Bloke tells me “There’s no combi Boiler in the world that’ll let you run the bath AND a shower.

Rosa_Damascena

Jeepers_Creepers said:

Hi Brian. To sum up pretty much what's been said above...

1) There are some combis powerful enough to supply two hot outlets at the same time, and they would typically need to be 38kW or over, to supply DHW at 16 litre per minute or more. That will provide for two showers, but bear in mind that showers don't just work off hot water, but blend with cold to provide a stabilised temp. What this means in effect is, your incoming mains water flow rate would really need to be very decent, around 20 lpm or more. And also at a reliable pressure - at least 2 bar, ideally 3+.

That is one heck of a boiler! When planning for a 3-bath / shower home last year, I was told that a 24kW would be enough (standard rather than a combi, though) for the future build, and that is what I had installed. Now I have visions of a wet UFH system on the g/f, I am told that I may need to go as high as 35kW. So I don't really get why the OP would need such a powerful boiler if they have 3 bathrooms running at the same time unless the advice is specific to combis.

Doozergirl

Rosa_Damascena said:

Jeepers_Creepers said:

Hi Brian. To sum up pretty much what's been said above...

1) There are some combis powerful enough to supply two hot outlets at the same time, and they would typically need to be 38kW or over, to supply DHW at 16 litre per minute or more. That will provide for two showers, but bear in mind that showers don't just work off hot water, but blend with cold to provide a stabilised temp. What this means in effect is, your incoming mains water flow rate would really need to be very decent, around 20 lpm or more. And also at a reliable pressure - at least 2 bar, ideally 3+.

That is one heck of a boiler! When planning for a 3-bath / shower home last year, I was told that a 24kW would be enough (standard rather than a combi, though) for the future build, and that is what I had installed. Now I have visions of a wet UFH system on the g/f, I am told that I may need to go as high as 35kW. So I don't really get why the OP would need such a powerful boiler if they have 3 bathrooms running at the same time unless the advice is specific to combis.

It's specific to combis.  Because it's heating water as it comes directly from the mains and providing it straight to the shower, it uses much more energy.  A standard boiler heats less water at a time, because is has more time - it stores it.   That's why it can 'run out' of hot water if the tank isn't large enough for the household. 

It's hot water that uses more energy than heating, so I'm not sure why you would need a bigger boiler for UFH that runs at a lower temp than radiators... 

Jeepers_Creepers

Rosa_Damascena said:

Jeepers_Creepers said:

Hi Brian. To sum up pretty much what's been said above...

1) There are some combis powerful enough to supply two hot outlets at the same time, and they would typically need to be 38kW or over, to supply DHW at 16 litre per minute or more. That will provide for two showers, but bear in mind that showers don't just work off hot water, but blend with cold to provide a stabilised temp. What this means in effect is, your incoming mains water flow rate would really need to be very decent, around 20 lpm or more. And also at a reliable pressure - at least 2 bar, ideally 3+.

That is one heck of a boiler! When planning for a 3-bath / shower home last year, I was told that a 24kW would be enough (standard rather than a combi, though) for the future build, and that is what I had installed. Now I have visions of a wet UFH system on the g/f, I am told that I may need to go as high as 35kW. So I don't really get why the OP would need such a powerful boiler if they have 3 bathrooms running at the same time unless the advice is specific to combis.

As Doozer says, that huge power rating is specific to combis, and that power is required only for the DHW side to 'instantly' heat the hot domestic water; the more powerful the boiler, the more hot water that can come out of your tap, or two. 

A typical 3-bed house will only need around, ooh, 15kW tops to heat it up and keep it warm, so it may seem that 38+kW is waaaay too powerful for this, but it isn't. What happens is, the boiler 'modulates' it's output so that it only provides what is actually required. (Like turning a gas hob ring up and down to boiler/simmer your tatties). What you would find with such a boiler is that it comes on fairly strongly to heat your rads, but then quickly backs down to a virtual tickover once the rads have got to temp; it doesn't pump out 38kW's worth all the time!

