combi size advice

[maciej.olchowik]

Hi Folks,

I need your advice on a combi boiler size. Let me describe my situation:

I live in 2-bed, 1-bath semi-detached upper villa (1st floor and loft) I plan to upgrade whole CH this summer by replacing boiler, pipework, radiators and controls. I know with combis it's flow rate and mains pressure that really matters. I measured all that:

flow rate: 28 ltr/min static pressure: 3.0 bar dynamic pressure: 1.5 bar (with one tap open)

I have 22mm copper gas pipe from meter point and I reckon it could be routed so that it will not exceed 16m in length between meter and boiler. I also have gas hob and fireplace in the proximity of a boiler (3-5m).

My question is:

1) Can I have Vaillant Ecotec 837 boiler and hob + fireplace fed off my current gas supply or should I rather go for Ecotec 831 ? I only have one bathroom but still would like to make the most of the flow rate.

or ...

2) If I wanted to upgrade my gas supply to 28mm what would that mean financially ? Has anyone done it ?

or ...

3) could I have Ecotec 837 without hob and fireplace (use electric ones) ?

your help is much apprectated,

mike

[The Natural Philosopher]

Y'know. I posted here a few weeks ago about a man who wanted a decent shower in a small place.

We assumed that the cheapest thing would be a combi,

Not.

In discussion with the plumber, it turned out that taking the old, but perfectly serviceable boiler and coupling it to a mains pressure tank and converting the rads to sealed Y plan would be the cheapest option of all.

Indeed, when I looked at the cost of a sufficiently high output combi (17Kw+) with the cost of a NEW uncased 10KW 'boilerhouse boiler' and pressurized tank, there was sod all difference.

To the point where I am struggling to find any situation where a combi is the best solution, or even the cheapest. Maybe a one bedroom flat of tiny dimensions with no shower..but that's it.

Not wishing to put you off, just double check that what you are doing IS the best cost-benefit..

I certainly have never met a combi yet that can match my peak hot water flows here, with a pressurized tank.

Nor yet one that delivers full pressure hot water even when the power goes out..well, for a while, anyway :-)

Or can have an immersion heater if you run out of oil or gas..

[maciej.olchowik]

thanks for your reply!

I take your point here. However my current system is really old and badly designed. It's open vented system with hot water cylinder and 15+ years old baxi boiler. I say badly designed bacause I think I have undersized radiators and pipes (8mm copper). All I can get during winter is less than 20 degrees air temp.

cost benefit is the main reason - here is why I think new combi is good idea:

1) my current boiler is old, I had few problems with it this winter and would like to avoid the situation where I have to replace it quickly in the worst moment (next winter) doing rads/piping/controls myself I can save some money. Remeber it's the labour cost that's the most expensive in CH 2) new boiler will be condensing model, which means savings on gas bill 3) heating on demand is more economical from storing water 4) I can free some loft space for future conversion

do I miss some important point here ? feel free to comment

I agree. However it's only two of us in the property, so what I am really after is a decent flow from a shower when someone is doing the dishes.

As I mentioned - I plan to convert loft at some point and have a second bathroom with electric shower as backup. As we shower quite a lot and hot water cylinder is a bit to small so we use electric shower at the moment.

can't argue with that =)

again thanks for your input,

Mike

[maciej.olchowik]

thanks for your reply!

I take your point here. However my current system is really old and badly designed. It's open vented system with hot water cylinder and 15+ years old baxi boiler. I say badly designed bacause I think I have undersized radiators and pipes (8mm copper). All I can get during winter is less than 20 degrees air temp.

cost benefit is the main reason - here is why I think new combi is good idea:

1) my current boiler is old, I had few problems with it this winter and would like to avoid the situation where I have to replace it quickly in the worst moment (next winter) doing rads/piping/controls myself I can save some money. Remeber it's the labour cost that's the most expensive in CH 2) new boiler will be condensing model, which means savings on gas bill 3) heating on demand is more economical from storing water 4) I can free some loft space for future conversion

do I miss some important point here ? feel free to comment

I agree. However it's only two of us in the property, so what I am really after is a decent flow from a shower when someone is doing the dishes.

As I mentioned - I plan to convert loft at some point and have a second bathroom with electric shower as backup. As we shower quite a lot and hot water cylinder is a bit to small so we use electric shower at the moment.

can't argue with that =)

again thanks for your input,

Mike

[maciej.olchowik]

thanks for your reply!

