Replacing an old Honeywell ST699 Timer?

[Apnomis]

I've been doing a lot of research on heating systems recently, and for those that have seen my other topic I've been having trouble with a cold radiator which I think will need new valves.

This has lead me to think that while I'm replacing valves anyway I may as well use the opportunity to fit Thermostatic valves to all the upstairs radiators. I always thought I would need to replace the boiler system to get these but (unless I'm very mistaken) it's just a case of swapping the valves on the existing system. Not so worried about downstairs but our upstairs rooms are very prone to overheating and I think TRVs would really help and save money.

But this has got me thinking, why stop there? Our boiler is controlled by an ancient looking Honeywell timer which from Googling I think is a Honeywell ST699 (except our version features a delightful brown/beige colour scheme!) There is also an equally old looking thermostat dial in the hall (just above the radiator which doesn't strike me as an ideal location!).

I would really like to get rid of the thermostat in the hall altogether for cosmetic purposes and I would love to have more flexible control over the heating than the draconian timer we have now that has one setting for all days for both the heating and the hot water at the same time.

Ideally I would like a modern digital timer with flexibility for multiple programmes for different days and times and a wireless thermostat in the living room to control the temperature. And one that doesn't lose all it's settings whenever there is a power interruption!

My question is how difficult would it be to replace just the timer and thermostat (with a wireless one) without touching anything else in the system? Would I need to replace it with a Honeywell model or are all the wires like for like?

Is it as simple as looking at a wiring diagram for the ST699 and attaching to the corresponding connection of a new timer?

Is it a DIY job or something a professional should do, baring in mind I don't want to replace the whole system, just the timer/thermostat of the existing system?

Any advice would be welcomed...

 

[stem]

I may as well use the opportunity to fit Thermostatic valves to all the upstairs radiators.

Good idea, but why not fit them downstairs too whilst the system is drained. If you do fit thermostatic radiator valves downstairs, don't fit one to the radiator in the room where the main thermostat is located. I take it that your hot water system already has thermostatic control via a motorised valve.

There is also an equally old looking thermostat dial in the hall (just above the radiator which doesn't strike me as an ideal location!)

Correct. Thermostats should be away from direct sources of heat and / or sunlight, and if possible, not on an outside wall

a wireless thermostat in the living room to control the temperature.

Ideally the thermostat should be in room where there isn't an additional heating source. If you have a fire of some sort in there that you use, you would be better with it in the hall. (but in a better position)

My question is how difficult would it be to replace just the timer and thermostat (with a wireless one) without touching anything else in the system? Would I need to replace it with a Honeywell model or are all the wires like for like?

It's not difficult if you have basic DIY electrical skills. Any make of programmer will be fine. When you have it, post details of it on here and someone will be able to advise you how to install it. If your existing programmer also controls the hot water, then make sure that the new one has the same facility. There are programmers for combi boilers that don't include hot water time control.

Is it as simple as looking at a wiring diagram for the ST699 and attaching to the corresponding connection of a new timer?

That's pretty much it, plus removing the wiring that goes to your existing
thermostat.

 

[Apnomis]

Thanks Stem, most helpful!

The only reason I wasn't going to bother with downstairs was because we don't really have trouble with the heat levels downstairs, if anything it's hard to keep it warm even when heaters are on constantly as it is a large open plan space and has a very large bay window at one end and French doors at the other.

Due to the size of the rooms I'm not sure TRV's would help as it would be cutting off before the whole space was to temperature. The curtains over the bay window are also floor length so would obstruct the TRV sensor (the low profile heater runs underneath the bay window). The other small radiator at the other end is back to back with the kitchen radiator but not sure if that would affect the TRV reading.

A room thermostat would probably be better, but I was mainly looking for quick (and cheap) fixes to improve the current system. To fix downstairs would probably require proper room sensors and better radiators for the size of the space it is heating, and new windows and doors wouldn't hurt either!

We do have a fire in the living room, but it's rare we have it on unless it's really really cold! On the plus side in the hall staircase and landing is not connected to the open plan space and the front door is a traditional wood design with thick curtain to reduce the draught in winter, so although the thermostat is directly above the radiator (small double) at light switch level it is still a cold space as it is next to the front door and the radiator has to heat up the stairs and landing too so it sort of still works.

Having said that I suppose it can't hurt to put a TRV on the kitchen radiator as that doesn't really need to be on while we're in there cooking, worth considering I suppose...

I will price up some boiler timers and see if it's worth doing or whether to add it to the wishlist of projects to tackle in future. Thanks for the tip about the combi timers not controlling the hot water, I hadn't considered that...

 

[D_Hailsham]

I would love to have more flexible control over the heating than the draconian timer we have now that has one setting for all days for both the heating and the hot water at the same time.

Does the timer allow you to have HW only in the summer but, in the winter you must have both CH and HW on at the same time?

If so, you gave a "gravity HW, pumped heating" system, which means it would have to be modified to give the flexibility you want.

 

[stem]

That was what I was referring to when I said.

I take it that your hot water system already has thermostatic control via a motorised valve.

The OP hasn't refuted my assumption....but then neither has he confirmed it either.

Apnomis, over to you!

 

[Apnomis]

I think you're getting a bit technical on me now lol.

You can have hot water only, but you can't have heating only - the hot water has to be on for the heating to work, although I have no idea how the two are connected from a system point of view...

