Nest 3rd gen query- how to install heatlink into Honeywell ST900 thermostat that has been removed

[nazzy]

Hi I am sorry for resurrecting this issue but I am a bit lost and need help. Have a nest 3rd gen heatlink and I am replacing the current thermostat, a Honeywell ST900. I have connected the L /N correctly and the heatlink powers up but the pickle I face relates to the 2 wires coloured black and grey which I assume is for the Hot water and Central Heating. Btw I have a megaflow system and the boiler is a Valiant eco tec pro. The current honeywell thermostat is connected to a Honeywell junction box. I have no room thermostat so the nest thermostat is going to be wireless. when I connect the grey and black wires to the heatlink, it does not power the boiler. I need to know the correct place to insert the wires. Previous thread suggests black goes into no 3 and grey into 2 on the heatlink. Is that correct? Please see pic below. Any advice much appreciated.

 

[stem]

I am replacing the current thermostat, a Honeywell ST900.

I'm not aware of this product? Do you mean a ST9000? If you do have a ST9000 this is a programmer (time control) and not a thermostat (temperature control)

It is a bit unusual to have a combi with a Megaflow but not unknown. As you have a Megaflow, I am assuming that you are replacing a ST9000 2 channel programmer for both Heating and Hot Water, coupled to two port motorised valves for control as opposed to a three port. If so, you can proceed as below. If you have something different, do not proceed and post back what you do have.

The wire that previously went to the ST9000 terminal number 4 is the 'central heating on' wire this goes to Nest terminal 3

The wire that previously went to the ST9000 terminal number 3 is the 'hot water on' wire this goes to Nest terminal 6

You then need to insert links between the 'Live' terminal and both 'common' terminals number 2 and 5 at the Nest.

 

[ban-all-sheds]

This isn't right:

The cable should be properly clamped, and no unsheathed conductors should be outside the enclosure.

 

[ericmark]

Megaflow is used to combine heating units together, like solar panels and wood burning stoves, it can become rather complex, there is often multi sensors so the heating which can be controlled does not fully heat the store so there is room for the heat from heating which is uncontrolled like solar. For larger houses the combi boiler is not really that good, as it does not lend itself to working with a heat store.

Even the simple condensate boiler has problems in insuring the return water is cool enough.

Rather than Nest likely something like EvoHome or another system using eTRV's would be better, but it all depends on what you have. To have a Megaflow means it does not follow what we think of as standard installation, it likely have multi-pumps and motorised valves.

 

[stem]

I wondered that, but looking at previous posts from Nazzy they do suggest it is a Combi hooked up to an S Plan which includes a Megaflow.

But whatever the system is, if the original programmer was an ST9000, then the Heatlink will provide the same switching functions.

 

[ban-all-sheds]

I'm not suggesting the OP falls into this category, but does the team think that sometimes people buy a Nest or Hive etc first because they think it looks cool, and only later do they consider if it will actually work with their system?

 

[stem]

Probably, much of the time. The folks I know that have them, also have to have the latest ipad and other techno devices, because that is what they are into. They can demonstrate to their mates at work, or down the pub that they can control their heating from their phone.

Some have even fitted them to antiquated systems with gravity hot water, which if they had wanted to improve their heating, they would have benefited so much more by spending the money converting them to being fully pumped instead.

I have installed / helped to install both types, and apart from the remote control facility which could I suppose be useful to some. When you are home, I don't think that they are as quick and easy to use as a simple programmer and room thermostat. So I don't have a Nest or Hive etc., because they don't really offer me any tangible advantages at the moment.

I do think the Nest Thermostat looks nice on a wall though...

 

[EFLImpudence]

I'm not suggesting the OP falls into this category, but does the team think that sometimes people buy a Nest or Hive etc first because they think it looks cool, and only later do they consider if it will actually work with their system?

They definitely do with RF in general - even when they already have all the wiring in place.

 

[winston1]

Some have even fitted them to antiquated systems with gravity hot water, which if they had wanted to improve their heating, they would have benefited so much more by spending the money converting them to being fully pumped instead.

So can you enlighten me on this? I have asked many people why a fully pumped system is better and the only answer I get is that the water heats up in half the time so it is twice as efficient.

Rubbish I say, that is not the definition of efficiency. It may take longer to heat the boiler but is the overall firing time of the boiler any longer? Also what about the extra electricity required for the pump?

Perhaps this should be in the plumbing section!

 

[JohnD]

Heating up the hw cylinder in half the time does, however, meet the definition of "better," which you say is the question you asked.

Perhaps there is also a saving in heat wasted from the circulating pipes, which only need to be hot for half the time, and are probably not well insulated.

 

[nazzy]

Many thanks to all that commented on my thread..your comments have been helpful. Special thanks to Stem..you a star! I followed your instruction and connected the nest.

As an aside, the reason for the nest was not because it was fancy, I don't have a room thermostat and needed a wireless one. This came in at about the same price for a honey well wireless thermostat so I opted for it.

 

[ericmark]

I looked at my brother-in-laws system, hot water solar panels, electric solar panels and immersion heat to use excess power, wood burner, and an oil central heating boiler all fed two megaflow units which were very large, the central heating was heated from the megaflow and using the heating only when home he never used the oil central heating boiler, it was fitted when the house was built, however looking at over £16,000 worth of kit, and you get a lot of oil for £16,000.

