Living with a combi boiler

Rosa_Damascena

ComicGeek said:

Rosa_Damascena said:

coffeehound said:

Time for a crisis meeting with your plumber, Rosa.

I'm not panicking, I have confidence in my team.

@ComicGeek - the screed will be at least 65mm, the floor level is high across the house so we are looking up to the point of the first breezeblock visible, ie after the darker lower section you see in the salamander shot. The piping isn't yet complete as there are a number of new radiators to be fitted (my request and everyone thinks its a bizarre one) so there is more pipework to come and insulation will come when the pipework is completed, with the plan for screed to be done on Friday. So whilst I get your point about more screed taking longer to heat up there was no way round this really but I don't see that pipes over the UFH tubes is the cause for alarm your reaction suggested. If it is they can be disconnected and forced over the copper pipes - although I think that will mean the level of the screed will come even higher and interfere with the fit of my kitchen, and that really would be a problem.

Tell me what is wrong with this plan.

Eta: the rest of the copper piping can be sleeved and buried in the Celotex I am sure. 

The way round it is that you install thicker insulation to bring the floor level up without increasing the screed depth.

Fitting the copper pipework (with a plastic push fit bend) above the UFH pipework is a very basic error. Those are your key gas and water pipes, they just don't go there. IMO it shows a complete lack of understanding and planning.

You are attributing what you are calling an error to "they just don't go there" suggesting its not custom and practice rather than technical reasoning behind this deduction. What you are completely right about is my limited scope of knowledge in this area. Can you spell out the implications of this so I understand what the actual risk is? 

ComicGeek

But you're also limited by the 15mm hot and cold water pipes installed out/into the boiler. Large flow cylinders would have 22/28mm pipes for this exact reason.

You won't have 3 bar mains pressure if they want to install the mains boost pump - that's a serious red flag.

The 16 litre/min combi output is normally at a lower temp, so you'll probably use most of this in the 1st shower. 

You really need to get some straight answers on what they are giving you.

Jeepers_Creepers

Rosa_Damascena said:

Jeepers_Creepers said:

If it turns out that your cold mains supply is borderline or inadequate, and if upgrading your incoming supply pipe cannot fix it, there is an ultimate solution, and that is a mains booster accumulator. This is roughly the size of your hot cylinder (!), but can be fitted in your garage or outhouse or even a purpose-made box next to your house. What this does is to pump the mains water into a pressurised tank up to 3 bar or above, and then this will supply your house with as much flow and pressure as you could shake a stick at. These are made by a few folk such as Challis, Salamander, Grundfos, etc., and will sort out any lack-of-flow issue. Budget around £1k or so.

My garage? You are looking at it, complete with Salamander

But if the water pressure and flow rate (>3bar and 24L/min respectively) are sufficient I'm happy that I'll get a 12L/min shower in each if all 3 are running concurrently.

These are rough ballpark figures - perhaps the interweb has proper calculated sums? - but say your 40kW combi does deliver 16 lpm at around 60oC, by the time that's blended with, ooh, ~8 lpm of cold (around 8oC in Summer, perhaps 5 oC in Winter?) to provide a ~40oC temp (a nice hot shower!), then in theory you should have around 24lpm in total available.

For a single shower at full chat, that is a soaking! For 2 showers, it's a theoretical 12lpm each which is a perfectly good and pleasant wetting, but only around 8lpm each if all three are running, which is nothing special but still a lot better than an electric shower.

These are all theoretical and, I'd suggest, the best possible scenario. There are plenty of reasons why it might not be that good.

Seriously, tho', how often would you have 3 showers running at the same time?! A more realistic issue is if someone flushes a loo during a shower - this will grab a significant amount of cold water, so your shower's thermostat will knock down the hot supply to match so the output temp remains constant.

This best possible scenario is also dependent on your having this solid, powerful mains delivery. If that is compromised or borderline, then the overall performance will be as well.

I cannot see any plumber suggesting a 40kW combi, tho', without being confident that the mains supply is up to the task - it would be a fundamental mistake if they got this wrong.

