12 Litres Per Minute - OK for a combi?

[gerbils]

I would not expect any leaks on an 8 mm microbore system.

But achieving enough flow can often be a problem.

I have heard that before but I don't follow. I assume if the rads stay the same size we'd be OK? (They all get super hot currently but they are all small and single panel).

Our existing system is pumped via a grundfos 15-60 pressure pump if that helps.

 

[Agile]

If well designed and installed there should not be any problems.

The main difficulty is that if there is much dirt in the system it can form blockages in the 8mm pipes.

 

[gerbils]

If well designed and installed there should not be any problems.

The main difficulty is that if there is much dirt in the system it can form blockages in the 8mm pipes.

Thanks. We are having a magnaclean and filter installed, hopefully this helps flush it out. Our rads are currently ok so far as heat goes but the water is as black as oil.

 

[Agile]

If system water is black it needs to be flushed out and then best treated with a little citric acid.

 

[gerbils]

It's rather low for that size of combi really, probably wouldn't get the benefit of such a large input.

Ideally, dynamic flow pressure would be measured, not static, as it's not just HW output rather HW output with other outlets on the mains being used at the same time.

Hi @Madrab, so I took your advice and went back to the installer and requested a quote for the 30kw Vallient - they are saying to stick with 35.

The 30kw will power 20 radiators (we have 14) and max HW flow of 12L/min (which matches our mains flow).

The only benefit I see is if we ran 2 showers (rare) or maybe getting the water hot enough on a very cold day?

I suspect though we are simply being fleeced?

 

[Agile]

Combi boilers are sized on the water delivery.

You could always upgrade the water flow rate a little. And 12 li/min open pipe is more like 9 li/min out of a shower head.

You are the customer and you should be able to get what you want when clearly a 30 kW will be totally adequate. Although the difference in boiler price may not be much. But there could be further inefficiences when using a larger boiler than actually required.

There might be gas pipe size implications as the gas pipe to a larger boiler might need to be larger too.

 

[Madrab]

Hi @Madrab, so I took your advice and went back to the installer and requested a quote for the 30kw Vallient - they are saying to stick with 35.

The 30kw will power 20 radiators (we have 14) and max HW flow of 12L/min (which matches our mains flow).

The only benefit I see is if we ran 2 showers (rare) or maybe getting the water hot enough on a very cold day?

I suspect though we are simply being fleeced?

I think you need to be careful about any advice when it comes to the way that a specific boiler is being 'sold' to you, especially given the mains constraints that the system seems to be under. If you wish to take educated decisions about the best way forward, then you need to get as much un-biased information there is and then ultimately make the call, based on that information.

The long and the short of it is, unless it is a very large combi or a storage combi then it will only be able to deliver it's quoted figures (12L/Min @ 35deg rise). Please note that 35deg rise. That ultimately that means that if the incoming cold mains, in the winter, is say 5deg then the HW output will be 5deg + 35deg, with this example ~ 40deg @ 12L/min. That then needs to be coupled with the output of any given outlet (how much does the shower flow). Most system can have some diversity applied to it but when I look as a specific clients requirements then that has to be considered @ potential max use requirements, some showers can use all that 12L/Min without breaking a sweat. Most people use a shower running at 38deg as a min too. As suggested, with a combi, 1 HW tap at a time will always be the norm and a lot of families do live with that but is that what is wanted?

With a mains supply @ 12L/Min and again - I cant stress this enough - we still don't know if this is a dynamic flow or just one single full flow outlet, then I would consider your system to potentially be hamstrung before it even gets out the blocks. Not being onsite and being able to measure certain systems then obviously it's all just best guess. Though if I came across that system, with the potential demand there may be and with the presumption that the mains flow is not 12L/min dynamic, that last thing I would be recommending is a combi without some serious re-designing of all the outlets to water saving/restricted flow items or warning that no more than 1 HW tap will be able to be used at a time.

 

[gerbils]

I think you need to be careful about any advice when it comes to the way that a specific boiler is being 'sold' to you, especially given the mains constraints that the system seems to be under. If you wish to take educated decisions about the best way forward, then you need to get as much un-biased information there is and then ultimately make the call, based on that information.

Thanks, yes that's where this forum and good people such as yourself are proving invaluable. It's genuinely appreciated.

With a mains supply @ 12L/Min and again - I cant stress this enough - we still don't know if this is a dynamic flow or just one single full flow outlet, then I would consider your system to potentially be hamstrung before it even gets out the blocks.

That is static as measured by one of the local plumbers, it's written clearly on the quote (the one who suggested a 40kw combi or an unvented cylinder which seems odd from what I have since learned on here). There are no dynamic readings on there just 12.2L/min and 1.8 Bar.

This appears to be perfectly in line with my unscientific test of timing the outside tap which I measure at 12L/min and the Kitchen tap is 10L/min.

If I open the outside tap fully and also run the kitchen tap, this goes down to 5.5 L/min - is that closer to a dynamic flow rate? If so I assume that's bad...

 

[Madrab]

If I open the outside tap fully and also run the kitchen tap, this goes down to 5.5 L/min - is that closer to a dynamic flow rate? If so I assume that's bad

Oh dear .... Yes, unfortunately that is closer to a dynamic flow figure and that's just 2 outlets open, so that really would be like one decent shower running with nothing else on. The long and the short of that is when the combi's running the shower, then that cold mains dynamic figure has to supply both hot and cold - can you see where this is going?

There was a suggestion earlier about finding out what the district main can deliver, make a call to the water transporter and say you're having a problem with the cold mains and can then come out and test the mains in the street please and let you know what mains pressure and flow they find, most will do this FOC. That then lets you know what may be achievable, if a mains upgrade was considered. This may give you other options. The alternative, to give a reasonable hot and cold service, would be to stick with stored, which is no bad thing, if designed and implemented properly.

