Central Heating Zoning controls (was Are room thermostats out of fashion?) longish question

[Ian]

Hello All,

I have been following the recent excellent threads and am now coming out of my confusion, I think. Our existing solid fuel boiler is not going to cope with the additional requirement of our extension and will be replaced by oil in the nearish future. This will allow some degree of control (yippee). My original plan was simple DHW stat and TRV's on all rads, but this , I now see, has difficulties in achieving 'boiler interlock'

From pointers given by various recent posters I believe I want:

1) two CH zones, since I have identified that all our rooms fall into one of two distinct heating requirement patterns. 2) separate control of hot water.

Now... someone (BillR, I think) mentioned the Honeywell CM67's. Having investigated, these look excellent, allowing each heating zone to be controlled to different temperature levels throughout six different periods of the day. I particularly like the look of the RF versions and, AIUI, by linking to an additional HC60NG to be used as the boiler/pump controller - as per honeywell installation guide - I could then have TRV's on all the radiators except those in the 2 rooms (1 room in each zone) where the CM67's are installed and this would provide the required 'boiler interlock'. I think I'm OK with this so far (?) Someone tell me if I'm not!

Then I come to the DHW.... I cannot see an equivalent beast to the CM67 for DHW purposes.... I guess I can use a simple cylinder thermostat - although I don't necessarily want to have a tank full of hot water permanently. Even if I did, how do I get this stat to talk to the HC60NG that is controlling the boiler?? I am sure I am missing something very simple here but cannot put my finger on it. It may become clearer when I have the boiler installation information to hand but in the meantime, I'd really appreciate if some kind soul can enlighten me.

Perhaps there is something to be said for the old anthracite boiler... a) light it once a year - DHW control is sorted b) switch on the pump when you want the CH c) twiddle with the TRV's d) switch the pump off at bedtime e) heat leak rad keeps the bathroom warm for the overnight tinkles and f) the heatloss from the boiler keeps it warm for breakfast! arhhh the simplicity!

TIA Ian

[Christian McArdle]

Not really. Interlock is easily achieved. Any argument is about whether it is worth achieving or not. The benefits of implementing the interlock depend largely on the attitudes of the occupier. If the occupier doesn't mind turn it off and on manually, then the interlock is not as effective in increasing efficiency as when they are not. I prefer a system I can just leave well alone, yet works efficiently.

It is possible to have an all-TRV system, even subzoned for timing. You simply plumb as S-Plan, with a 3 channel programmer, or with separate programmers conveniently located. But rather than using the 2 port valve microswitches, you achieve interlock using a flow switch after the bypass, but before the heating zone valves. The output from the flow switch can be combined with the HWC zone valve microswitch to provide a boiler call for heat.

Personally, I prefer to use a programmable room stat, as it allows different temperatures at different times of day. Having implemented it, it very effectively maintains temperature in all rooms, including those with TRVs, provided the system is reasonably balanced. The rooms with the programmable thermostats are those used most frequently (master bedroom and lounge). This provides very accurate temperature in those rooms (measured away from the radiator) and a handy nearby override mechanism.

The only problem is the kitchen, which doesn't get to temperature. However, this isn't really a control issue, as the room is 5m x 2.5m, with 2 external solid walls, 3 large windows, 1 door, french doors and a single tiny radiator at one end suitable only for a little bathroom. I'm planning to put a fan convector (i.e. Myson Kickspace 800) in to resolve this. Unfortunately, there's no room for insulation, although the draught proofing could be seriously improved.

Do you mean an RF cylinder thermostat? I'm sure I've seen one somewhere. However, I prefer to use proper cable when possible. My experience with other RF stuff is that it isn't quite as reliable as a bit of twin and earth. OTOH, if running cable is very difficult, then they are tempting.