That Salamander remains a mystery, tho', and I think we are all curious about it :-)

Rosa_Damascena

Jeepers_Creepers said:

Rosa_Damascena said:

Jeepers_Creepers said:

If it turns out that your cold mains supply is borderline or inadequate, and if upgrading your incoming supply pipe cannot fix it, there is an ultimate solution, and that is a mains booster accumulator. This is roughly the size of your hot cylinder (!), but can be fitted in your garage or outhouse or even a purpose-made box next to your house. What this does is to pump the mains water into a pressurised tank up to 3 bar or above, and then this will supply your house with as much flow and pressure as you could shake a stick at. These are made by a few folk such as Challis, Salamander, Grundfos, etc., and will sort out any lack-of-flow issue. Budget around £1k or so.

My garage? You are looking at it, complete with Salamander

But if the water pressure and flow rate (>3bar and 24L/min respectively) are sufficient I'm happy that I'll get a 12L/min shower in each if all 3 are running concurrently.

These are rough ballpark figures - perhaps the interweb has proper calculated sums? - but say your 40kW combi does deliver 16 lpm at around 60oC, by the time that's blended with, ooh, ~8 lpm of cold (around 8oC in Summer, perhaps 5 oC in Winter?) to provide a ~40oC temp (a nice hot shower!), then in theory you should have around 24lpm in total available.

For a single shower at full chat, that is a soaking! For 2 showers, it's a theoretical 12lpm each which is a perfectly good and pleasant wetting, but only around 8lpm each if all three are running, which is nothing special but still a lot better than an electric shower.

These are all theoretical and, I'd suggest, the best possible scenario. There are plenty of reasons why it might not be that good.

Seriously, tho', how often would you have 3 showers running at the same time?! A more realistic issue is if someone flushes a loo during a shower - this will grab a significant amount of cold water, so your shower's thermostat will knock down the hot supply to match so the output temp remains constant.

This best possible scenario is also dependent on your having this solid, powerful mains delivery. If that is compromised or borderline, then the overall performance will be as well.

I cannot see any plumber suggesting a 40kW combi, tho', without being confident that the mains supply is up to the task - it would be a fundamental mistake if they got this wrong.

That Salamander remains a mystery, tho', and I think we are all curious about it :-)

Seeing as I live on my own, I can confidently say the probability is nil. And a 40kW boiler for one person is crazy!

But that situation is likely to alter in the future, and it is for that reason I wanted to know about whether the immersion heater and HW tank could be left as a contingency.

ComicGeek

Rosa_Damascena said:

ComicGeek said:

Rosa_Damascena said:

coffeehound said:

Time for a crisis meeting with your plumber, Rosa.

I'm not panicking, I have confidence in my team.

@ComicGeek - the screed will be at least 65mm, the floor level is high across the house so we are looking up to the point of the first breezeblock visible, ie after the darker lower section you see in the salamander shot. The piping isn't yet complete as there are a number of new radiators to be fitted (my request and everyone thinks its a bizarre one) so there is more pipework to come and insulation will come when the pipework is completed, with the plan for screed to be done on Friday. So whilst I get your point about more screed taking longer to heat up there was no way round this really but I don't see that pipes over the UFH tubes is the cause for alarm your reaction suggested. If it is they can be disconnected and forced over the copper pipes - although I think that will mean the level of the screed will come even higher and interfere with the fit of my kitchen, and that really would be a problem.

Tell me what is wrong with this plan.

Eta: the rest of the copper piping can be sleeved and buried in the Celotex I am sure. 

The way round it is that you install thicker insulation to bring the floor level up without increasing the screed depth.

Fitting the copper pipework (with a plastic push fit bend) above the UFH pipework is a very basic error. Those are your key gas and water pipes, they just don't go there. IMO it shows a complete lack of understanding and planning.

You are attributing what you are calling an error to "they just don't go there" suggesting its not custom and practice rather than technical reasoning behind this deduction. What you are completely right about is my limited scope of knowledge in this area. Can you spell out the implications of this so I understand what the actual risk is? 

Just a few below!

1) Increased screed depth to cover these higher pipes will mean lower surface temp (ie less heat output into room) or higher water flow temperature (ie higher gas consumption), as well as taking longer to warm up. It's 25mm minimum cover above a pipe with wrapping/insulation.