I take your point here. However my current system is really old and badly designed. It's open vented system with hot water cylinder and 15+ years old baxi boiler. I say badly designed bacause I think I have undersized radiators and pipes (8mm copper). All I can get during winter is less than 20 degrees air temp.

cost benefit is the main reason - here is why I think new combi is good idea:

1) my current boiler is old, I had few problems with it this winter and would like to avoid the situation where I have to replace it quickly in the worst moment (next winter) doing rads/piping/controls myself I can save some money. Remeber it's the labour cost that's the most expensive in CH 2) new boiler will be condensing model, which means savings on gas bill 3) heating on demand is more economical from storing water 4) I can free some loft space for future conversion

do I miss some important point here ? feel free to comment

I agree. However it's only two of us in the property, so what I am really after is a decent flow from a shower when someone is doing the dishes.

As I mentioned - I plan to convert loft at some point and have a second bathroom with electric shower as backup. As we shower quite a lot and hot water cylinder is a bit to small so we use electric shower at the moment.

can't argue with that =)

again thanks for your input,

Mike

[maciej.olchowik]

thanks for your reply!

I take your point here. However my current system is really old and badly designed. It's open vented system with hot water cylinder and 15+ years old baxi boiler. I say badly designed bacause I think I have undersized radiators and pipes (8mm copper). All I can get during winter is less than 20 degrees air temp.

cost benefit is the main reason - here is why I think new combi is good idea:

1) my current boiler is old, I had few problems with it this winter and would like to avoid the situation where I have to replace it quickly in the worst moment (next winter) doing rads/piping/controls myself I can save some money. Remeber it's the labour cost that's the most expensive in CH 2) new boiler will be condensing model, which means savings on gas bill 3) heating on demand is more economical from storing water 4) I can free some loft space for future conversion

do I miss some important point here ? feel free to comment

I agree. However it's only two of us in the property, so what I am really after is a decent flow from a shower when someone is doing the dishes.

As I mentioned - I plan to convert loft at some point and have a second bathroom with electric shower as backup. As we shower quite a lot and hot water cylinder is a bit to small so we use electric shower at the moment.

can't argue with that =)

again thanks for your input,

Mike

[The Natural Philosopher]

Is it though? If you have to run a big f*ck-off boiler flat out to heat the water, is it as economical as a modulating boiler running at tickover?

The size of a smaller boiler and small (150L tank is not much greater than a 20KW boiler.

Neither is the cost a great deal different in my case. YMMV.

To get the same sort of peak flows you need something like a 30KW combi..this may not be an issue for you though.

You won't get that from a combi. I stayed in a house once..with someone with a teenage daughter. The HOWLS of anguish when I turned on a hot tap to wash my hands from the teenage daughter showering..

We are a two person house, and believe me even a 250 liter tank only lasts one bloody good bath or shower..but at least with thermostats the flow may drop off, in the shower when you run a tap, but it doesn't go cold..

Mmm. The pressurized tank will go anywhere..doesn't have to be in the loft. Could be in the garage.. As could the boiler.. Mines in a corner of the loft as it happens..

Seriously look closely at what peak hot water rates you CAN get with a combi.

I was seriously disappointed. The ones that did the rates were all 1500 quid plus. So was a new boiler and PHW tank. About 750 each. less if you shop around.

And remember, IF your gas supply can't do the peak rates, then you have ye more expense to upgrade it.

[maciej.olchowik]

apologies for multiple post last time - I was getting the message to try again and again ... from google

ok - to heat up X litres of water from temp A to temp B you need certain amount of calories from gas. As far as I understand it, it doesn't matter if you heat it fast (combi) or slow (storage tank) - you still have to burn the same amount of gas. Big f*ck-off boiler will run flat out for say 15mins while you take shower. At the same time modulating boiler will tickover for 1 hour in reality burning the same amount of gas. Now, when you store the water in the tank heat will escape and you have to burn some more gas to keep water at temp B all the time. This is where combi wins I think (at least in terms of energy savings) Additionally if you work irregular hours you don't have the trouble of programming when exactly you need to have your hot water.

Yep - I agree. But you also have to have run additional pump (press tank) which presumably is noisy and cost you electricity.

flow from a shower when someone is doing the dishes.

hmm... - no good, will have to discuss that with my girlfriend then =(

anguish when I turned on a hot tap

how about using thermostatic mixer showers ? Flow would drop but you won't get temp fluctuations ... Just like you have with your tank setup.