Again it really comes down to what you want or are happy to live with, what is it you are looking for from your system?

 

[gerbils]

Oh dear .... Yes, unfortunately that is closer to a dynamic flow figure and that's just 2 outlets open, so that really would be like one decent shower running with nothing else on. The long and the short of that is when the combi's running the shower, then that cold mains dynamic figure has to supply both hot and cold - can you see where this is going?

There was a suggestion earlier about finding out what the district main can deliver, make a call to the water transporter and say you're having a problem with the cold mains and can then come out and test the mains in the street please and let you know what mains pressure and flow they find, most will do this FOC. That then lets you know what may be achievable, if a mains upgrade was considered. This may give you other options. The alternative, to give a reasonable hot and cold service, would be to stick with stored, which is no bad thing, if designed and implemented properly.

Again it really comes down to what you want or are happy to live with, what is it you are looking for from your system?

I will call the water board up.

Our needs are simple. All we want is a more efficient boiler and a better shower experience for the kids.

I like the idea of never running out of hot water as it happens every weekend here when 2 or 3 people want a bath, hence a combi and the space saving would be a small bonus, but it wouldn't be the end of the world if we did a straight swap + new bigger cylinder and some kind of shower pump; or a couple of digital showers which I am being told are even better.

The latter I fear would be more costly but haven't had it priced up. If we use one of the cheaper online outfits such as heatable or BOXT (undercutting local installers by at least £1k) they will only do a boiler install, not the other bits. Minor hassle though, ideally I just want the best system for the money.

Edit: Called them up, they are coming to test though said they want to test in the house too....

 

[MNW67]

Our needs are simple. All we want is a more efficient boiler and a better shower experience for the kids.

I have been in a very similar situation. 2.0 bar at the road and 12l/min at the kitchen tap. It is massively frustrating. Moving to a place with very good water pressure has made things so easy.

I would get a quote for another regular boiler and a new cylinder, for comparison. Some modern cylinders can reheat in the time it takes to have a bath.

If you are only wanting a modest increase in shower performance, have you thought about a power shower with an internal motor. Cheap and cheerful, but not stylish as they look like an electric shower. I would have been happy I think with one of those. Instead we fitted a massive shower pump which I never actually enjoyed.

 

[Johntheo5]

As above, a shower like the Triton AS2000XT (I think) has its own internal pump that takes HW from the cylinder and cold from the CW cistern and can deliver up to 14LPM.

Re dynamic flow&pressure, I would suggest to buy a pressure gauge that you can fit on the end of the outside tap (or kitchen tap) then open the other cold tap (or taps) and measure this via a bucket and a 1L container to give you the flowrate in LPM and note the pressure. But if, as you said in your first post that the standing (static?) pressure was only 1.8bar then you can forget about your combi but no harm in taking the readings yourself at different times during the day as you still need to fill/refill the CW cistern.

 

[Madrab]

I will call the water board up.

Our needs are simple. All we want is a more efficient boiler and a better shower experience for the kids.

I like the idea of never running out of hot water as it happens every weekend here when 2 or 3 people want a bath, hence a combi and the space saving would be a small bonus, but it wouldn't be the end of the world if we did a straight swap + new bigger cylinder and some kind of shower pump; or a couple of digital showers which I am being told are even better.

The latter I fear would be more costly but haven't had it priced up. If we use one of the cheaper online outfits such as heatable or BOXT (undercutting local installers by at least £1k) they will only do a boiler install, not the other bits. Minor hassle though, ideally I just want the best system for the money.

Edit: Called them up, they are coming to test though said they want to test in the house too....

You really need a professional resource that will happily go into an end to end design for your requirements and give you all the available options that could deliver what it is you want or need. To go down that on a forum would take a long time and posts that would be war and peace unfortunately.

Digital showers are nice but pretty expensive, as is a whole house pump and power showers would be an alternative as suggested - baths on a combi take a time to fill but again if you were willing to wait the 20 odd minutes for a decent depth then it could be worked with. With your requirements then the ideal would be a cold water cistern and a good sized cylinder on Hot Water Priority system. That can recharge the cylinder in jig time. The cylinder is then pumped on a whole house pump or as suggested, 2 power showers. Showers would need to be controlled as far as time is concerned of course and that would be driven by the size of the HW cylinder to ensure everyone could get one. Baths would be the same but again as suggested, re-heat time would be quick.

Price, of course, is a major consideration and budget would play a big part in deciding so that would need to be understood.

Fair enough to let the transporter in to run some tests, get the tea and biscuits out and get friendly and get as much detailed information around flow and pressure - dynamic - as possible.

 

[gerbils]

I have been in a very similar situation. 2.0 bar at the road and 12l/min at the kitchen tap. It is massively frustrating. Moving to a place with very good water pressure has made things so easy.

I would get a quote for another regular boiler and a new cylinder, for comparison. Some modern cylinders can reheat in the time it takes to have a bath.

If you are only wanting a modest increase in shower performance, have you thought about a power shower with an internal motor. Cheap and cheerful, but not stylish as they look like an electric shower. I would have been happy I think with one of those. Instead we fitted a massive shower pump which I never actually enjoyed.

Why didn't you enjoy the shower pump?

Yes a power shower would be an option - they seem pretty cheap £250-£400, not sure how much they cost to install though.

That said, they seem to deliver around 12L/Min which is what our mains is and what a combi would assumedly give, but obviously without the issue of 2 sources using the water at once.

 

[Johntheo5]

These Tritons or whatever take stored water so the mains doesn't have to keep up with the demand.
What are the capacities of the CW cistern and the HW cylinder?.