CH and HW are on : which takes priority

[jives11]

Hi,

I'm trying to further my understanding of my central heating system. I have read the various FAQ's in this forum but one question I have.

My system is the 'classic' fully pumped gas fired system with a hot water tank. One radiator is in the HW circuit as a fail safe, along with some towel rails, to warm them in summer even though the main CH system may be off.

My Question is when the timer has both CH & HW on, how is this conflict resolved. Does the controller prioritise HW, or is the switch in a Tee position where it routes the water from the boiler to both HW & CH ? IS HW only selected when the CH is OFF i.e in the summer ? I tend to rely on a timer stat so my CH is always ON and I allow the programmable stat to determine when the pump gets enabled

My tank has a moulded foam insulation plus an additional lagging jacket. I'm finding in cold weather the house takes a long time to get up to desired temperature, if at all. While this may be affected by insulation etc, I wonder if the conflicting requirements of HW & CH reduce the responsiveness. perhaps I should schedule HW much earlier in the day and rely on good tank insulation to keep it hit until evening ?

[Bob Mannix]

Yes

Yes

Yes or have HW on all the time (if tank well insulated)

[Andrew Gabriel]

It depends on how the system was designed and installed. If you have a mid-position valve selecting heating and hot water (which is probably most common), then the system can do both together. If you have a prioritsed system, then hot water gets priority as it takes a finite time to heat up, which is usually less than it takes for you to notice the house cooling down.

My H/W tank is heated from 05:00 to 05:30 each day. That easily lasts a day unless I have several visitors. I have a button to heat it for another half hour on demand in such cases. If there's only me in the house, it lasts the following day too.

[Roger Mills]

You haven't really told us enough to be able to answer that question. There are several flavours of 'classic' systems, and they all behave slightly differently. Have a look at

and try to work out which 'plan' you have. [It won't be C-Plan but it could be either S-Plan, W-Plan or Y-Plan depending on the number and type of motorised valves present.]

A W-Plan system will heat *either* the HW *or* the CH, but not both at the same time. If both are calling for heat, the CH demand gets suppressed until the HW is hot. The other 2 plans can heat both the HW and the CH at the same time, usually with equal priority.

A well insulated cylinder will keep the water hot for many hours, so you don't have to heat it immediately before using it. If you have a programmable stat on the CH, use that to time the CH (with CH on the main programmer set to 'constant', and use the main programmer to time just the HW - at a different time from when the CH is on.

[Ed Sirett]

Good Q.

Does the controller prioritise HW, or is the switch in a Tee

Systems can either the one-or-the other using a 3-way diverter valve or both together if needed using a mid-way capable valve.

Older installations tend to have mid-way valves (Y-plan). More modern ones ave diverter valves (W-plan) or more likely 2(or more) 2-port valves (S-plan/extended S-plan).

What happens during both HW+CH demands depends on how the system was installed. even with a mid-way valve the system may still effectively prioritize the HW tank. The modern idea is that the HW cylinder can be reheated very quickly typically (10-15 minutes for a bath full) and so the house heating can be switched off while the HW cylinder is reheated.

Possibly. Test 1. does the system heat the house when the HW is switched off? perhaps I should schedule HW much earlier in the day and

If the tank has a thermostat then it should only take a small proprtion of the time to do HW.

There are dozens of reason why the heating is not adequate, HW coil short circuiting the HC probably does not make the top 5.

[jives11]

at

try to work out which

thanks. Examining the inside of my airing cupboard with a torch I can see 2 2- Port ACL lifestyle valves. One is definitely on the Tank coil pipe and the other appears to be on the main CH circuit which the coil circuit joins further up , if that makes sense. I'd guess that 2 * 2-way switches can implement a 3 way mode where tank is open and HW is open too.

This thread has been very useful. I have been tending to leave the CH permanently on 365 days and regulate the house with the timer stat. While this is fine in winter /cold weather , this may mean that during Warm weather I'm probably briefly heating the house to create hot water, as the CH will be still on as will the HW, for a couple of hours a day at least.

[Dave]

Now I am confused :-(

It looks like my set-up is an S plan with a valve missing from the hot water tank heating. All mine consists of is a valve that is controlled by the room stat that open/closes the pumped supply to the central heating. (Both hot water and rads are pumped with the same pump) There are two separate circuits that are fed by one pipe from the pump, then split for hot water and CH, with a valve in-between the rads and the boiler and on their return to the boiler. Is this an S type?

I have a slightly different question to ask. When the new boiler was installed, the plumber didn't wire it up as it should have been and the pump runs all the time that the boiler is powered up (not just when it is fired up). It it better to leave it like that, or should I find the pump terminal, on the boiler, that shuts down

15 minutes after the boiler ceases firing each time/cycle?

Dave

[Roger Mills]

What you have described doesn't equate to *any* of the standard plans! It sounds as if you have a system which, while being fully pumped, suffers from many of the disavantages of a gravity HW and pumped CH system - in that there is apparently no control of the HW temperature, and no boiler interlock to turn the boiler off when HW and CH demands are both satisfied. You would need another valve and a tank stat, and some adjustments to the wiring to convert it to a pukka S-Plan system.

The pump needs to run whenever there is a heat demand on the boiler - including times when the boiler's internal thermostat has turned the burner off. The pump shouldn't run when there's no demand on the boiler. If it does, it's wired wrongly. If the boiler has dedicated pump connection terminals to enable it to control the pump, that's where it should be connected.

