Old Mistral oil boiler! Configuration question

[darthcareful]

Hi everyone

I've just had someone in to fit my Nest to an old Mistral oil boiler. It's working (sort of) but not in the way I expected.

The boiler is very basic, with an on/off switch for the heating pump and an on/off switch for the water. However, due to the way it's configured, the water needs to be on for the heating to work. For example, my old programmable timer didn't come with a thermostat and gave me the options of (water) or (water & heat).   

This means that the Nest works like this:

if the water is on, the thermostat works normally. 

If the water is off, the thermostat will continue to control the heating circuit (and I can hear the click as the heat pump starts) but the boiler will not fire up, as the water circuit is not on. 

So, I have a couple of questions:

- is there any way to configure the nest so that when the stat triggers it turns on both water and heat simultaneously?

- or, if I have a situation where the heat circuit is on, but the water circuit isn't  (and therefore the boiler is off) is this a problem?

Thanks for reading... 

Rob

[Frustratedddd]

Was the install fitted by a pro installer, as they should know what they are doing?

You should be able to heat the water on it's own without the radiators getting warm.

You  also you should be able to heat the radiators, but the boiler will come on obviously at the same time. When you are heating the house, you will get hot water as well obviously as the water is circulating round the system.

Your oil boiler I assume has a separate pump supply, and a separate burner supply, is that correct? You should just have a cable coming from your pump, and a cable coming from your burner, and that's it?

The heating only works when the burner is working and the pump is running. The hot water only needs the burner running. Does that make sense?

The Nest temperature control should turn the pump on when called for heat, and the burner will cut in when it needs to heat the boiler. When you just need hot water, just the burner will operate and no pump.

Your switches if still installed just need to be left on all the time, and the Heat Link and thermostat will do the rest.

I take it you are UK with a Heat Link?

Should the answer be yes you do have a Heat Link, then the switching contacts need to be connected accordingly. I expect from the sounds of things you have  standard terminal connections. The  relays have normally open or normally closed connections, and I expect yours has been incorrectly installed!

Can you take a picture of the terminations?

[DragosC]

Hi there, I'm just following up on our last message. Do you still need our help? Keep us posted. 

[DragosC]

Hi there, @darthcareful

Please make sure it's wired correctly. You can find the wiring guide for your specific heating system in here: Learning Thermostat Installation Guide. Also, it would be for the best to have some pictures of the wiring. 

[darthcareful]

Hey! And thanks for chasing this up. Apologies for not replying sooner... I have a bit of a building project happening and things moved on.

I'm sure the wiring is fine - it was installed by a professional electrician - but I'll get photos and see what you think. I've also been in contact with Mistral, who tell me the boiler is heat only and over 35 years old - in order to get water, a separate pump has been installed. They can't give me any specific detail, as it sounds like an old-style installation, but they suspect that this is why boiler only gives the options of (water only) or (water & heat) - and, in fact, the old timing clock gave exactly those options. There was no (heat only) option previously.

So, I may have to live with it. But I'll certainly post pictures of the wiring to see if anyone has any ideas. 

@Frustratedddd thanks for your super-quick input. Yes, it's UK with a Heat Link, and you describe it perfectly (to the best of my knowledge anyway) - I've sent your comments to the electrican to see what he thinks.

Just one question: 

"Your switches if still installed just need to be left on all the time, and the Heat Link and thermostat will do the rest."

- yes, I can see that that will work. Essentially the water circuit would be  'on' all the time and temperature controlled by the boiler thermostat, and the heat kicks in as controlled by the Nest. But that seems quite wasteful of oil - I'm heating the water to a set temperature all the time, rather than a few hours a day. So did I understand you correctly there?

[Frustratedddd]

For a simple explanation, so long as you have the burner for your oil boiler, and a pump to circulate the water round the radiators the Nest will work. Just leave the switches on 24hr, and the Nest will do the rest via it's schedule.

My boiler was installed in 1984, so you can see there is a similar install here. I hasten to add, I have installed dozens of these thermostats, but mine is like your one - Old!!!

The connections to the pump and burner are just 2 flexes normally, ie L&N to the pump, and L&N to the burner, and that's it.

The connections in the Heat Link are the critical bit, and this is where installers get it wrong. Your system will have to be connected slightly different, due to the simple install.

The connections you will be using in the Heat Link, assuming you have the install as descibed above, will be:  N- L - 2 - 3 - 4- 5 - 6  and T1 - T2 if hard wired to the display.

Regarding the boiler thermostat, you are correct, that looks after the boiler, but if you are not drawing any water or heat, the boiler will not be firing up. The Nest schedule when setup will also help you, as that can be set on/off as many times as you like per day.

I have an oil sender unit on my oil tank, and that gives me an idea of the litres per day used. I am using about 4 ltrs per day currently. 7am - 11am and 3pm - 11pm daily.

I hope that helps a little, and I would be interested in seeing what the connections are used in the Heat Link.

[darthcareful]

That's incredibly helpful, thanks - I'll let you know

[DragosC]

Thank you very much all for letting me know. One important mention to your observation. If you have any other programmer for the heat pump or the water tank or another device, please make sure it's set to "Always on". 

[Frustratedddd]

I have just read your first post again, and I am convinced the wiring install is wrong!

I think the installer has wired like a standard system, your's is not like that. I think the connections if you look at them have been wired like this....

Term 2 = Live Feed

Term 3 = Live out to pump

Term 5 = Live feed - could be looped from Term L or Term 2

Term 6 - Live out to oil burner

The other Terminals 1 & 4,  I suspect will have nothing connected.

The critical part is the switching of the oil burner section, and I think those connections are wrong.

Can you just remove the Heat Link cover, and make a note of the connections?

DragosC, as a specialist, what are your thoughts? The user has stated no programmer anymore, just on/off switches which I advised them from the start have to be on 24hr.

[DragosC]

Thanks for your observation @Frustratedddd . In order to be able to tell if it's installed correctly, we'll need a picture of the wiring from @darthcareful . 

[CristianC]

Hi there. Just checking in. How's it going with your thermostat? 

[CristianC]

It's been a while since this thread was last updated so I'm going to lock it. If anything comes up, feel free to open a new one.