Direct Heat Banks - why no zone valves (and other questions)

[Dave]

I need to make a decision about the new heating system in the next week or so; after all the debate and advice here (and lots of web crawling) a DPS GXV heat bank with a Worcester Greenstar 29HE are still the current favourites. A couple of questions:

- I need 3 heating zones, what's the advantage of using a pump for each zone rather than one pump and 3 zone valves?

- Is there a significant disadvantage in joining the heating returns and running back to the cylinder in one long'ish pipe or should I run 3 separate returns?

- Because the cylinder will be in the garage I want a recirculating pump on the DHW but how can this work when the water is just being recirculated around pipework and not through a heat source?

- Having a directly heated cylinder presumably means there's more delay between the room stat demanding heat and the rads providing it - is this noticeable in practise?

Dave S

[Christian McArdle]

Personally, I don't see one. I would prefer a single pump and multiple zone valves. The zone valves allow for easier wiring up, without relays or additional anti-gravity valves. It also lends itself to shared tappings on the cylinder and is less noisy. Imagine a multizone system with 5 or 6 pumps whirring away.

No disadvantage, provided the pipework is correctly sized for the expected heat capacity. If slightly undersized, it will be noisy, rather than ineffective, although gross undersizing will require high pump speeds, excessive noise and may never reach full capacity.

Basically, it would require the DHW pump to go full time, which would destratify the heat bank, so I wouldn't recommend it. Consider installing an additional plate heat exchanger near the taps and running a primary circuit out to it, heavily insulated (well in excess of statutory requirements). Then rig up a low flow pump to keep the heat exchanger and primary warm, with the main pump firing when a flow switch activates.

Your premise is incorrect. The radiators will become hot almost immediately, as the heat bank water is already hot. Such a system will provide the quickest response to a heat demand. Indeed, the radiators would probably be getting hot by the time the boiler has managed to ignite.

Christian.

[Dave]

... snipped That's what I thought but the DPS website makes a point of suggesting pumps without valves - I'll get to the showroom and ask them.

... snipped

That's an interesting idea. One of the heating zones is for towel rails and airing cupboard so I could probably use that - it won't be running permanently but it should be sufficient.

immediately,

OK, I was being dumb. I guess it takes a little longer from a cold start but that's not a very frequent event.

Many thanks. Dave S

[Set Square]

With 3 pumps you can match each pump to the requirements of a single circuit - but the control logic is a lot more complicated, particulrly if the boiler needs pump over-run (which pump over-runs?)

A single pump has to be able to run all 3 circuits - but on occasions will only need to run one. Maybe one of these fancy variable output devices (Alpha or somesuch?) would be worth considering. The control logic is much simpler - using an S-Plan+ setup.

It's ok as long as the pipe is big enough for the total flow and as long as they only combine *after* the last rad return in each circuit. [If any rads return after the join, you might get reverse circulation round bits of the system].

I don't understand this bit! Surely the idea of a heat bank is that DHW is at mains pressure - and gets heated by the heat bank on its way to the taps? In this case, how can it be circulated round the taps when there is no outflow?

Is this heat bank being used only for DHW, or for heating as well? If it's also used for heating, you'll need an additional pump and control logic to make sure that the heatbank is always hot. The room stat will then simply open a zone valve (and switch on a pump if necessary) to circulate water through the already hot heat bank to the radiators.

[Dave]

...snipped

Good point. So the pumps would be standard 3-speed types, each set to the optimum speed for that zone and controlled directly by the timers/stats (possibly with relays).

...snipped

I don't understand it either but it's a "standard option (!)", Christian has made a good suggestion so I'll investigate further.

... snipped

Thanks. Dave S

[Ed Sirett]

Each pump can be adjusted individually but this can be got around by balancing the whole system with all the zones active. There is a huge down side which is that you may get some reverse flow in the other zone(s) when only 1 or 2 pumps are going. You can add nonreturn valves to each zone (these may buzz). You can mitigate the effect by making sure the common feed to all the pumps is as fat and short as possible.

None p[rovided the pipes are suitable sized. However be careful not to return any radiator seperately to the common return from others in its zone.

From the furthest HW tap (that you want enhanced) or HW usage point take a small (10mm hep? 8mm microbore?) pipe back to near the heat bank. Insulate every piece of DHW to very best standard you can manage, including the return pipe. Obtain a BRONZE circulating pump (this is like the CH pump but made of bronze and 5x cost). Put small DHW pipe into the pump inlet. The pump outlet into the mains cold water inlet. Put pump on very lowest setting, also only open one of the pump isolating valves a tiny bit. You want a trickle of water to flow out of the DHW supply form the heat bank but enough to make the DHW pipe warm/hot but not so much that the returned DHW is actually warm. Some poeple say you should put a non return valve in the return circuit, I think that length of small pipe work and the restriction on the pump isolating valve should be enough to stop much mains cold being drawn back and mixed with the HW. Consider putting pump on timer or PIR so that it only operates at peak times. It is a compromise between energy saving and convenience. You will however save a stack of water as you will have instant(ish) HW. The heat bank is the heat source and occasionally the boiler will have to fire to replensih it.

