Dual Combi's ??

[marvin]

Hi,

I'm having some work done, extension for a utility room and swapping my old conventional boiler for a combi. I've been reading through posts and understand that combi's can't do more than 1 thing at a time.
To improve this would it not be possible to have 2 combi's installed, 1 controlling the heating and hot water downstairs and 1 doing the same upstairs ?

This if possible would get round the showering and washing machine on rinse cycle problem

I really want to get rid of the hot water tank from the bathroom to free up space so a combi seems the ideal solution.

What are your thoughts on this and is there anything else I can look other than a combi that still frees up the space in the bathroom ?

Cheers.

Marvin.

 

[AdamW]

Perhaps instead of having two combis you should have one combi for CH and washing machine type tasks, and a different type of water heater dedicated to your shower?

Imagine using two combis to power a single shower. That would be an amazing shower.

 

[taydo]

how big is your attic could you not put a hot and cold tank up there and heat the tank on a trv, supply the shower with a pump.
or opt for a sealed system boiler not combi with the hot cylinder in the loft?
the way your looking at the moment will give you very expensive running costs for no apparent reason unless of course you live in a mansion

 

[breezer]

are we not forgetting something.

doesnt matter how many combis you have, you still only have one incoming water supply, so its flow / pressure will be divided by number of combis on (same as it would if only one combi, and more than one tap on )

 

[goodtohelp]

A combi can cope with hot water and CH in the average household, it wouldn't cope in a busy bed & breakfast with more than 2 bathrooms/showers running at the same time. Then you would need 2 combi, each running its own independent part of the system. I would go for a combi any day

 

[AdamW]

are we not forgetting something.

doesnt matter how many combis you have, you still only have one incoming water supply, so its flow / pressure will be divided by number of combis on (same as it would if only one combi, and more than one tap on )

Ah, yes, but surely the combi can only heat as much water as its rating allows... now for a temperature rise of 40 degrees, specific heat capacity of water being 4.2 kj/kgC...

eg one 30kW combi would only be able to heat 10.7 litres of water per minute from 15 degrees to 55 degrees, but if you had two then you could combine the two flows and have 21.4 litres per minute at the same temperature, at the expense of using twice as much gas.

However, in this case one combi would be given over to the shower, producing its hot water whilst the washing machine runs off the second combi. There is (hopefully!) enough mains water to go round (usually around 20+ litres per minute isn't it?) for two.

I would go with a sealed system personally, you can get massive amounts of hot water stored up in them. I am not a plumber but I am sure that when I stay in hotels they must use these as the shower pressure is great and I can't imagine having gigantic combis!

 

[DP]

By far the best option is to have an unvented system coupled to the combi with combi supplying hot water to kitchen

 

[ollski]

A combi can cope with hot water and CH in the average household, it wouldn't cope in a busy bed & breakfast with more than 2 bathrooms/showers running at the same time. Then you would need 2 combi, each running its own independent part of the system. I would go for a combi any day

The problem for me is not so much that they can't supply the hot water, although in cold conditions with a low temp incoming main this would quickly get annoying, but the fact they are hot water priority. In a house with 3 girls it would always be freezing.

 

[DP]

Last time I looked as possibilty of linking two outlets from a combi, the makers tecky department advised it would not work. The installation finished with two Baxi 105e and 300l UV HW cylinder. The only time the boilers are not heating the rads is when they are supplying HW to kitchen/ utility room

 

[AdamW]

The problem for me is not so much that they can't supply the hot water, although in cold conditions with a low temp incoming main this would quickly get annoying, but the fact they are hot water priority. In a house with 3 girls it would always be freezing.

Sounds to me like DPs suggestion of a combi feeding an unvented system in your loft is ideal for your situation. Even with 3 girls you won't have the shower going ALL the time (never figured out what they're doing in there personally). The unvented system will act as a buffer.

Perhaps a plumber can answer this question: if you set the unvented system to refill relatively slowly would the combi then run itself at 50% to the hot water and 50% to the CH simultaneously, or does a combi only do one thing at once? This is the key to whether or not your finished system does the trick.

 

[oilman]

goodtohelp wrote

I would go for a combi any day

I agree, heating and hot water might be wanted by the end user, but don't forget the needs of service engineers to have good employment prospects. After all if we got rid of combies, there'd be much less work about.

AdamW wrote

Sounds to me like DPs suggestion of a combi feeding an unvented system in your loft.......

In fairness to DP he did not mention the loft. It is difficult enough getting people to have their unvented systems checked anyway; if they are in the loft they'd never get done. There is also the expense of living with an unvented cylinder. Vented cylinders require very little maintenance, I make part of my income fixing unvented cylinders.

....if you set the unvented system to refill relatively slowly .......

This can't be done. The unvented cylinder has to have a water supply at a defined rate, and one of its functions is to provide the cooling water in the event of an overheat situation.

 

[AdamW]

OK, DP didn't mention the loft. I made an assumption there because Marvin said he wanted to get rid of the hot water tank in the bathroom. Whilst this is a fairly safe assumption, it is an assumption nonetheless.

OK oilman, if you want to have the gold plated hot water/ch system that will let you have high pressure, silent showers, lots of them in quick succession with 2 or 3 running at once and a decent CH performance, what do you get? (other than a huge bill when the plumber finishes it!) Oh, and it has to be cheap to run as well.

I just wonder as I love the pressure but hate the appearance of a power shower (although don't mind hidden pumps provided they are quiet) but in a couple of years will have a house with 2 or 3 showers to be running at once.

 

[oilman]

You can't have quality, speed and low cost at the same time as they are conflicting demands. If you want cheap to run, try a bucket on a stand with holes in the bottom, if you want bees knees, then here is a possible solution, but not the only one. If you have a power shower it will need servicing, and the same goes for an unvented cylinder. Both will cause you to use more water than you would for a bath (so much for concern for the environment and saving water by taking showers) so they will be more expensive than the bucket solution. Between these extremes there are solutions which vary in installation and running costs and performance.

 

[AdamW]

Tried the bucket, pressure wasn't too great.

It is surprising just how much water any power shower uses oil. My bath waste was running slow once and because of my work load it got to the stage where I would shower in the morning and it would only just be empty the next morning. When I had a shower the next morning the bath got about as full as it would when taking a bath!

So, these Dualstreams probably require your own reservoir. No matter, if you have one of them you can generate your own hydroelectric!

I like the statement "Wholesome water to all cold taps". That's one way to put "no tank"!

I definitely want an unvented system now!