15.2 ltr/min for big f*ck-off Vaillant Ecotec 837. Initially I thought this would be enough for shower + kitchen tap (i.e. dishes).

Yes I am sure I could find some corner space even with a loft converted. Thanks for your input I will definitely consider this option.

Mike

[The Natural Philosopher]

not necessarily. Boiler efficiency varies with output power. Also duty cycle (unburnt gas up chimney before it lights sometimes)Lots of conflicting evidence on the net on this. No axe to grind, jutrs be aware that boiler thermal efficiency is not 100% and isd not constant with outut power.

A lot depends on tank insulation. Usually this is pretty good, and a pressurized tank has about as high a volume to surface area ratio as you can get. So it doesn't lose heat very fast at all. In any case you cvan swich it off overnight after the shower goes cold and have it come on half an hour before you get up.

? what pump? No pump at all. It runs off mains pressure. The only pump is the circulation pump for the CH and HW primary, and that only runs when the boiler runs in any case.. and boilers make a fair amount of noise. A combi doesn't use a pump when heating hot water, but it still has one for rads etc. So theres just as many pumps.

flow from a shower when someone is doing the dishes.

anguish when I turned on a hot tap

Well lets put it this way, when I went to wash my hands, I didn't get enough hot water to fill a basin in under 5 minutes, and that was tepid. OK she had a small wall mounted combi, nit a big fuckoff one.

I have been into this in depth twice now..and each time the combi didn't work out cheaper or better in any sense.

Hot water is a pretty small element of my heating budgets, and our tank stays hot for a couple of days so its pretty well insulated.

I think where I would install a combi is a bachelor flat of small dimensions, with a single occupant and a single bathroom there I can see the virtue of the very small foot print. And overall heating requirements are not huge.

Once you get to a family situation, I think the approach loses all merit.

[David Hansen]

On 23 May 2007 08:14:20 -0700 someone who may be [e-mail] wrote this:-

There are two assumptions here. Firstly, that hot water cylinders lose a lot of heat. One with double or triple insulation does not.

Secondly, that a combination boiler does not have a small not particularly well insulated hot water cylinder inside which the boiler will fire periodically to keep warm. Many do have one, though it can be turned off.

Thermal store. Instant hot water when wanted.

Connect the central heating to it as well and one gets instant heating by flicking a switch when entering the building. No need to wait for the boiler to warm up the radiators. No need for a time clock which then heats up the building even if one has been delayed.

[Tim]

What would be typical products (and prices if possible) for us to look at?

[Jim Alexander]

Despite asking for advice you seem to have already made up your mind so now you need to do the pipework calcs. Basically you asked about pipework gas carrying capacity. The answer can be found here...

'm not going to do the calcs for you but one I did earlier suggests that

16m of (mainly) 28mm is not adequate for a 28kW boiler, cooker and stove.

Jim A

[Ben Blaukopf]

We stuck one in a one bed flat, two people.

The space saved by not having a cylinder meant we could move the washing machine out of the kitchen, which made space for a dishwasher. Which solved the dishes problem. We vented the boiler through the roof, which meant we could put it in a big cupboard.

Still couldn't shave while my wife was showering, but other than that it was great. Including the shower. It had a 5 litre heated store for proper instant hot water, which worked well.

[John Rumm]

Using the tables here:

get a 15m run of 22mm pipe will give a max gas flow rate of about

3.4m^3/hour - and that assumes a straight run with the maximum allowed pressure drop (1 mBar). Bends and elbows add more effective length to the pipe.

My 35kW combi boiler claims to take a maximum gas rate of about 3.7m^3/hour

The short answer is no.

What power input is the boiler, hob, and fire? You may well be able to get the specs from the makers web sites.

Your supply into the house is probably big enough (it runs at higher pressure as well, so even if the pipe size looks no bigger than 22mm it will have higher capacity). Hence the main cost is that of the pipe and time to fit it.

That is a 37kW output boiler IIUC. So power input will be getting on for 41kW max, or 147.6MJ/hour. If you take a minimum natural gas energy value as 37.5MJ/m^3 that could equate to 3.9m^/hour.

At that rate 12M (effective length) is your limit on 22mm pipe.

Which I guess answers your question.

28mm pipe would deliver upto 7m^3 at 15m, which is going to be adequate I would have thought.

