Adding Room Stat

[Mark Trueman]

Hi,

Current system - gravity HW, pumped CH. HW must be on for CH to be on. Runs off a Drayton Lifestyle LP241 timer. No stats or TRVs on rads, when its on, its on full blast.

Anyway, im thinking about adding a room stat to try to control the temp a bit better. Didnt mind it getting to 25 degrees previously, but now my new marine fish tank is not liking the increase in room temp.

Anyway, i digress. Looking at installing the Drayton Digistat 2. Looks easy enough to wire up from looking at the LP241 instructions.

What i want to know is; is there a better/cheaper alternative room stat (24 hour version is fine, 7 day version is even better). And if there is, is the wiring likely to be the same?

Regards

Mark

[Christian McArdle]

Almost any wired programmable room thermostat will have simple contacts and are wired up in the same manner. A very few have permanent L and N for power, but the overwhelming majority are battery powered using latching relays.

There's a huge variety of designs to choose from at various prices, appearances, sizes, shapes and features.

Christian.

[Set Square]

With your existing setup, all you can really do with a room stat is switch the pump on and off - so, whenever heating is selected by the main programmer, the boiler will cycle on its own stat - and the DHW will continue to get hot(ter) by gravity circulation.

A room stat will help to stop the house from getting too hot - but it won't prevent the boiler from wasting engergy by cycling needlessly, nor will it prevent the DHW from overheating.

If you're wishing to improve the overall control of your system, you should seriously consider installing a zone valve in the gravity HW circuit and a cylinder stat on the hot water cylinder, in order to convert it into a C-Plan system. [See

]

This will provide a boiler interlock to ensure that the boiler only runs when needed - and will provide much better control over DHW temperature.

[Christian McArdle]

That isn't quite true. On an old boiler setup like this, you would set the programmable stat to cut power to the boiler, so it won't short cycle. Even without any additional zone valves at all, you can install a cylinder stat.

The main disadvantages compared to a zone valved system would be:

1. Loss of temperature control of HWC when space heating is called for (but only when the thermostat is active, not just because CH is on).
2. Possibility of parasitic gravity circulation on heating when only HW called for.

It still won't comply with Part L1, but it will be massively better than before, for just the cost of two thermostats and a bit of cable.

Christian.

[Set Square]

But surely, if you do this without additional relays, you have to power the boiler and pump from the same feed - effectively connecting them together. When (in the summer, say) you turn the boiler on to get DWH, the pump will come on too.

[Christian McArdle]

Well, one relay will be required, now you come to think of it, although no doubt someone will come up with a clever relay-less design if we wait long enough, using the NC terminals of the cylinder stat.

Christian.

[Mark Trueman]

Which i think it does now anyway, we tend to set the boiler to "1" (its an old baxi back boiler), and let it heat the water as it sees fit.

If i install the stat, is this no longer going to happen?

Will look into this, thanks.

Picked up a Digistat 3 tonight, going to fit it at the weekend

Mark

[Set Square]

A room stat on its own won't fix it - although it will prevent the radiators from getting hot when they're not needed.

The problem with having no controls on the DHW temperature is that you can't run the hot water and radiators at different temperatures. Ideally, the water going to the radiators needs to be at about 80 degC - which means that the boiler needs to run at this temperature. However, this will result in the DHW being heated to approaching 80 degC - but it needs to be limited to

60 degC, otherwise there's a severe risk of scalding. If you run the boiler cooler to limit the hot water temperature, the radiators won't get hot enough.

If you control the gravity hot water flow in the way I suggested in my previous post (by using a zone valve and cylinder stat) you can have the best of both worlds - hot radiators without the DHW getting too hot. You also get the added bonus that it shuts the boiler off when both CH and DWH demands are satisfied, thus saving energy (and money!).

[Mark Trueman]

You have just summed up my problems well. Yes, the water gets too flipping hot in the winter as we have to turn the temp up on the boiler in order to get the rads up to a nice temp

So im looking at converting to a c-plan, which doesnt look too hard, once i get my head round the electrics. I think the way this is wired up currently is a total botch job so it may take some working out!! Is there an easy way to wire in the cylinder stat and 2 port valve by just using the connections available to me in the LP241 controller. This would also make wiring runs a lot easier. Connecting anything to the pump is definitely out of the question, as its not easily accessible.

Im pretty competent at electrics and plumbing but havent ever done any ch wiring before so any tips would be happily received

[Set Square]

Sorry, I'm not familiar with the LP241 controller - but the best way is often to bring everything together (boiler, pump, valve, thermostats) in a

10-way junction box or wiring centre - similar to that shown in the C-Plan wiring diagram in if you can't easily get at the pump, you may be able to get at the other end of its cable - and divert or extend it into the junction box.

When you fit the zone valve, you'll have to be careful to locate it somewhere where it won't interfere with the fill and expansion pipes having clear runs to the boiler. Very close to the cylinder is often a good place - in either the flow or return pipe, between the coil and the point where the F or E pipe connects in.

Note also that the zone valve needs to have a change-over (as opposed to on/off) switch which is independent of the motor connections, and which switches when the valve is fully open. [Maybe they all have this anyway - but check to be on the safe side, 'cos it won't work without!]

[Mark Trueman]

OK,

So im just trying to work out how to connect this new stat up (Drayton Digistat 3)

Its got 3 wires, common, heat satisfied and heat not satisfied. Ideally i would like to be able to just wire this into the timer control and not have to do a run to the pump, as the pump is in a stupid position under the floor and is a pain to get to. Plus there is no junction box, it seems to be wired up by someone with a deathwish!! Its a Drayton Lifestyle lp241 controller. Just wanted to check the following....

Im pretty sure that common on the stat can be put into to the "ch on" (pin

4) in the timer, but im not sure about the other 2.

Any help is appreciated before i have to pay someone to do it :(

[Set Square]

There's a very good chance that the wire currently connected to CH ON in the programmer goes to the pump. You need to connect your stat *in series* with this pump connection. This means disconnecting the existing wire, connecting CH ON to Common on the stat, and connecting the disconnected wire - now floating around in space! - to 'heat not satisfied' on the stat. Don't connect anything to 'heat satisfied'.

[Mark Trueman]

Cheers mate, simple when you know how. That worked a treat. Now i just have to work out how to wire up the 2 port and the cylinder stat (which i will buy at the weekend maybe).

Of course, the problem now is that as the central heating is on all the time (pump is controlled by the stat), the hot water is on constantly. The sooner i get this cylinder stat and valve in the better i think.

So expect some more questions soon....

Thanks again

Mark

[Set Square]

The pump is controlled by the stat - but only when the original programmer tells it to - so you can still have it on only at specified times if you wish. In other words, if the programmer says "No Heating", then No heating it is, regardless of what the stat says.

[G&M]

Cheapest 7 day unit I know is the Maplin one. Has mains rated normally open and closed contacts.

[mike ring]

"Mark Trueman" wrote in news:41781e17$0$1397$ [e-mail]:

One from me, Mark; where do you actually get the drayton stat - it looks the right size and shape to replace my mechanical one, but there seem few suppliers, and I worry about new web suppliers (tho' the ones I _have_ tried have been fine....ish

mike