a problem on the lighting circuit.

Just double pole switch the rooms using one side for the rooms light and the other side for the boost to the mvhr

So basically wire a three core round to all sws and up to the mvhr

If 2 pole lighting switch plates exist, in 2 & 3 way versions, then I've certainly not seen them! Imagine the amount of cabling in the back of one. But electrically speaking that would also work, and be much simpler.

 

[Knobhead]

Rock, you can get them in grid format. But I think hasel should take a reality check, loads of wiring where two relays does the job.

 

[Rockingit]

Rock, you can get them in grid format. But I think hasel should take a reality check, loads of wiring where two relays does the job.

I'm in complete agreement! An intermediate switch in a 2p format would have 8 terminations + cpc.....crazy way of working. I'm trying to get rid of cable on this design, not add to it!!

 

[Knobhead]

My brain hurts just thinking about it, to hell with trying to do it in practice.