No! the heat bank stays hot all the while (OK there is a bigger delay after you come back from holiday). So the hot primary water is ready to send to the radiators instantly.

[Pete C]

Or, use a pipe stat as well to run the pump so it only circulates DHW when the pipe has cooled down.

This will help to keep the DHW in the heat bank stratified, and has the advantage that it is more responsive to timer or PIR control.

Another option is to save on an expensive pump is to put a microbore pipe alongside the heavily lagged DHW pipe or even inside an oversized DHW pipe.

This would have it's own pump and pipe stat and be connected across the DHW coil of the heatbank. When the DHW pipe cools the pump would circulate water through the DHW _coil_ of the heatbank, so using a small amound of DHW heat (but not the DHW water itself) to keep the DHW pipe hot

cheers, Pete.

[IMM]

It is simple. use a relay for each pump. Maplins sell a 5A 240v DPDT miniature relay for 32.99 and 31.50 for the base. It is easy to arrange DHW priority .

DHW pump, or heat bank pump.

Not so.

Mains in -> check valve -> to plate heat exchanger -> out of plate to DHW taps -> returns to the mains in after the check balve -> check valve before the tee -> bronze pump just before the tee. On section just before the tee foit a pipe stat. when the pipe get cold the pump kicks in to circulate hot water around. The flow will active the DHW heat bank pump. Unt to temp and the stat cut out and all is off.

[IMM]

Yep.

[IMM]

One should be there.

[IMM]

No DHW in the heat bank, only primary water.

Heat has a plate heat exchanger, no coil.

[IMM]

Pumps valves are available with integrated check valves. See the Gledhill piccie below. The upper pump valves are larger than the lower.

Pumps for each zone is far better. Gledhill use this method see:

two pumps to the left could have been one and a mid position 3-way valve.

Best get DPS to install a heating flow and return on the heat bank for each zone, then totally independent zones feed from a neutral point (heat bank).

A pump is cheap. A replacement relay at £2.99 is cheap. All win, win, having a pump for each zone. Much better balancing too.

See my and Eds post on this.

Wow! expensive and complex.

immediately,

[IMM]

Look at the Gledhill Gulfsteam 2000 CPSU. All in one box, boiler and heat bank. They also have instant electric backup for DHW (Switch).

say only large orders are entertained, but TP can get individual boilers. They supply one offs to selfbuilders.

With Gufsteam you will need three zone valves to have 3 zones.

[IMM]

Should be: It is simple. use a relay for each pump. Maplins sell a 5A 240v DPDT miniature relay for £2.99 and £1.50 for the base. It is easy to arrange DHW priority .

[Pete C]

OK, is it a thermal store that's heated by a coil?

In that case you'd connect a DHW 'preheat' circuit across the heatbank's primary water connections.

I wonder if 4 10mm pipes in a 22mm pipe would make a good counterflow heat exchanger...

cheers, Pete.

[IMM]

Yep. The termninolgy, which has not settled, is that a thermal store has a coil and a heat bank a plate heat exchnager. Range call their Flowmax a thermal store yet it has a plate heat exchanger. They are about the only one who does, and as they started with coils and went to plates I think they just kept the names up.

Possible. You could have a second plate heat exchanger on the flow fromthe boiler with the cold mains running though it. It is better to have a blending valve on the flow/return which ensures only 75-80C water enters the store at the top and stays there. The store heats up "top down" being direct. As the DHW plate takes its heat from the top of the store you gain nothing by having a pre-heat plate on the flow.

There are hybrid stores that have a coil as a DHW pre-heat and a plate. Mains into the coil, out of coil and into the plate. The flow switch is adjusted so that the DHW plate pump only energised on high water flows as the coil can easily do low flows. So no pump hunting.

Sounds like it would restrict flow.

[Pete C]

Hi,

Sorry, by preheat I don't mean heating DHW before it reaches the heat exchanger, just keeping the DHW hot in the pipes to save water and have a quicker response when the hot tap/shower is turned on.

I was suggesting using a non DHW circuit alongside the DHW pipe and separate pump connected to the heatbank to keep the DHW hot. Though as a pump is used on the secondary side there might be a way of getting it to do this too.

cheers, Pete.

[IMM]

When the pump kicks in, heat is taken from the heat bank to the plate very quickly giving an excellent response.

Read my post on this thread on this point using a pipe stat and secondary bronze pump.

[Pete C]

True, but if the tap is some way away from the exchanger you'll still need to run a couple of litres of water before hot flows.

Would the DHW have it's own pump or share it with the CH?

How much is a bronze pump, I'd expect they are pricy. Pumps with a plastic impellor in an iron body are cheaper though might seize up if used for water with no inhibitor in.

cheers, Pete.

[IMM]

Its own broze pump.

Grundfos make the Comfort dedicated to secondary circulation. It has a built in thermostat, so the pump does not continually run. There is also a model with a time clock so it does not run overnight or on hols etc.

may be cheaper to buy a Bronze pump from another cheaper pump maker and use a cheap pipe stat