Hot Water Tank Balancing valve???

[news]

A friend of mine has just had a (leaking) 25 year old HW tank replaced - standard vented HW system gas heated by a secondary coil.

The house is just like mine and on the output of the secondary heating coil there was a tap like body - neither hers or mine has ever had a handle and I assume that it is the equivalent of the lock-shield valve on the rads. It is designed (I assume) to stop the HW tank hogging all the heat when both HW tank and rads are both demanding heat - but once set during installation it can be forgotten (in just the same way that the lock-shields are forgotten).

Anyway as part of the new tank installation this tap has been removed and not replaced with anything. Is this normal for modern installations or has there been some sort of drop off?

We wont find out how balanced the system is until much later in the year and if its wrong I think it would be better to get the installers back sooner rather than later.

Thanks for any advice

[harryagain]

In days of yore, the hot water cylinder was often heated by a separate gravity/convection circuit that was uncontrolled.

If it is to be part of the pumped system, either it will have to have either some sort of manual control valve to balance the system OR more usually it has a separate circuit controlled by a motorised valve/s (thermostatically controlled) so that it can be shut off or the entire boiler output goes to quickly heat the water. This is a lot more useful and efficient.

If the new tank has no means of balancing/restricting the primary water flow, you will likely find that most of the primary water goes through the coil and almost none through the radiators.

[news]

Thanks, It does have a three way valve, so I am not worried about when only the radiators or only HW is calling for heat. However, I suspect that now if both HW and rads call for heat she will have to wait until the water is hot and the three way valve shuts off the cylinder until anything goes into the rads.

This seems like a bit of a retrograde step to my very first house which had a "Hot water" or "CH" priority system. In this case if set to HW priority there was no CH until the tank was hot.

[Andrew Gabriel]

Problem with older cylinders is they didn't have many turns in the coil, so ended up sending the water back without much of the heat being removed. So yes, they short-circuited the heating system without dumping much heat into the tank, so boiler output was much reduced when running both heating and HW.

Newer cylinders tend to have larger coils, or even fast heat recovery where the coil splits into several thinner coils for larger surface area and higher heat input into the cylinder. So although this will short out the central heating, it will absorb most of the boiler's output resulting in a very fast reheat of the tank. The tank stat will switch back to heating-only before the house starts cooling down. Some systems nowadays don't even have the mid-position for both heating and HW - when there's demand from the cylinder, they divert 100% of the boiler output to a fast recovery coil, which will reheat the cylinder in minutes, before switching bask to central heating - the radiators won't even have time to get cold.

OTOH, if the cylinder only has a few turns of the coil, without the balancing gate valve, it won't work optimally. I don't know if these cylinders are still available - maybe someone else in the group might?

[news]

Thanks for that explanation - makes perfect sense.

[Dave Liquorice]

If it was a modulating boiler which are a relatively modern invention. Old boilers are on or off. OK it'll cycle on it's overheat stat but not until once all the system water is hot enough. That's not the same as the delta T flow/return not being high enough.

This "tap" may also have been a one way valve to stop gravity circulation around (part of) the radiators. It's position doesn't sound right for that though.

We need more detail from the OP, pumps, valves and their location in the plumbing, control system (stats etc) and when things opearate.

[Roger Mills]

As others have said, if you're using a fast recovery cylinder which can absorb the full boiler output, it doesn't take long to heat up.

Also, depending on what timer(s) are being used, it may be possible to arrange things so that HW and CH are not normally on at the same time. For example, my HW comes on earlier than the CH each morning - so that they are not competing.

That would been a W-Plan system with a 3-port diverter (as opposed to mid-position) valve - which could *only* do one thing at a time. By default, that would have provided HW priority - I don't think there was an option for CH priority. A Y-Plan system - with a mid-position valve -

*can* heat both HW and CH at the same time. If the HW *does* hog all the flow until the cylinder is hot, it's still no worse than a W-Plan.

The problem with fitting a 'balancing' valve to prevent the HW system from hogging all the flow is that it will also restrict the flow in the HW-only situation, making it take longer to heat the HW with unnecessary boiler cycling.

[John Rumm]

One consideration to keep in mind is that modern part L compliant cylinders will have a much higher power heat exchanger than old cylinders. On many old ones the maximum transfer rate was often limited to less than 5kW, so you needed some load from the rads to prevent the boiler cycling endlessly during a cylinder reheat. A modern one may be able to sink 15kW or more, and hence the need to provide additional load is no longer there, and you can get a much faster reheat (in fact, you can reheat fast enough to match a smallish demand (e.g. a shower) in real time, giving combi style "unlimited" hot water.

[John Rumm]

On mine it does one or the other, even though its S+ plan and has multiple two port valves, so in theory could run the DHW in parallel with other zones. That's because it runs a much higher flow temperature for the cylinder reheat than it runs for the (weather compensated) rad zones.

I would expect most stuff would be part L compliant now...?

[news]

It is a fully pumped system, with a mid position valve just down stream of the pump from where there is a split to HW/CH. Thermostats on the HW , ambient Air and most radiators. Separately programmable HW and CH timing cycles. Two year old modulating condensing boiler.

Unless this changes replies radically I think I have enough helpful advice from the group about the design of modern HW tanks that we can rest easy that there has not been a drop off.

Thanks to all.

[ray.alexander]

Well I'd get the installers back, no point I guess wondering what could work/not work I suppose. Mind you they might charge a lot ? Well if they do might be easier getting some prices in from some other firms.

That's what I did and I was amazed at how much I'd ended-up saving when I'd used STL Heating. Just goes to show it's best to shop around!

[Geo]

5 posts today in this group from this spammer for STL - "amazed at how much I'd ended-up saving " "blog that someone made" "rang some local North West based firms"

- I smell a rat.

[GMM]

That's an interesting perspective which, of course, makes a lot of sense. Being on the verge of dealing with my DHW (I have been for a while but it's really going to happen soon), it's a current area of interest and the heat transfer rate could be a significant factor in choosing one cylinder over another and perhaps a factor in deciding on what size to get (although I suspect that 'enough to fill the bath' is a good rule of thumb) . Very few suppliers seem to quote a figure for heat exchange, however, and instead give reheat times or just say "it's good, 'onest guv", if anything. Similarly, I couldn't find a specific minimum performance in part L, though I'm sure there should be and I could easily be looking at the wrong document.