CH/DHW plumbing layout

[Frank Stacey]

Sorry this is rather long.

Currently, my house heating system has pumped CH circuit and a gravity feed DHW circuit. I want to pump the DHW circuit because the convection in the system seems very sluggish - a substantial boiler takes a long time to produce much hot water and spends an awful lot of the time switched off while it is doing so.

At present the CH circuit has a hot flow from one side of the boiler and the pump is on the return to the same side of the boiler. The DHW has separate hot outflow and cold return both on the other side of the boiler. It has been suggested to me that I could put a pump on the DHW return and a motorised valve in the loop and all would function properly - I would, of course, fit a boiler thermostat to control this circuit. I am not sure where the best place for the valve would be - presumably close to the pump?

This arrangement is not quite the same as any of the X-plans usually considered in this context. The S-plan is similar and only has one pump. The system I am proposing would be very convenient because most of the existing pipework, pump and valves are in a very cramped space beside the boiler whereas the proposed arrangement with an extra pump would allow the new pump and valve to be inserted in a place where there is more than sufficient room to work.

Are there insurmountable problems which I have not foreseen?

2 supplementary questions!

The valves currently in the system are of an older(?) type. (There is a frost protection circuit for the greenhouse as well as the CH system - hence

2 valves.) They are 2-port valves with a live signal to open them and a separate live to close them. Do these valves have any advantage over the more modern type with a single open control and spring return to close?

The system includes some pretty hefty relays to switch power to the pump and to the boiler. Is it not the case that the switches built into the motorised valves can readily cope with the load of driving a pump and a boiler? The specs of the pump and the boiler talk about 0.5 amp and about 100W which doesn't seem much to a novice in these matters. Could I just remove these clunky bits of kit and use the valve switches?

Any help would be much appreciated.

-- Frank

[Set Square]

What you want to do is quite do-able and, as you say, comes quite close to an S-Plan.

Where it differs from an S-plan is that you will have 2 pumps, which need to be controlled separately.

In an S-plan, the room stat opens the heating valve and the cyl stat opens the HW valve. Each of these valves has a pair of volt-free contacts which close when the valve opens. These are wired so as to switch on the boiler and pump whenever either or both of these contacts art closed.

In your case, you need the CH valve to switch on the CH pump and the HW valve to switch on the hot water pump. You almost certainly need some additional relay logic to switch on the boiler whenever either or both pumps are on. The relay(s) are needed not for their current switching capacity - but to ensure that the boiler comes on when required without inadvertently bringing the other system on.

[BigWallop]

This sounds like a standard combination boiler layout. The feed side to the DHW part of the boiler isn't strong enough because it is only at gravity head height pressure from the cold water storage tank in your loft (?) so it should be possible to switch this feed over to the mains water pressure and this should give it more oomph, gusto, or whatever you'd call it, to supply the hot water you want at the taps.

Is it true that the boiler only fires up when your turn a hot tap on and demand water ? Or do you have a hot water storage tank as well ?

[Set Square]

I don't think you read it very carefully! This is NOT a combi. It is a (now old fashioned) system where the primary circulation in the CH circuit is by natural convection (often referred to as "gravity") rather than being pumped. Hence it takes a long time to heat up a tank of hot water - and only uses a fraction of the boiler's capacity in so doing.

Nothing at all to do with the rate at which water flows out of the hot taps!

[Andy Hall]

I think you're on the money.

There are probably several solutions to sort out the logic.

In fact, you can avoid a relay altogether if spring return zone valves are used.

The OP was talking about (I think) an older pattern that I used in one of the first heating installations that I did.

This had motor control to open and to close it and there were microswitches and a cam. The thermostat had to be a changeover type. Basically, as the themostat switched over, the motor would run to the opposite end of its travel until a microwswitch turned off the power. As it switched back, the microswitch would run to the other end. It worked well and I don't know why they fell out of favour.

Anyway for this one, it would be possible to hook up the relevant pump and the motor of the relevant valve to the relevant thermostat directly. Then wire the auxiliary contacts of the valves together and to the boiler.

When a thermostat demands, it's valve will open and its pump will start. THe boiler will be told to fire.

This is obviously a simple "OR" arrangement, and means that there is not priority for HW over CH which is more normal these days. However, it will be a fair bit better than the current arrangement.

In fact one could omit the motorised valves and put a gravity system no return valve on a vertical run of each circuit. THis would stop natural circulation and the flow would only take place when the relevant pump is on. However, two relays would be needed. The approach might save a little cost, but the motorised valve route is probably much easier to do.

.andy

To email, substitute .nospam with .gl

[BigWallop]

I have to admit to not knowing a lot about all the different types and layouts of heating systems. But was only going on the description of the pipe layout given by Frank (the OP).

[Set Square]

This would certainly avoid the use of relays. My only concern would be that the pump would start *before* the valve opened - giving the water nowhere to go for a short while. I don't know whether this matters.

[Aidan]

It sounds like a fairly old cast-iron boiler, so it might be cost-effective to consider a more efficient replacement; you knew that, of course!

I have modified several domestic installations that had pumped HTG and gravity DHWS circulation as you describe. I used one pump and a 3-port mid-position valve, so that's obviously my preference. It could be done the way you describe. I don't see the point of using a pump and a motorized valve. The pump would be switched by the timer AND the cylinder thermostat, so the valve isn't necessary. A spring-check non-return valve in the return, to prevent overheating the HWS by gravity circulation might be useful,

These installations usually have the cold feed on the HWS return to the boiler and the open vent on the HWS flow. You need to ensure there are no valves between these connections and the boiler. Putting a pump in this circuit could involve a risk of pumping-over through the open vent, if the pipe arrangement isn't properly thought out.

Would the valves the Satchwell Mini-valves (now made by Sunvic, I think)? They're motor-open motor-closed valves. They were supposed to last longer than the motor-open spring-shut valves, which normally operate with the motor stalled, but the life of the conventional valve actuators is pretty good, so I don't think it's now a big advantage. I think you'll find spares for the original valves are fairly costly.

I'd be concerned about the risk of the greenhouse circuit freezing if a component failed, and associated risk of losing the water from the rest of the system and then dry-firing the boiler. If you have to retain the greenhouse circuit, I'd prefer to isolate it with a plate heat exchanger, if possible. This circuit would have it's own expansion vessel & pump and could be filled with anti-freeze. A low pressure cut-out on the boiler should be the minimum protection.

The valve switches generally make relays unnecessary, but you'd need to check your wiring. They might be multi-pole relays that were necessary because they may be switching more than the one circuit that the valves' switches could.

[Christian McArdle]

I get hot water priority on my S-Plan simply by having the cylinder right next to the boiler/pump plumbed in 22mm with no balancing valve. I'd like to see the radiators get any hot water with that lot shunted across the boiler output!

Christian.

inadvertently