[Roger Mills]

Yes indeed. That is an S-Plan system which enables you to have just HW, just CH or both at the same time.

If it's wired correctly, the radiators won't get hot when you're just heating the HW (like in the summer) because the CH valve won't open unless the programmer and room stat tell it to.

With an S-Plan system, the HW valve is controlled by the programmer and cylinder stat, and the CH valve by the programmer and room stat. Secondary contacts in the valve actuators close when a valve is fully open - switching on the boiler and pump. So the boiler and pump only run when either or both valves are open.

[jgharston]

My system at home looks nothing like any of those diagrams. For a start, those diagrams have three or four water pipes going to the boiler, my boiler only has two water pipes, see

if your system is the same as mine, then:

If 'CH+HW' is selected, then at the programmed times the CH switch activates. When the roomstat determines that the temperature is below the set level it turns on, so turning on the value that lets water to the radiators, and, via the unactivated HW switch, turns on the boiler and the pump.

Looking at the plumbing diagram shows that this causes hot water to be pumped through the whole system, hot water tank and radiators.

If 'HW' is selected, then at the programmed times the HW switch activates. This turns on the pump and the boiler, but, as the CH switch is not activated, not the valve.

Looking at the plumbing diagram, this shows that how water will be pumped around just the hot water tank part of the system.

I don;t known what "plan" this is called, it doen't match any of the refered diagrams, but is the system used in every house I've ever lived in.

-- JGH

[jives11]

Thanks Roger, I hope it is wired as you describe as that would seem the ultimate, and would suit my preferred current method of permanent CH regulated by a CM67 programmable stat. HW is controlled in the 'normal' way i.e 2 shorts bursts - one before I get up and one in the late afternoon.

The only unusual aspect is that I have 4 bathroom towel rails in line with the HW , and a landing radiator which has no shut off valves so I ssume is always in circuit in case of a "China Syndrome" type malfunction. A couple of the rails are shut off

I can't say if the whole thing is balanced or not but I guess that is a whole new area to discover

[Dave]

I use the boiler stat to control the heat of the water, so that if the power shower handle points vertically, you get water coming out that is neither too hot, or too cold. The seams to work OK

I did have a tank stat on the old system, so I'll dig it out and wire it in.

That is what I thought.

Many thanks for your info. I'll get round to correcting all this when the weather warms up a bit and we can live on one tank of hot water for a while.

Regards

Dave

[Roger Mills]

It will work after a fashion, with two major drawbacks:

1. You have to run the boiler at a suitable temperature for the DHW (about

60degC) so the radiators can't get any hotter than this, whereas they need to be at 80+degrees to generate their full heat output

1. The boiler will waste energy by cycling on its own stat even when the DHW is hot enough and the CH valve is closed.

Wire it in to what? Unless it has a valve to control, you *still* won't have a proper S-Plan system.

[Roger Mills]

The diagrams are *schematics* - showing how they *function* rather than exact representations of the pipework. With the exception of W-Plan they all show 3 connections to the boiler. But it doesn't take a lot of imagination to visualise what they would be like if you combined the HW and CH returns before taking then back to the boiler. There would then only be 2 boiler connections, and they would look more like your diagram.

any of the standard Honeywell 'plans'. In the 60's and 70's, the 'norm' was to have a gravity HW circuit and pumped CH circuit. What you have is one step up from that in that it's fully pumped. But it still doesn't provide independent control of HW and CH (you can't have CH without HW) nor boiler interlock - as provided by most of the 'plans' - and the thing is that it would be so easy to update it to take advantage of a much better control system.

No it isn't - he's got a proper S-Plan system.

[Dave]

Yes, I understand that. I have always been worried about the constant pump running and didn't consider this.

We have a tame plumber that will do the pipe work, so that is not a problem. Thanks for that.

Can anyone advise just how far up the tank should I fit a thermostat. I used to have it about 1/3 above the bottom of the tank. We are having a new hot water tank some time this year and it will be pre insulated with foam. I can cut a hole for it and insulate it again after I fit the thermostat.

Many thanks to all who have posted

Dave

[jgharston]

I know they're functional schematics, but, probably because I'm an electrician, I tend to expect functional schematics to have some relationship to the equtipment they represent. A central heating pluming circuit is essentially a parallel lighting circuit - live feed out, tee off to emitters in parallel, return back to source.

Everywhere I've lived since 1969 has been like that (except the one place I lived which had no heating at all. Get up, fill coal bucket, get fire started...)

My boiler's been wearing out and I plan to get it replaced, and moved to a more useful location. The plumber can put in whatever system is appropriate - as long as it still give me a tank full of hot water :)

-- JGH

[Roger Mills]

Indeed - and it matters not one iota whether the returns join before going back to source, or whether they go separately - this does not affect the

*function* of either an electrical or a water circuit.

[dennis]

But may make a difference to the pipe sizes needed.

[Roger Mills]

Yes, of course - and to cable sizes in the electrical analogy. But my previous post was in the context of a discussion as to the *functional* equivalence or otherwise of heating schematics which show either separate or combined HW and CH returns - so I didn't feel it necessary to mention pipe sizes.

[dennis]

The big difference is that you really need to run the maximum size cable everywhere but that isn't so with water. You could get away with running smaller cables, like water, but you would need a lot of fuses in odd places.