He told me is was a nightmare to set up, because of using the megaflow as a heat store for the central heating, the normal way to use a condensation boiler is for flame hight to vary to maintain the return water at the correct temperature, however with the megaflow it was a case of flat out until the tank had a set amount of heat, leaving room for the solar and wood burner to add more, then switch off. The boiler heat exchanger was at the bottom of the megaflow so the return water was cold and the sensors were near top of megaflow to switch boiler off.

It seems the Wilis system is far better, where the heat exchangers are not in the main tank, but only the Irish seem to be able to work it out!

Anyway with his system the room thermostat connected to the motorised valve and circulation pump, not the boiler.

Nest or Hive would have worked well for him, except there is intermittent internet and no mobile phone coverage. Which it seemed stopped the Nest or Hive working. Plus it had an over ride to switch on the central heating if the Megaflow got too hot, which could happen if you had a roaring wood fire going. The insurance required the system to be inspected once a year, which was not cheap, and he also had UPS to ensure pumps did not fail. I would not want to play around with a system like that.

It was one of those systems where the rich could boost on how environmental friendly they we are. But it could not have saved him any money.

Personally I did not want Nest, I know when I want heat weeks in advance, no earthly need to alter heating with the phone, however the programmable wireless thermostat simply did not work, it did not do what it said on the box, only way to be sure a wireless device does as it should is two way coms. So sending a signal switch off boiler without getting a signal back to say OK got that is useless, well OK it could simply be if no signal received in an hour turn off boiler, but there is nothing in the thermostat details which tell you what it does, but if you can read the room temperature and it says "Last seen less than 1 minute ago" you know coms have not been lost. With my hard wired programmable thermostat OK, however even with that it has failed due to batteries too low even when the low battery light is off, but my eTRV reports "Battery Voltage: 2.94141V (Reported 28 minutes ago)" it actually tells you the battery voltage.

So it's nothing to do with being able to program with phone, it's all down to actually working. And I have every intention of fitting NEST because it works, not because I can alter it from phone, just because it works. However until we know how the Megaflow is plumbed and wired into the system, advising on how to wire is really not possible. And if the poster knows how it is wired then he would not be asking the question.

 

[flameport]

have asked many people why a fully pumped system is better

Little or no control of the temperature of the water, excessive heating times, pipework has to be of large diameter and carefully designed to ensure proper flow between boiler and cylinder, totally incompatible with modern low water content boilers.

Also what about the extra electricity required for the pump?

Insignificant, even with an older design pump it's only 50W or so, modern ones substantially less.

 

[stem]

So can you enlighten me on this? I have asked many people why a fully pumped system is better.

1. More consistent / accurate control of the hot water temperature
2. The hot water heating shuts down when the hot water cylinder is up to temperature and the prevents continual boiler cycling
3. Different on and off times selectable for the hot water and central heating (Possibility of having the heating on without the hot water being on)
4. Prevents the stored hot water reaching the same temperature as the boiler thermostat setting with a possibility of scalding
5. Reduced heat emitted to the environment helps keeps your home cooler in the summer
6. Possible to balance the flow from the boiler to the heating and hot water according to the charisteristics of the system
7. The hot water cylinder heats up much more quickly
8. Saves energy
9. Allows the full functionality available from wifi driven intelligent thermostats

Also what about the extra electricity required for the pump?

It won't be running for very long at all with just the hot water on, and in the winter it will be running already for the heating anyway. It will be compensated in part because the continual cycling of the boiler which occurs with a gravity system will be reduced as the boiler is shut down electrically when the system is fully pumped and up to temperature.

 

[winston1]

1. More consistent / accurate control of the hot water temperature
2. The hot water heating shuts down when the hot water cylinder is up to temperature and the prevents continual boiler cycling
3. Different on and off times selectable for the hot water and central heating (Possibility of having the heating on without the hot water being on)
4. Prevents the stored hot water reaching the same temperature as the boiler thermostat setting with a possibility of scalding
5. Reduced heat emitted to the environment helps keeps your home cooler in the summer
6. Possible to balance the flow from the boiler to the heating and hot water according to the charisteristics of the system
7. The hot water cylinder heats up much more quickly
8. Saves energy
9. Allows the full functionality available from wifi driven intelligent thermostats


It won't be running for very long at all with just the hot water on, and in the winter it will be running already for the heating anyway. It will be compensated in part because the continual cycling of the boiler which occurs with a gravity system will be reduced as the boiler is shut down electrically when the system is fully pumped and up to temperature.

1. You can still have a cylinder stat.
2. As it does on a gravity system with a cylinder stat.
3. Also can be done with a gravity system.
4. Not if there is a cylinder stat.
5. Why is less heat emitted to the environment.
6. ?
7. Agreed, but this is nothing to do with efficiency, but agreed is is desirable.
8. So it is often said, but how?
9. OK.

It seems that most of the argument assumes there is no cylinder stat or control of hot water temperature. I had a gravity system many years ago, there was a cylinder stat and 3 port motorised valve. When the cylinder was up to temperature the boiler was switched off. It was also possible to have heating without hot water.

So perhaps the fully pumped system "more efficient" argument is based on earlier gravity systems with little controls.

But thanks for your input.