combi boilers again!!

[richman]

We converted a five bed house in London for students and the plumber put in a gas Vaillant turbomax plus 828 to serve a shower and a bath. All the old pipe work from the old conventional system is still insitu just blanked off. Frankly the combi is just poor; baths are just not worth having and the shower flow is disturbed so much by washing machines, dishwashers etc.I thought I could get away with it because we all know students dont wash but this lot do....

I have read what you all say about combis but think that they are useless for more than one old lady in a flat, I am afraid.

We now want to put in a an en suite shower room on the second floor, exactly where the old airing cupboard/immersion heater was. The boiler is on the ground floor in the kitchen and the existing bathroom is on first floor.

I want to put a megaflow or similar unvented storage with an upgraded new boiler.

The vaillant is nearly new and I wonder if some clever internal rearrangement will turn it into a sealed system boiler or should I just bite the bullet and buy a new proper one.

What do you reckon?

[John Rumm]

There is nothing to stop you using the combi just like a coventional boiler with water heating driven (via zone valves etc) from the CH side of it.

You could then either not use its DHW capability at all, or more probably use it either for one of the showers (where it should work well with no other demands on it to upset things), or perhaps things like just feeding the kitchen where the (probably) shorter distance to the taps would save having to run off so much before getting hot water.

[Andy Hall]

You could pension off the combi into just feeding part of the requirement such as washbasins or sinks perhaps - depending on the plumbing arrangements.

You can then use the CH part of the combi in the same way as any other boiler to provide heat for the cylinder's heating coil. This requires a diverter valve, thermostat and possibly a timer/programmer. No big deal and nothing lost.

Before you do this, however, do check the cold mains flow rate and pressure and make sure that they are adequate. Typically a combi has poor performance when its power rating is inadequate for the requirement - simple physics of heat transfer. However, if the water flow rate is also low, then a pressurised cylinder wouldn't help and you would need to use a system with roof supply tank.

[Christian McArdle]

You can (and probably should these days) run off cold fill only.

No problem whatsoever. A combi boiler is just as capable as any other of heating a water cylinder.

Christian.

[Doctor Drivel]

I'm afraid you and your "plumber", so not know much about combi's. :-) High flow models are available; I doubt if he has seen one. This sounds a typical cheapo landlords effort to me. Well firstly I would have properly assessed the DHW demand. How many people are using this house? Why a bath, when students only need a shower? A bath takes space and somthing else to maintain and keep clean.

There are high flowrate combi's around and I would have gone for one of those. Also I would have arranged the pipework properly. This sounds like

15mm cold water mains pipe snaking around the house and all appliances teed off this, of which the combi is one. The combi, ideally should have it own dedicated cold supply from the stop c*ck, with the cold supply to the shower taken off this combi supply just before the combi inlet. This way when a toilet is flushed it doesn't affect the combi DHW.

The same applies to an unvented cylinder regarding the pipework. A 22mm supply pipe back to the stop c*ck. Go for a low pressure heat bank rather than an unvented cylinder. No explosions, and students can do silly things when drunk, like pissing about with the cylinder controls.

The Vaillant can be turned into a system boiler. That is easy as that is what it is with a DHW section added.

CH demands? CH zoned? If not is it better to have separte timed zones? A mains pressure system: combi, unvented, heat bank, is dependent on the flow and pressure of the mains

Start at the beginning. Give me your DHW demands: number of people in the house, Number of showers, sinks, size of cold water mains, idea of flowrate, etc.

[John Rumm]

Probably better for the clothes / dishes as well these days (modern detergents and all that). Note though that even cold fill only can still knacker a shower in some cases. If you mains flow is marginal, turning on a cold tap will reduce the available flow to the combi causing a temperature rise in its output.

[Christian McArdle]

If it is that marginal, then a mains pressure cylinder/heat bank, would suffer the same effect as a combi, too.

Christian.

[Doctor Drivel]

Depends on how it is piped up. And if still a problem you get a proper combi shower mixer with integral pressure balancer, or fit an external one on the hot and cold to the shower.

The w/machine and dishwasher can be piped up to take cold only. In reality the amount of hot water taken from the DHW system is negligible, as most of the water taken is cold via the DHW dead-leg pipe. Bt the time hot comes through the appliance is near full of water anyway.

If you have a secondary circulation loop then a hot fill can save money in energy.

In this case, the OP would be better sorting out the pipework, getting the combi supply dedicated back to the stop c*ck and piped up as I suggested, get the appliances on cold fill, get rid of the bath, have a flow restrictor on the sink tap (available from BES), and put in another combi for the new ensuite. This is "probably" the most cost effective route for him. I await his DHW demands.

[Richard Conway]

I was wondering when a second combi would come into the discussion :)

[Doctor Drivel]

Exactly. More thought is generally given to unvented cylinder and heat bank installations. Far too many combi's are just slapped in teeing off the nearest cold mains pipe for its supply, giving them a poor reputation.

[Andy Hall]

Unnecessary expense and would take a very long time to recover the cost considering that dishwashers and detergents used in them are designed for cold fill and gradual heating, and washing machines, detergents and clothes are also mainly warm wash only these days.

In other words alter the requirements to meet the limitations of the combi. This was not what was being requested.

[Mr Fizzion]

My dishwasher has only a cold inlet. Certain proteins harden when "cooked", so you wouldn't want to bake them on. (egg for example).

Washing machines may be warm wash only these days, but unfortunately food is much more colourful than it used to be. So if saturday night's popadoms somehow managed to eject some lime pickle onto your pure white shirt, you're going to have problems if you wash it at 40C! That kind of stain needs a minimum of two washes (or some sort of non-chlorine based bleaching agent) at modern temperatures.

Mr F

[andrewpreece]

The Vaillant Turbomax 828 can deliver 11ltrs/min wth a temperature rise of

30C. It'll run a shower easily, and a bath in less than 10minutes.

As others have stated, perhaps your pipework is not very good. Test it by seeing what the cold bath tap will deliver at full whack, or even the kitchen cold tap. It should be considerably more than 11litres/min to give you some margin. My cold water taps can deliver 19litres/min. Even so, a pressure equalisation valve will help stop slugs of cold water coming through a shower in a house wih so many people in it.

Andy.

[Doctor Drivel]

It may be the best solution for him, as it is for many homes. Have you anything constructive to add?

[Doctor Drivel]

A secondary circulation pump can also save much money in reduced water waste. Also it is a great convenience to have hot water immediately at the taps in any water system.

Nope. Fit the combi properly. The same will apply to any water system; fit it properly. It is clear this combi was just slapped in. Boy do you have a strange logic. Must be Little Middle England logic.

[Richard Conway]

Not really, no

[Doctor Drivel]

Then no need to clutter the thread.

[Andy Hall]

A couple of litres? Are you serious?

Even if it were running perfectly with unlimited supply of water it would still be inadequate since this is an 11 litre/min boiler.

[Andy Hall]

In the winter, 11lpm would take at least 15 mins to fill a bath and would provide a marginal shower.

[Doctor Drivel]

??

That is one issue, the main one in this case. Another is priority of water. The system should be piped to ensure the shower has little, to no, interference from other draw-offs.