When wiring this system up, you'll still need a normal programmer. The hot water side handles HWC timing. The heating side acts as a master controller for your heating zones. You normally set this side to either "OFF" or "24H" and allow the programmable thermostats to handle any timing. There is good reason for maintaining a separate programmer. Firstly, you can turn off the entire system simply, without traipsing round the house to all the room stats. Secondly, wayward teenagers wanting 30C in the middle of the night (and having a programmable stat in their room) can be defeated. It is best to leave on 24H, if possible, though, as the programmable thermostat can keep a reasonable overnight temperature, such as 12C, rather than turning off completely. However, if you go overboard in balancing up TRV radiators so as to starve the master room, this may not be as effective. Doing so just a little, though, helps maintain steady temperatures.

Christian.

[Andy Hall]

I think that a better and more convenient solution to this would be the Danfoss Randall TP75-RF (about to be replaced by TP7000-RF) for the heating zones and a WP75-RF for the hot water. This would give you completely independent time and temperature operation for each zone and the hot water.

These have a common receiver type and you can get up to 3 channel receivers, although probably to order.

should be able to or any heating supplier.

You would sit the receiver at the boiler position and hook all the demand outputs to the switched input of the boiler and possibly the pump if the boiler doesn't have an overrun output for the pump.

If the motorised zone valves need to be in a position remote from the boiler and you can get power there, then you could have additional single channel receivers if you wanted. The receivers are trained to the transmitted signal during installation and you can have multiple receivers listening to one transmitting programmer.

Obviously this would cost a bit more to implement, but the trade off is against wiring convenience and time .

.andy

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[Set Square]

What you need is, in effect, an S-Plan-Plus [see

with 3 zones, 2 for heating and one for hot water.

Use a standard programmer and cyl stat to control the zone valve for the hot water - with 2 or 3 on/off periods per 24 hours, as desired - but always the same temperature unless you manually twiddle the cyl stat (which is probably acceptable). On the same standard programmer, set CH to permanently on and use its CH ON contact to feed the programmable room stats. The actual on/off periods (and different temperatures if desired) are then controlled by the room stats - which in turn control their respective zone valves.

The volt-free contacts on all 3 zone valves are connected in parallel - with the output side controlling the boiler and pump. That way, the boiler and pump will run whenever one or more zones requires heat and will shut down when they are all satisfied - achieving the desired interlock.

[Andrew Gabriel]

You will need to allow for pump overrun with most boilers. The Honeywell schematic does not.

[Set Square]

This is true. In fact, it's only a small change from the Honeywell diagram. The basic logic remains the same, but the output from the zone valves' volt-free contacts drives just the boiler rather than the boiler and pump in parallel. The pump is then connected to the boiler's pump control terminal - which is live whenever the boiler is firing plus whenever the boiler's pump over-run stat calls for circulation to continue.

[Ian]

Very many thanks to all who responded. My missing link was the need to still have a normal programmer to handle the DHW - not realising that the programmable room stats do not replace the programmer. I now have a fairly large bulge appearing on my forehead!

I don't really have a problem running cables everywhere since the whole place will be redecorated following the extension. I I've had a look at the Danfoss site and can see that that would meet the bill but have noted the comment from Christian concerning the potential reliability of the RF stats. Since one may be >20m from its receiver , I think it makes more sense to go the wire route.

Is there anything to choose in reliability of the electronics or TRV's between various manufacturers?

Thanks again all

Ian

[Christian McArdle]

Electronics are normally reliable. I'd buy a room stat mainly on the basis of appearance, provided it had the required functionality. Brand in itself wouldn't matter to me, except in that a good brand is more likely to physically look better.

TRVs should be from a reputable manufacturer and expect to replace every 7 years or so when they start to get a little slow. Cheap ones may fail in a couple of years.

Christian.

[Andy Hall]

You don't.. You can have a programmable thermostat for the cylinder as I mentioned - either wired or wireless.

If you do this plus programmable room controller(s), you don't need to have an additional programmer at the boiler at all.

On the other hand if you are going for wired connection to the cylinder, you could cover that using a simple cylinder stat and a programmer at the boiler instead.

They are pretty solid in fact. There are specific standards that they are required to meet and incorporate error detection to avoid errors due to interference problems.