2) Most UFH manufacturers have a maximum screed depth and won't guarantee operation beyond this. I've done a couple of projects at 100-120mm screed but we had to use a special screed for this.  

3) The cold water pipe is now in the heated zone so will sit and get warm, in the ideal temp zone for legionella/bacteria issues. Any needed insulation around the pipe will increase the issue with screed cover. 

4) The push fit plastic elbows are going to be covered under the screed, not a good idea. Why hasn't that been continued in copper, or all installed below screed as a flexible plastic pipe?

5) There's nothing supporting the pipework over the UFH loops, so is likely to bend slightly once the screed is poured, and really test those plastic elbows.

6) This is a gas pipe and needs to be treated with respect. It is more likely to be damaged during screening works in its current position, why take the risk? Just cut into the insulation, lay it in with proper protection, and then sit the UFH over the top. Probably take an hour or so to sort out now, plenty of time ahead of the screed pour on Friday.


My guess is that they installed the UFH pipework before they did the pipework from the boiler, and then just took the easy route after realising their mistake. 

Rosa_Damascena

^^^^ That's what I like, solutions! Thank you 

fezster

If your incoming mains flow rate is above 12 l/min, then that pump is redundant. It simply cannot be used.

If its below 12l/min, you'll be able to run one shower, and it'll be just "adequate".

Even if your incoming main is >25l/min, the combi can only supply 16l/min hot. Combi flow rates are generally quoted at a 35 degree temperature rise. In summer, this may be sufficient to run 2 showers (16l/min hot mixed with 8l/min cold) to provide 2x adequate showers at a comfortable 40 degrees. However, in winter, when the incoming cold is 5 degrees, you'll manage a pathetic 8l/min at 40 degrees to each shower.

Be interesting to hear your teams thoughts on how they intend to achieve 3 good showers with this setup.

On the point about keeping the gravity fed tanks. This is not possible as everything will be mains pressure driven. Generally, you either have one or the other, although in theory you could leave some showers or taps gravity fed and others mains fed, but it's added complication with little benefit. If you wanted mains pressure to multiple outlets, an unvented cylinder would have been a wiser choice. Dependent on sufficient pressure / flow rates. 

Jeepers_Creepers

Rosa_Damascena said:

Seeing as I live on my own, I can confidently say the probability is nil. And a 40kW boiler for one person is crazy!

But that situation is likely to alter in the future, and it is for that reason I wanted to know about whether the immersion heater and HW tank could be left as a contingency.

Nah. Rip it oot

You've gone 'combi'. Job done.

Rosa_Damascena

Jeepers_Creepers said:

Rosa_Damascena said:

Seeing as I live on my own, I can confidently say the probability is nil. And a 40kW boiler for one person is crazy!

But that situation is likely to alter in the future, and it is for that reason I wanted to know about whether the immersion heater and HW tank could be left as a contingency.

Nah. Rip it oot

You've gone 'combi'. Job done.

I will take this as the final word, and accept as a different point that I do need to get my head around the theoretical physics of water pressure in my system so I can at least understand the scientific logic of it. And look into the placement of the copper pipework (credit to @ComicGeek ).

NSG666

Looking at the pics posted poses a couple of further potential issues. I thought that the insulation boards needed a membrane over them to protect it from the screed as that's how I've seen it done. In EcoTherm's Q&As they provide this:

Yes, and EcoTherm Eco-Versal is ideal for this application. We recommend placing a sheet of polythene (1000 gauge / 0.25 mm) over the insulation before pouring your screed. This will protect the foil surface of the insulation from the alkalinity / dampness of the screed and prevents it entering the joints between the boards.

They do a product called Eco-UFH which doesn't need protecting but this isn't it.

Water Regs exist but are hardly ever referred or adhered to. I can't find my book but I think they stipulate that no joints should be buried in screed and also that mains fed hot and cold water pipes should be in conduit and replaceable without having to dig up a screed floor. There are plastic pipe in pipe systems which allow this although I used to make my own using a conduit from Toolstation.

ComicGeek seems to have experience of laying screed floors so might be able clarify these points.