Heating, pros & cons of Gravity vs pumped

[Clive M]

Current heating is pumped with gravity HW,

What are the pros and cons of Gravity with controls (i.e Tank stat & valve) vs fully pumped with 3 way valve?

Is one more efficient that the other?

[Christian McArdle]

Advantages of pumped hot water:

1. Energy efficiency (in spades)
2. Reliability of circulation.
3. Rapid recovery of cylinder.
4. Complies with building regulations.
5. Allows sealed pressurised operation.
6. Works with modern boilers

Advantages of gravity hot water:

Christian.

[Dave Plowman (News)]

I suppose it still works when the pump fails. Or divertor valve, etc. But certainly not enough to outweigh the disadvantages.

[fred]

In article , Christian McArdle writes

Apart from the fact that 1, 2 and 3 are basically the same thing (Reliability of circulation gives rise to Rapid recovery of cylinder, which gives rise to Energy efficiency), I am inclined to agree. From o/p's previous post it appears he already has a pump in the system so there is no cost penalty and so no reason not to go to pumped h/w. My personal choice would be to use 2 separate 2port valves instead of 1 3port.

Faster recovery is by far the greatest advantage of pumped h/w.

[Owain]

"Christian McArdle" wrote | Advantages of gravity hot water:

Allows solid fuel boilers to dump heat to cylinder. Works in power cut.

Owain

[G&M]

It isn't legal to fit a new gravity system anyway so efficiency isn't an issue.

[Clive M]

Exisiting system is gravity, with the of leaving as it adding a 2port valve to control HW or converting to fully pumped. The question should really be , would a fully pumped system be more fuel efficient than a current gravity system? Current HW recovery time is not an issue at the moment.

If I go for the fully pumped route what is the best pump plan (honeywell) to use.

W Plan - seems to be least favored as it gives HW priority Y Plan - 3 way valve S Plan - 2 2port valves

The boiler manual (netaheat) states "if a 3 poer valve is used a bypass is not nessesary since one circuit is always open, where a pair of 2port valves are used a bypass is necessary. the total length of the bypass circuit should be greater than 4m of 22mm pipe, it should be fitted with a lockshield valve and be adjusted to maintain a minimum flow through the boiler of 4.5 ltr/min

Would this bypass circuit reduce effiency - i.e by allowing the boiler to cycle heating a short length of "bypass". I suspect the the easiest option is to go with a 3 way mid position valve?

[Set Square]

If you are getting fairly fast hot water recovery with your existing gravity system, by far the easiest mod is to convert it to a C-Plan - for which you will just need a 2-port valve and cylinder stat. If this is wired up correctly, it will provide a boiler interlock, so that the boiler only fires when either or both HW or CH are calling for heat.

In HW mode, if less than the total boiler output can can tranferred to the hot water via gravity ciculation, the boiler will simply cycle on its own stat. I suppose that with a fully pumped system, hot water recovery will be faster and the boiler would have to keep itself hot for a slightly shorter time. However, the additional increase in efficiency would be fairy small compared with the saving achieved by going from nothing to a C-Plan.

If I were converting from gravity to fully pumped (which I'm not - because my house already had a Y-Plan system when I bought it) I'd go for an S-Plan for several reasons:

• W-Plan doesn't provide concurrent CH and HW which can be a pain - even with a fast recovery cylinder

• Y-Plan is ok, but 3-port mid-position valves are notorious for going wrong - and contain much of the overall system control logic within the actuator head. If their microswitches fail, all sort of funny things start happening which are not always easy to diagnose

• S-Plan is easy to understand, is less prone to single points of failure, and lends itself to adding additional zones (S-Plan-Plus) if, for example, you want separate control over your upstairs and downstairs radiators, or maybe over a conservatory

With regard to a by-pass loop, I'm not convinced that you need one. By-pass loops are usually used in fully pumped systems in conjunction with pump over-run. That is, the pump keeps ruuning to cool the boiler down after it stops firing, and the water has to have somewhere to go, even if all the zone valves are closed. However, in your current setup, in gravity hot water mode, the pump isn't running at all, so can't over-run - and the boiler apparently copes ok.

If you *do* go for an S-Plan, and *do* decide to add a by-pass loop, use an automatic by-pass valve rather than a gate valve. These remain closed under most circumstances, only opening when the water *really* has nowhere else to go.

HTH.

[N. Thornton]

I couldnt figure out what you mean by that.

Only if the rest of the system is non electrical too, and there arent many of those around now. Thermostat, all valves, fuel flow... would have to be a crude old system.

Regards, NT