Combi or System boiler

[Bev]

We are in the thought process of removing night storage heaters and having new full central heating and hot water from a kerosene fired boiler.

The issue is whether we go for a combi boiler or a system boiler with a much smaller HW cylinder than at present and with the smaller cylinder relocated to the loft.

The boiler will be an outside boiler.

The length of hot water to taps/shower pipe runs from a combi boiler will be great and need a lot of running off before the hot water comes through but it avoids the need for a cylinder.

A system boiler would give a heated water store that would be almost directly above bathrooms and kitchen but would be in the loft (single storey dwelling) so supplies to taps would run down walls rather than up from underfloor.

I'm more tempted by a system with cylinder as I have also read some reviews that say that kerosene combi are not as good at producing HW as a gas fired.

Any thoughts please on system vs combi and also efficiency of kerosene combi for the supply of HW.

Thanks

[Harry Bloomfield, Esq.]

Bev submitted this idea :

I had a cylinder for stored water installed back in mid-80's (gas boiler) and have resisted suggestions to replace it with a combi twice since then. Combi boilers are just about OK for houses which are not much occupied, but they have great disadvantages for most people..

1. They waste water until it heats up.
2. Increased wear on the boiler, which has to fire everytime HW is needed.
3. They are more inefficient heating water.

Stored systems are more complicated and costly to install, plus take up extra space. Unused HW looses heat from the cylinder.

Heating engineers like replacing stored with combi systems because they get the value of the scrap copper.

[Brian Reay]

We stayed with a relative for a few days this week who has a combi, it was noticeable that if we were having a shower and someone turned on a hot tap in the kitchen, you got a ?cold surprise?. Conversely, we have two showers which are pump assisted and we can run both at the same time and you don?t notice if someone turns on a hot tap. We have a hot water storage tank.

True, the hot tank ?sits there? and is losing heat but, in practice, it is so well insulated it seems to get heated up and require very little energy to maintain it. It isn?t unknown for me to turn it off when going away for a day, come back, remember to turn the heating on but not the water heater, and not notice until residual hot water from before the short trip has been used- perhaps a day.

[AnthonyL]

We experience this to an extreme with our combi as the kitchen is quite a way from the boiler, though thankfully the shower is near the boiler.

What would suit us is a small tank above the kitchen warmed by the CH system and maybe a thermostatically controlled element. At a previous property a similar arrangement worked well from an open coal fired CH system. Do such systems exist?

[mechanic]

Not just at first start either - if you are using hot water in short bursts you get a slug of cold water followed by excessively hot each time you turn off and on again; ok if you're running a bath, wasteful otherwise. Avoid combi if you can.

[John Rumm]

There is nothing to stop you running a combi with a conventional Y plan system as well. So stored HW for baths etc, then on demand mains pressure potable water for kitchen / showers etc.

[John Rumm]

Why a smaller cylinder?

HW production with a combi really just comes down to the maximum instantaneous power. Smaller boilers (e.g. 24kW) will do a nice shower but will be slower filling a bath. A more powerful one (say 35kW) will do better all round and can cope with two showers at once (assuming the cold mains supply is up to it).

Have you considered a system boiler and an unvented cylinder? That gives the advantages of both at once - lots of stored hot water and mains pressure delivery, with no need for an additional header tank.

Guidance on choosing a system here:

Unvented systems here:

[Dave Plowman (News)]

IMM is alive and well. ;-) Why would you heat up and keep hot a cylinder just for baths?

[Tim Lamb]

Also consider a thermal store. Avoids the issues of a combi but retains flow rates through small bore piping.

[Bev]

Because the one that is in the cylinder cupboard is 263l and is a low pressure system fed from loft header tank and it takes up far too much space. My thoughts were to go with a pressurised unvented cylinder (which, I think, is what you describe below---) in the loft with the relief being discharged outside into the drain. The discharge route is easily achievable.

Thanks for the links (and to others for their responses too)

[Harry Bloomfield, Esq.]

I understand you can buy an electrically heated local system (to where it is consumed), which stores the combi heated water keeping it hot. Thus you get hot water quickly, without the usual delay of a combi.

[NY]

At our old house, we had a Grant combi oil boiler, mains-pressure fed. And you could run the hot tap into the bath at full flow (same flow rate as cold tape, since it was mains-fed) and the water was red hot. So it definitely did not take a long time to fill a bath.

At our present house there is a gas-fired stored-HW system. That takes

*ages* to run a bath, because the HW is fed from a header tank so there's only about 10 feet head of water - and the idiots who installed the HW/CH system used 15 mm pipe (the same as for the cold water) whereas normally you should use 22 mm for hot to give a decent flow rate to compensate for the much reduced head of water compared with the cold.

When this boiler eventually needs replacing, we might go for a combi and abandon the header tank and the hot water cylinder. Or at the very least get the HW cylinder replaced with one that is rate for mains water pressure and abandon the header tank.

[NY]

The delay before you get hot water is a function of two things: how quickly a combi starts to heat cold water to make hot water, and the length of the pipe runs between the boiler or the cylinder and the tap.

Our present house has a hot water cylinder, so hot water is always available immediately from there. But... it has *very* long pipe runs, so it can take several minutes for the water to run warm, though that is at a very low flow rate (compared with the cold water) because the cylinder is fed from a head tank rather than mains and the idiots who installed the system (long before we bought the house) used 15 mm piping for the HW (instead of 22 mm for header-tank systems).

[Tim+]

You can get horizontal unvented cylinders if lack of headroom is limiting your choice of cylinder capacity. No idea how they compare price wise but I would expect them to be more expensive just because demand is lower. Still, it might give you more options.

Eg.

Tim

[Bev]

Thanks but headroom is not a problem.

[Tim Lamb]

You will miss the waste heat in your now spacious airing cupboard:-)

[The Natural Philosopher]

The one thing I have noticed is that oil boilers have never failed to provide decent hot water.

I'd put a mains pressure tank in the loft - then you have all the advantages of the combi - mains pressure hot water, no shower pumnps - and all the advantages of a system boiler with a tank - high flow rate of hot water.

If your water is hard, put in a water softener to protect it.

[Andrew]

15mm copper, or 15mm 'plastic' push-fit ?, which may have an even smaller internal cross section than 15mm copper.

A decent shower pump will fix that and also allow you to use 'modern' taps in the bathroom intended for continental use where they do not have tanks in the loft, but rely on mains pressure.

You might have these 'high pressure' taps now if you didn't fit them and that is part of the problem.

[stephenten]

One oft overlooked point is that domestic natural gas supplies to domestic houses is typically 66kW maximum unless you have a commercial supply instead, a bit like the electricity board's 100A 1 phase supply or 3 X 100A 3 phase supply.

Why is this important? Well my house has a 38kW vaillant combi. So that leaves me a 28kW supply margin for my two gas ovens and 6 ring hob.

It is possible to build combis with higher rated flat plate domestic hot water heat exchangers and burners. However you run the risk that trying to consume gas at a faster flow rate than the gas meter and governed can supply, the internal gas pipework pressure will drop below 22 mBar.

Supply Vs demand issues for LPG or oil tanks may not be a factor as presumably they can be designed to be able to supply what the peak demand of the house is going to be, i.e. the boiler going at full pelt if rated higher than a domestic gas boiler and with the cooker going too in the case of LPG.

I am sure an OFTEC or LPG accredited engineer can offer more comment.

So to the original op, you may find that a decently sized combi on a decently sized LPG/Oil tank may not have the same issue as encountered by natural gas combis.

[Bev]

I think the boss is looking to put a heater in it (just joking)