I would regard them as a convenience if wiring is awkward, because obviously they are more expensive as well. However, if you can wire easily then that makes more sense.

I've used Danfoss and Honeywell controls and not had a problem with either. Similarly their TRVs.

Since replacing a TRV typically involves a drain down if the problem is in the base, then I would avoid the cheap products and again go for Honeywell, Myson, Drayton, etc. Honeywell seem to be the most commonly available at the merchants.

.andy

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[Set Square]

Whilst this is true, using a conventional programmer with its CH output connected to the programmable room stats has the advantage that the heating can be turned off from a single point (e.g. for the summer) without having to re-programme the stats.

[Andy Hall]

True, although a simple switch near the boiler connecting the switched outputs from the room programmers would achieve the same thing.

.andy

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[Christian McArdle]

Better to supply the feed to the programmable room stats, rather than the switched returns. Then you only need a SP switch for the lot, instead of a pole per zone.

Christian.

[Andy Hall]

No because they all come together at a common point at the boiler anyway, to form the switched live to fire it up.

If you have two (or more) room programmers, you could bring their demand signals and connect them together, then go through the switch, bring in the HW demand and then into the boiler.

This is an "or" function of the room programmers followed by an "and" created by the switch followed by an "or" with the HW.

Boiler = ((Prog1 OR Prog2 OR ...... ProgN) AND Switch) OR DHW

.andy

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[Andrew Gabriel]

That's not the problem. The problem is that when the last zone valve closes, there's no water circuit for the pump overrun. You have to make some provision for that.

[Neil Jones]

I installed a Honeywell CM67NG RF recently. It has an 'off' setting on the programmable thermostat part - which is not really off but a user-selectable frost-free setting which is 5 deg C out of the box, IIRC.

Neil

[Christian McArdle]

You couldn't put the switch on the output of the programmers, as that would short the outputs of them together, causing the zone valves to open in unison, rather than being individually controlled. You could put the switch on the microswitch outputs of the 2 port zone valves. However, this would not be ideal, as if the boiler fires due to the HWC zone valve demanding heat, then the heating zone valves may already be open, causing unwanted heat in the radiators.

You could always add relays. However, the standard (cheaper and simpler) method is the sequence:

Programmer -> Room Stat (programmable) -> Zone Valve -> Boiler (via microswitches) (or switch)

Christian.

[Set Square]

Ah, I see! You're talking about the by-pass loop rather than the wiring. You are perfectly correct - the Honeywell schematic doesn't show a by-pass loop and ought to do so.

My point is also valid. The Honeywell electrical diagram shows the boiler and pump feeds connected togther - so that the pump stops when the boiler demand is turned off. For a by-pass to work, the pump needs to be controlled by the boiler so that it can continue to run after the boiler has finished firing.

[Set Square]

Sure - but the earlier discussion related to a system with 2 or more heating zones, each controlled by a programmable stat. So you would have to turn each of these off rather than doing it in just one place. Also, I'm not sure that with some programmable stats you can effectively turn it permanently off with disturbing the other settings. Maybe you can?

[John Laird]

Does your last sentence make sense ? If the heating zone valves are already open, then this can only be because one or both of their timers are on and one or both of their stats are demanding heat. In what way is heat in the radiators then "unwanted" ? Apart from that, yes, the outputs can only be commoned at the boiler after the zone valves, or independent zone control will not be achieved.

[Christian McArdle]

It is unwanted because it happens despite you switching the central heating off. This switch should override the programmable thermostat.

So, the idea is to provide a master switch to turn off the heating. If that master switch is after the zone valve microswitches, then the programmers may call for heat and turn on the zone valves. The switch only prevents the zone valves firing up the boiler. However, if the boiler fires up for DHW, then the valves are already open and will pass water.

To work properly, you need to put the switch before the zone valves, so they aren't open unnecessarily. This is easiest done by switching the feed to the programmers, as doing so after the programmers requires relays, or a multi pole switch.

Christian.