Combi or Unvented for a 6 bed house?

Go unvented there's to much demand for a combi.

 

Agree.

With a combi, you'd not only need a huge beast of a machine, almost certainly requiring an uprated gas supply pipe from your meter, but your cold mains water supply would also need to be able to supply your two showers, and you may well find that this is borderline.

 

Totally unvented, house is far to big for a combi even a storage model. Look at using a system boiler in conjunction with it. Yours is going to be bordering on the commercial/industrial sized boiler. Probably about 38Kw

 

To answer you fully we would need to know your mains water pressure and flow rate.
But yes, probably too much even for a big combi.
Consider though a storage combi, Veissman do one with a 130 litre store, quality bit of kit although on the expensive side. If you do go for one you'll need to get it fitted by a Veissmann registered gsr but then you'll get access to a long warranty.

 

Valliant do a 37kw system boiler EcoTEC plus 637.

 

Firstly, a mains pressure water system depends on the supply coming down that one water pipe from the street. That is a combi or an unvented cylinder.

You know your requirements. You say only ever two showers would be on at the same time. A big combi can cope for sure. Look at ATAG, etc. The advantage of a big combi is that it never runs out of hot water. Say you get a 20 litres/min non-stored water combi, that will give two showers approx 10 litres each of hot water, then mixed with cold, will be a higher flow again. Fine. That is 600 litres of hot water in 30 minutes delivered. That means you will need at least a 500 litre unvented cylinder, which is BIG and expensive and heavy with heat standing losses. They also more complex piping with zone valves and the likes. With a big combi two showers can be had and showers after with no waiting for reheating of a cylinder. Also an unvented cylinder costs for an annual service.another disadvantage. Unvented cylinders also require large overflow pipes. They also hold pressurised water A combi will do your house in heating unless it is really massive.

That is briefly the pros and the cons. I would be inclined to lean towards a big non-storage combi.

 

A storage combi has an unvented cylinder inside.

 

Fit two smaller combi's.

Seen a few big houses with this setup.

 

As an alternative, you could fit a:

1. Worcester Bosch Greenspring instantaneous water heater delivering 21 litres/min. It is 50kW. Cost about £1,300.
2. Fit a small, cheap, system boiler doing the CH - size it up.

You are then dividing and ruling with each independent system being simple. Also if the gas meter rate is being exceeded, have a flow switch on the cold feed to the Greenspring which switches out the system boiler (via the boilers's stat circuit) when DHW is drawn off. Then you have a simple priority system and the 6 cu metres/hr (62kW) gas meter limit is not exceeded. The meter diversity factor calculation may put you within the meter's gas limit. If they are side by side, size the gas pipe for the Greenspring and tee off for the system boiler. The two will not be on at the same time.

Worth pricing up.

 

So have I. Good suggestion.

 

I thought that unvented cylinders - eg Megaflo - had a diaphragm inside which was air-pressurised on one side like an expansion vessel, so that the stored hot water was kept under compression so could supply a number of outlets simultaneously - until the pressure ran out, of course?

 

Hi DA
From what I know, the Megalo utilises a floating baffle that allows the air bubble and water to come into contact with each other (that is the weakness of the Megaflo because the air bubble dissipates over time and needs to be replenished) Fabdec have introduced a cylinder that overcomes this issue with their own patented device http://www.fabdec.com/productlist/Excelsior
As to the OP's original question, the answer will be there are many answers and even more possibilities.
To size an unvented cylinder properly you will need to calculate it's recovery time vs peak demand. For instance if two showers are being used first thing in the morning and the tank is sized to cope with that demand and then you have 4 showers in the evening then that's the peak demand.
it's sized not about the number of showers used simultaneously but on demand over a short period of time. how about when someone uses a shower (or two) and someone else draws a bath?
No matter what solution you go to you will always hit the issue of do I want to supply enough hot water to satisfy (totally) peak demand (largest vessel or combi of all) or do I allow a sensible calculation that can cope with 70-80% of peak demand and reduce the tank size accordingly.
Dont forget that a large cylinder (300 litre) normally comes with two immersion heaters, so for peak load you could supplement the recovery time from the indirect coil with two 3kw heaters running at the same time (very quick recovery time that is). Expensive in terms of energy but satisfies demand
It's all down to what is most important to you and what you want.
I hope this helps
Russell
PS just a plug (sorry) If you go down the unvented route and want to connect the PRV discharges to local drain, consider using my newly invented product hotun
It provides a visible point of discharge, an air break to drain and stops foul odours coming back up from the drain, all in one product - and it's less expensive to install than any other product combination currently available on the market
hotun - not just a tundish

 

Hi
I have installed systems that suits the sort of property described,
I installed a combi for kitchen and ensuite shower, and using the s plan system on the heating flow from the combi, install a stored hot water system to the other baths,
Customer was very happy with the performance.

Hope this idea may help?

Regards
Peter

 

The system we used for a slightly larger property is Megaflo Commercial and two system boilers one for hot water and the other for heating, if one boiler broke the system could sill work on the one. The owner has not complained to this day and happy with the system. One main factor was that we had to increase the bore of the in coming main for the system to work to full potential no biggie with the moling machine.

 

Took the words right out of my mouth so to speak.

 

A large combi boiler is probably cheaper to run, I do know it is cheaper to buy and install, the performance will not be there. The speck you gave was 6 Bed, 5 showers, 15 radiators. Property is over 3 floors(ground, 1 and 2) and 2 showers running at the same time, so in my opinion the unventilated is the way to go due to the spec you have given. I can throw some thing out there unvented can be run on solar as well. You need to work out cost and performance, I am not to sure if the combi will cope with the showers on different floors if both were running at the same time.

 

Commercial. Expensive!

 

Yes. It has stored water standing losses.