Will Nest Hello work with my current set-up.

Benny5 · 19 Jun 2019

I currently have a wired door bell. The chime is pretty old (Micromark MM8815) as is the transformer that powers it. The thing is, the transformer is somewhere under floorboards and short of moving furniture and lifting carpets I can’t inspect it. I have measured the output at the chime and the push button and both return 14.6V.

Given Nests statement ‘Nest Hello needs to be powered by wires delivering 12 - 24 V AC and at least 8 VA’. Can I get away with my present arrangement if I fit one of these devices?

Any help really appreciated.

flameport · 19 Jun 2019

Very unlikely - most doorbells are 8 volts.

14.6V is the open circuit voltage, that will drop significantly once a load is connected. Check the voltage stated on the transformer label, or measure it at the chime with the button pressed.

bernardgreen · 20 Jun 2019

Benny5 said:
The thing is, the transformer is somewhere under floorboards and short of moving furniture and lifting carpets I can’t inspect it.

Do not overload the transformer , short of moving furniture and lifting carpets you will not be able to put out the flames if the transformer catches fire.

Get it out from under the floor boards

Benny5 · 20 Jun 2019

flameport said:
Very unlikely - most doorbells are 8 volts.

14.6V is the open circuit voltage, that will drop significantly once a load is connected. Check the voltage stated on the transformer label, or measure it at the chime with the button pressed.

Measurement coming in at 11.8v.

This coupled with the comment from bernardgreen suggests a change is required. Clearly a replacement, placed in a more suitable location, is desirable.

My plan is to use one of these; https://www.screwfix.com/p/british-general-8-24v-8-va-bell-transformer-module/8707p#_=p . And if I can’t locate the old one for removal, I plan to terminate the wires coming from the transformer.

Without any current draw will the transformer be ‘safe’?

Once again thanks for your input.

Benny.

Just to update. Found the transformer, its a Friedland 753. (8V)

bernardgreen · 20 Jun 2019

Benny5 said:
Without any current draw will the transformer be ‘safe’?

In over 50 years I can recall only two or three "bell" transformers that failed catastrophically.

I know one similar to this that has been powered since 1964 to supply a couple of small lamps. It is mounted on a wall and fused sensibly so if it did decide to self destruct it would not be a serious problem.

Items manufactured in recent years are not always as well made as those from the 1960's

Benny5 · 20 Jun 2019

bernardgreen said:
In over 50 years I can recall only two or three "bell" transformers that failed catastrophically.

I know one similar to this that has been powered since 1964 to supply a couple of small lamps. It is mounted on a wall and fused sensibly so if it did decide to self destruct it would not be a serious problem.

View attachment 166304

Items manufactured in recent years are not always as well made as those from the 1960's

I can easily relocate this old unit where it is visible and will be protected by a fused switch. The transformer is cool to the touch; the problem is this issue ‘Nest Hello needs to be powered by wires delivering 12 - 24 V’ whereas mine measures 14.6 or 11.8v under load.

It may work; I could always give it a go.

The other problem, maybe my bell will not respond well to the higher voltage produced by a 24v transformer.

Once again thanks for your help.

Benny.

bernardgreen · 20 Jun 2019

Benny5 said:
by wires delivering 12 - 24 V’ whereas mine measures 14.6 or 11.8v under load.

14.6 is more than 12 volts so will be OK. With most equipment ratings for input voltage levels have a tolerance of at least 5% ( hopefully Nest has )

Hence an input requiring nominal 12 volts will still operate at 12v -5% ( 12v -0.6 = 11.4 v )

Benny5 · 20 Jun 2019

bernardgreen said:
14.6 is more than 12 volts so will be OK. With most equipment ratings for input voltage levels have a tolerance of at least 5% ( hopefully Nest has )

Hence an input requiring nominal 12 volts will still operate at 12v -5% ( 12v -0.6 = 11.4 v )

Bernard, you have made my day.

I shall give it a go and should problems arise I can always add a new transformer. I am now in the process of relocating the old one.

Thank you so much.

Regards,

Benny.

bernardgreen · 20 Jun 2019

I am not sure if the Next is OK with an AC input

From Nest website

"" Nest Hello needs to be powered by wires delivering 12 - 24 V AC and at least 8 VA (in Europe)or 16 - 24 V AC, and at least 10 VA (in North America).""

Which suggest the bell transformer is supplying all the power for the Nest system and not just a signal from the door beel push being pressed

That said it should work.

Benny5 · 20 Jun 2019

bernardgreen said:
I am not sure if the Next is OK with an AC input

From Nest website

"" Nest Hello needs to be powered by wires delivering 12 - 24 V AC and at least 8 VA (in Europe)or 16 - 24 V AC, and at least 10 VA (in North America).""

Which suggest the bell transformer is supplying all the power for the Nest system and not just a signal from the door beel push being pressed

That said it should work.

Yes, the only source of power for the Nest unit is that supplied by the existing doorbell transformer.

Have just finished the relocation exercise and its now in a cupboard with its own fused supply; all measured outputs remaining the same.

Thanks for all your assistance.

Regards,

Benny.

bernardgreen · 20 Jun 2019

Please to help.

cwinson · 1 Oct 2019

Hi Benny,

I have the same transformer as you have, measuring 14.2V with (without button pressed). Did you get the Nest working with that transformer in the end, or did you switch to the new one you mentioned earlier in the thread?

Also, what sort of Chime did you have? I think I have a Friedland Underdome D792 8V AC, so hoping that will be workable.

Chris

Benny5 · 1 Oct 2019

cwinson said:
Hi Benny,

I have the same transformer as you have, measuring 14.2V with (without button pressed). Did you get the Nest working with that transformer in the end, or did you switch to the new one you mentioned earlier in the thread?

Also, what sort of Chime did you have? I think I have a Friedland Underdome D792 8V AC, so hoping that will be workable.

Chris

Hi,

My chime is a very old Micromark MM8815, dates back to the late 80’s. When I first installed the Nest all worked well except for the chime which struggled to activate the ‘Ding Dong’ mechanism. So I purchased the Screwfix supplied transformer and all worked a treat.

Benny.

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Will Nest Hello work with my current set-up.

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