[The Natural Philosopher]

The best online deal I found for a stainless pressurised tank was a Tempest something or other. Lifetime gurantee. About 650 quid for IIRC

150 liters. There were some slightly cheaper ones, but they didn't say 'stainless'; and didn't have a lifetime guarantee.

The plumber said 'Tempest? yeah. Done a few of those. Happy with that' so that's the one we got.

I just googled like mad..also look up 'boilerhouse boilers'

I found a dainty little uncased one for £585..however in this case we are using the existing boiler.

I did find sub £1k combis, but they were usually gas, and none were really capable of more than about 10-15KW. since the friend was already very unhappy with his 7KW electric shower, that didn't seem enough to me.

In the end the opportunity cost of the new hot water tank to the existing boiler - which I didn't think was possible, but the plumber said most definitely was - was below the cost of going to a combi.

and the final result was definitely better..the tank goes in the boilerhouse, clearing a load of clutter from the bedroom. Which will then be used to store the clothes currently hanging in a room off the bedroom, which will be the ensuite shower.

[The Natural Philosopher]

I think you have made the most useful point here for the OP to consider.

A small cheap combi will not do more than one taps worth of hot water at a time.

If that is a non starter, then a small cheap combi is not the way to go, and a large not so cheap combi, becomes almost the same footprint and cost as an alternative approach.

and of course, since the combi was inside the insulation layer, any loss from that is heating to the house.

[Jim Alexander]

Maybe, maybe not. The 28mm upgrade I referred to retained 22mm near the meter because I didn't fancy re-coring the wall.

0.43 of the allowable pressure drop occurs in that section and the 22mm final leg to the boiler.

The lesson is the OP needs to do the calcs for his proposed installation and it may be necessary to take 28mm as close to the meter as possible. Also I said 28kW but the limiting boiler output for the installation is 33kW.

Jim A

[maciej.olchowik]

guys - many thanks for all the useful comments, I now have much better understanding of what can be achieved with what =) I don't want to get into the battle which is better combi or tank? I just want to understand requirements that I have to meet to install both systems.

found here...

pipes and answer original question myself

these are max gas requirements for applicances: Vaillant 837 =~ 4.0 m3/h Vaillant 831 =~ 3.3 m3/h Cooker =~ 1.0 m3/h Fireplace =~ 0.5 m3/h

so if I want 837 model I need to have 5.5 m3/h on some pipes between meter and boiler. points of interest are: M(meter), B(boiler), F(fireplace), C(cooker)

M --- A --- D --- B | | C F

distances are effective (i.e. including tees and elbows), I have calculated pressure drops from cda tables: M-A 10m need 5.5 m3/h and 28mm pipe gives pressure drop of 10/25 =

0.40 mbar A-D 3m need 4.5 m3/h and 28mm pipe gives pressure drop of 3/30 = 0.10 mbar D-B 5m need 4.0 m3/h and 22mm pipe gives pressure drop of 5/12 = 0.42 mbar A-C 3m need 1.0 m3/h and 15mm pipe gives pressure drop of 3/15 = 0.20 mbar D-F 3m need 0.5 m3/h and 10mm pipe gives pressure drop of 3/9 = 0.33 mbar

total pressure drop from meter to appliances: boiler (MA+AD+DB) = 0.40 + 0.1 + 0.42 = 0.92 mbar cooker (MA+AC) = 0.40 + 0.20 = 0.60 mbar fireplace (MA+AD+DF) = 0.40 + 0.1 + 0.33 = 0.83 mbar

all is within 1 mbar limit of pressure drop so should work ok. Please could you sanity check my calculations and let me know if I am being stupid ?

The pipe coming out of a meter is 22mm. Can I (or rather CORGI man) cut it just after the meter and expand it to 28mm ? Or do I have to replace the whole meter ?

great group - thanks.

[Jim Alexander]

checked this using my spreadsheet and it seems to agree quite well. My boiler calc says 0.44 + 0.09 + 0.44 = 0.97. The slight difference is probably because I calculate and apply an exponential function from the table. However I suggest you take into account any existing 22mm at the meter it's not practicable to change. Anyway you have 5m of 22mm to play with at the other end.

Still within spec but for many surprising just how close to the limit. Some people (not me) even advocate 2 combis but I doubt he understands the calcs.

yes

unlikely

Jim A

[olo]

thanks Jim - much appreciated

yes - I have used nearest values from the table

will do

excellent - thanks for clarifyng

mike