Likewise with a hot tap - if you only turn on one tap and that one is half open, the boiler will only fire up part way. Opens two taps/showers full, and the boiler will take off. The actual water temp exiting the boiler will remain fairly constant in both scenarios as the boiler constantly monitors and regulates this, turning its burner up and down as required. 

They are pretty cool. Espec when they go wrong... 

You could install either a 24kW or a 38kW combi boiler in a 2-bed house, and they'd heat the house in a very similar manner. It's only when you turn on a hot tap, bath or shower that you'll notice the difference - the much increased hot water flow from the latter. I have a 30kW combi in our 3/4 bed house, and it gives a truly great shower - but only one at a time. 

Rosa_Damascena

Doozergirl said:

Rosa_Damascena said:

Jeepers_Creepers said:

Hi Brian. To sum up pretty much what's been said above...

1) There are some combis powerful enough to supply two hot outlets at the same time, and they would typically need to be 38kW or over, to supply DHW at 16 litre per minute or more. That will provide for two showers, but bear in mind that showers don't just work off hot water, but blend with cold to provide a stabilised temp. What this means in effect is, your incoming mains water flow rate would really need to be very decent, around 20 lpm or more. And also at a reliable pressure - at least 2 bar, ideally 3+.

That is one heck of a boiler! When planning for a 3-bath / shower home last year, I was told that a 24kW would be enough (standard rather than a combi, though) for the future build, and that is what I had installed. Now I have visions of a wet UFH system on the g/f, I am told that I may need to go as high as 35kW. So I don't really get why the OP would need such a powerful boiler if they have 3 bathrooms running at the same time unless the advice is specific to combis.

It's specific to combis.  Because it's heating water as it comes directly from the mains and providing it straight to the shower, it uses much more energy.  A standard boiler heats less water at a time, because is has more time - it stores it.   That's why it can 'run out' of hot water if the tank isn't large enough for the household. 

It's hot water that uses more energy than heating, so I'm not sure why you would need a bigger boiler for UFH that runs at a lower temp than radiators... 

Don't really understand this either but will make sure I will get to the bottom of it. The boiler will need to be relocated and I would rather not replace it quite so soon.

Doozergirl

Rosa_Damascena said:

Doozergirl said:

Rosa_Damascena said:

Jeepers_Creepers said:

Hi Brian. To sum up pretty much what's been said above...

1) There are some combis powerful enough to supply two hot outlets at the same time, and they would typically need to be 38kW or over, to supply DHW at 16 litre per minute or more. That will provide for two showers, but bear in mind that showers don't just work off hot water, but blend with cold to provide a stabilised temp. What this means in effect is, your incoming mains water flow rate would really need to be very decent, around 20 lpm or more. And also at a reliable pressure - at least 2 bar, ideally 3+.

That is one heck of a boiler! When planning for a 3-bath / shower home last year, I was told that a 24kW would be enough (standard rather than a combi, though) for the future build, and that is what I had installed. Now I have visions of a wet UFH system on the g/f, I am told that I may need to go as high as 35kW. So I don't really get why the OP would need such a powerful boiler if they have 3 bathrooms running at the same time unless the advice is specific to combis.

It's specific to combis.  Because it's heating water as it comes directly from the mains and providing it straight to the shower, it uses much more energy.  A standard boiler heats less water at a time, because is has more time - it stores it.   That's why it can 'run out' of hot water if the tank isn't large enough for the household. 

It's hot water that uses more energy than heating, so I'm not sure why you would need a bigger boiler for UFH that runs at a lower temp than radiators... 

Don't really understand this either but will make sure I will get to the bottom of it. The boiler will need to be relocated and I would rather not replace it quite so soon.

Unless you're extending, whilst the method of heating it changes, the actual heating demand of your house doesn't.  The mode of heating it doesn't require any more energy.   Underfloor heating is also a constant, low heat, not high heat on instant demand.  

