Combi v HW Tank

[Andrew]

But you do have a large tank of water at ?much more than 60C, so is there a possible safety issue ?.

[Andrew]

Then when you built your house you should have insulated all the hot water pipe runs properly and not just with a roll of horsehair lagging.

[Andrew]

Unlike the pipes connecting it to your bath and sinks

[Harry Bloomfield, Esq.]

It happens that

formulated :

I doubt there is much difference at that stage, but reheating a tank from cold at the colder stages, will be much more condensing efficient. Stored hot water means that if there is a boiler issue an immersion heater can still be available to supply hot, if water goes off and you have a cold water storage tank, you still have HW for a while.

A combi has to fire up everytime even a tiny amount of HW is needed, so much more where and tear, plus less efficient. A combi has all the failure prone bits inside it, so a specialist is needed for every failure. The combi puts the system under slightly higher pressure so more likelihood of leaks and the slightest leak will involve lots of regular topping up.

We had an open vented system installed 40 years ago and now on our third boiler. Each time replacement was due, they wanted to move us to a combi, but I insisted we should remain open vented - which allows me to easily fix most of the issues which arise. HW flow is also much faster, than provided by most combi and a bath can be filled much quicker with both hot and cold running at the same time.

Remember - an open vented system is much more expensive to install than a combi, once the open vented is removed it will cost a lot of money to reinstall.

The big advantage of a combi, is space saving - great for compact flats and having instant HW water, rather than having to wait 20 minutes for the cylinder to heat.

[Harry Bloomfield, Esq.]

John Rumm explained :

Isn't that the type which needs a G3 qualification?

[www.GymRats.uk]

That was the other thought but I'd forgotten about that option.

Would be nice to get a wood burner in the lounge (also on the to-do list) but plumbing would be tricky as the house is upsidedown and I have UFH in all the upstairs rooms so no direct route downstairs to the boiler area.

Cheers - Pete

[www.GymRats.uk]

But they have to be Japanese boilers if I remember correctly...

In fairness to Dr.Evil I did go with his suggestion of boiler at home. "EcoHometec" AKA M.A.N /Micromat which has been superb and to date everything has been repairable, and the more I'm forced to repair the simpler it becomes. So far the last repair of pin-holes in the back plate patched up with repair putty is still holding up. That was the first time I've taken the boiler off the wall but all went well. :)

[www.GymRats.uk]

Mums house built early 60's had a huge floor standing gas boiler, gravity fed, hot water tank and header tanks in the airing cupboard. We had to replace the indirect HW tank once (might have been late 70's) when internals rotted out and the boiler was replaced about 25 years ago with a little wall mounted thing. Never missed a beat. The HW tank was a pain in the days before we had an electric shower though as it would only do one bath and the next had to wait an hour or more to bath. :)

[Paul G]

I too have a Stuart Turner pump (hot & cold feeds) for our showers. Unfortunately the pressure to our kitchen hot tap (downstairs) is poor and I'm under spousal pressure to improve it. It could be simply at the end of a long pipe run.

Are there any gotchas in adding another pump for the DHW ?

[Dave Plowman (News]

I discovered with my system boiler and fast recovery cylinder - having come back off holiday where it was switched off - that I got an acceptable shower very shortly after switching it on. Might have not been in the depths of winter, though, where the header and storage tanks would have had colder water.

[Fredxx]

This has a big coil in it, I'm afraid I don't know much more. It does mean you have a lot of primary water and a bigger expansion tank.

The hot enters a mixing valve, with cold to reduce the temperature to a constant safe level.

I suppose I save an expansion vessel and all the pressure relief hardware, drain etc, but then add a header tank with all it's issues and hardware.

[Fredxx]

Yes, but that's the same as an unvented tank, except mine operates at a lower pressure and doesn't need the pressure relief valve hardware.

[SH]

A half decent combi like a Vaillant 838 will take 38 kW of gas...

So two of these is 76 kW which is far more than a domestic gas meter can supply, that being around 66kW...

and thats before you add in gas hobs, gas ovens, gas fires etc.

You could have 2 off 30 kW combis but the water won't be quite as good as on a 38 kW boiler.

[Fredxx]

<snip>

I remember those days and his posts!

[John Rumm]

In my case the unvented cylinder heats to 60 most days, and 70 once per week (anti legionella cycle). I put a blending valve on the output so it can't feed the house at more than about 50 or so.

Unvented cylinders have a number of over pressure and over temperature safety valves that will dump water to a waste pipe in an emergency.

Heat banks are typically at atmospheric pressure anyway. Extraction of heat usually being by means that allow the DHW maximum temperature to be limited.

[John Rumm]

No reason why you can't. Only downside is a bit of added noise when you run the tap.

[John Rumm]

The losses from the cylinder itself are pretty small (couple of hundred watts perhaps) - it is well lagged with the lagging contained in an outer skin. Keep in mind that in many cases you want some of the heat loss from the cylinder, since it can then heat an airing cupboard. (I deliberately chose not lag some of the pipework so as to lose some extra heat into the airing cupboard)

Combi heatloss is in theory lower - but you do end up running off more partially heated water waiting for the output to get to temperature. Not all combis can run in condensing mode when heating the DHW. Also combis typically need to be near an outside wall, so pipe runs can be longer. A cylinder is often best near the middle of a property.

I installed a combi in my last place when I converted the loft. Went for a 35kW model. I found that it could do one really good shower or two "adequate" ones at once - at least in summer. (the 1/2" mains feed in that place could only muster about 17 lpm at best). All in all it was ok, and did better than many combis I had experienced in the past. (and the vented system it replaced was particularly poorly done, so I hardly lost any bath filling performance).

In this place there is a decent MDPE mains feed that can do 30 lpm or better, so the cylinder will do two reasonably good showers at once.

From the DHW point of view I was designing it for performance, and gave no consideration to cost of running TBH. However I did focus on cost and efficency for the system as a whole - going from a single heating zone off the back of an old Ideal Mexico floor stander that was probably less than 70% efficent, to a modern condensing system boiler with 95%+ efficiency, split heating zones, external weather compensation, and split temperature operation (so the CH can run at 45 degrees when its mild out, but the DHW still gets heated to full temperature)

[John Rumm]

Since the building regs were updated some years back (for the purposes of heat and energy conservation) all hot water storage systems now come under part G and would need signing off by building control or by a suitably competent installer when installing or servicing.

[John Rumm]

Old cylinders typically only had coils large enough to reheat less than ~5kW. This made some sense on old gravity circulated (i.e. thermosyphon) cylinders, or when running fully pumped on old Y plan installs with a fix rate output boiler (where the cylinder load could be added to the CH and reduce cycling of the boiler)

With a modern one on S Plan or W plan, you can reheat a fairly standard sized cylinder in 15 to 20 mins.

[John Rumm]

As long as you don't draw heat at a faster rate than the boiler can supply, you should be able to get constant delivery of DHW in much the same was as you do from a combi. (it does help if the controls are set to fire the boiler soon after it detects a drop in temperature though)