I do know that there is a Grundfos pump in the airing cupboard next to the cylinder and two white boxes connected to the pipes which I believe are motorised valves, but that is the extent of my knowledge - the airing cupboard is a bit of a spaghetti junction or pipes and wheel valves, and I don't really know what any of them do! I just opened them all when I first moved in and hoped for the best, I've never touched them since lol.

Sounds like there would be a bit more work involved in sorting the flexibility side out, as well as updating thermostatic room control, so I think I might just leave that bit alone until the day comes that I install a whole new combi-system.

For now I think I'll just make do with installing the TRV's on the radiators...

As an aside my 'bay' window is more like a box on the side of the house, it's not full height and as far as I can see the large window sill is the only thing separating it from the outside, which seems like a horrible design from an energy saving/warmth point of view considering all the other walls have cavity wall insulation.

I had a closer look at it at lunch time and noticed that the previous owners built a new window sill that is sitting (loose) over the original sill - I lifted it up and the original sill is cold to touch (that window is North facing too which doesn't help matters - there is still snow remaining in front of it even though the rest thawed a week or so ago!). I'm wondering if I should get a piece of thin insulation boarding and slip it between the original sill and the wooden sill cover in order to help keep the living room warmer - there is definitely a cold void that emanates from that window and I think that must be the cause as the actual window is double-glazed...

 

[D_Hailsham]

You can have hot water only, but you can't have heating only - the hot water has to be on for the heating to work,

That says you have a gravity HW, pumped heating system,

But

two white boxes connected to the pipes which I believe are motorised valves

Do they look similar to this?

View media item 11762
If so, you have a fully pumped system, but it's not being used to its full ability because the ST699 has not been wired correctly. Fortunately the changes I mentioned (motorized valves!) will not be necessary. It's just a case of installing a new programmer, correctly wired, and a thermostat.

For convenience the Honeywell Sundial RF2 Pack 2 is your best bet. It consists of a 7 day CH and HW programmer, with complete independence between CH and HW, and a thermostat which communicates wirelessly with the programmer, so it can be located in another room. The programmer can be located where the existing ST699 is.

 

[Apnomis]

Yep like that - they're called 'Lifestyle 2-port motorised valve', there is one underneath the pump on a pipe that doesn't go into the cylinder, and one on a pipe that come off the other pipe and goes into the cylinder about halfway up it.

They've both got what looks like a switch with A - B settings, the one on the pipe running down past the cylinder is set to B, and the one on the pipe running into the cylinder is set to A, someone has also written on this one with a marker pen putting a C next to the A and an O next to the B (presumably meaning open and closed?).

Both the valves, the pump, and the cylinder thermostat have wires coming off them going into another white junction box which then has only 3 wires coming out the other end going underneath the airing cupboard (presumably to the timer downstairs?). Oh and the valve that doesn't go into the cylinder has it's wire going via a smaller round junction box screwed to the bottom of the airing cupboard - not sure if this is just to extend the wire though?

 

[D_Hailsham]

Any chance of a pic showing the two valves etc?. A wide-angle view is better than close ups of each item as then we can see how it all fits together.

 

[Apnomis]

Here you go...

View media item 89579

 

[D_Hailsham]

Turned and annotated

Untitled

• D_Hailsham
• 6 Feb 2015
• 1

The HW valve is controlled by the cylinder stat
The CH valve is controlled by the room stat
The bypass provided a path from flow to return if both valves are closed buit the pump is still running. The manual valve should be replaced by an automatic bypass valve.

As you say, the junction box on the floor is just to extend the cable from CH valve, so it doesn't hang in the air.

You have everything necessary to do what you want, but the wiring in the junction box on the wall needs to be checked, as will the junction box on the floor. Close-up Pics of these boxes (lids off ) should do.

Slightly off topic
You mentioned changing the boiler to a combi. You need to check the incoming cold water flow rate (litres/min). The best place to do this is the garden tap.

The existing HW cylinder will be wasting a lot of heat. If a combi boiler is not a runner (poor flow), it should be changed for a modern insulated cylinder.

 

[Apnomis]

Thanks for the info!

See attached inside of the junction box, not sure how it can help as to me it's a complete mess of wires! (Sorry it's sideways it's how my phone uploads them and I can't seem to find a rotate button on the site).

The one on the floor is more straight forward - it's just a like for like pass-through to extend the cable...

View media item 89590
We have good water pressure in the house so I don't think a combi would be a problem, however that's a couple of years off yet (or until the current one dies!).

Interesting point about the automatic bypass valve, I might consider doing that when I change all the radiator valves - would it make a big difference do you think?

 

[D_Hailsham]

See attached inside of the junction box, not sure how it can help as to me it's a complete mess of wires!

I agree. Don't see why two rows of terminal blocks are required; the norm is one row with 10 pairs of terminals.

I think you will have to get someone in to sort it out. Or, work out what is connected to each terminal and post the info.

The one on the floor is more straight forward - it's just a like for like pass-through to extend the cable

The colours are the same on both cables??

We have good water pressure in the house so I don't think a combi would be a problem

Pressure is not the same as flow rate.

Measure the flow rate at the garden tap using a marked bucket and a watch. Then repeat, but this time turn the kitchen cold tap on full. the lower the pressure, the greater will be the difference in flow rate.

Interesting point about the automatic bypass valve, I might consider doing that when I change all the radiator valves - would it make a big difference do you think?

The current gate valve will always allow some water to circulate straight from the flow back to the return. This means a higher return temperature, which is inefficient.

 

[Mucckker]

will the new ST699 fit into the old brown back plate and operate the same,