(The existing boiler would also need to conform to current regulations in order to be moved, btw, or it's headed for the tip.)

Rosa_Damascena

Doozergirl said:

Rosa_Damascena said:

Doozergirl said:

Rosa_Damascena said:

Jeepers_Creepers said:

Hi Brian. To sum up pretty much what's been said above...

1) There are some combis powerful enough to supply two hot outlets at the same time, and they would typically need to be 38kW or over, to supply DHW at 16 litre per minute or more. That will provide for two showers, but bear in mind that showers don't just work off hot water, but blend with cold to provide a stabilised temp. What this means in effect is, your incoming mains water flow rate would really need to be very decent, around 20 lpm or more. And also at a reliable pressure - at least 2 bar, ideally 3+.

That is one heck of a boiler! When planning for a 3-bath / shower home last year, I was told that a 24kW would be enough (standard rather than a combi, though) for the future build, and that is what I had installed. Now I have visions of a wet UFH system on the g/f, I am told that I may need to go as high as 35kW. So I don't really get why the OP would need such a powerful boiler if they have 3 bathrooms running at the same time unless the advice is specific to combis.

It's specific to combis.  Because it's heating water as it comes directly from the mains and providing it straight to the shower, it uses much more energy.  A standard boiler heats less water at a time, because is has more time - it stores it.   That's why it can 'run out' of hot water if the tank isn't large enough for the household. 

It's hot water that uses more energy than heating, so I'm not sure why you would need a bigger boiler for UFH that runs at a lower temp than radiators... 

Don't really understand this either but will make sure I will get to the bottom of it. The boiler will need to be relocated and I would rather not replace it quite so soon.

Unless you're extending, whilst the method of heating it changes, the actual heating demand of your house doesn't.  The mode of heating it doesn't require any more energy.   Underfloor heating is also a constant, low heat, not high heat on instant demand.  

(The existing boiler would also need to conform to current regulations in order to be moved, btw, or it's headed for the tip.)

Yes, a fairly sizable extension is in the planning. The existing boiler was installed last September, so unless the regs change between then and next year it should comply.

Snowdrops_Bluebells

Family of 6 here...

In our last house we had an (300l) unvented how water cylinder installed (which took about 30mins to heat up if all the hot water had been used) We only ran showers and you couldn’t tell by the temperature but the flow would drop if more than one was being used at once.

Current property has an 38kw combi and a 24kw assistant (lpg if it makes any difference) and it is the same problem- flow drops (this system is inherited, we are looking at options to alter) Our plumber seems to think we need an industrial boiler, which drained the colour from my husband’s face... Although we just tend to stagger showers to avoid the flow drop, and bang on the door really loud if the teens are taking too long! 

Replacing the shower you have with an electric one would probably be the most economic way to achieve what you want.

ComicGeek

Snowdrops_Bluebells said:

Replacing the shower you have with an electric one would probably be the most economic way to achieve what you want.

Not really, unless the mains water pressure is really good, as running both the electric shower and combi fed shower would still result in a mains water pressure drop and limit flow rate. It solves the problem with heating the water, but not delivering adequate water pressure to all outlets - that needs either an accumulator or cold water booster system.

grumbler

ComicGeek said:

Not really, unless the mains water pressure is really good, as running both the electric shower and combi fed shower would still result in a mains water pressure drop and limit flow rate. It solves the problem with heating the water, but not delivering adequate water pressure to all outlets - that needs either an accumulator or cold water booster system.

What matters is the max. flow rate, not pressure. Static pressure can be high, but can drop when the flow rate rises.

Also, the flow rate for electric showers is pretty small - just a little more than 10L/min for the most powerful ones

grumbler

Snowdrops_Bluebells said:

...it is the same problem- flow drops... Our plumber seems to think we need an industrial boiler,

This suggestion makes no sense to me. An industrial boiler can have higher power, but cannot increase the flow rate.