Combi or not combi - help!

[Harry Bloomfield]

Scott has brought this to us :

Yes! You should be able to put a hand on the two outlet pipes and if there is only call for one - HW or heating, only one should be hot.

Our boiler is in a cupboard on the wall and is almost inaudible. I have to open the door to hear if it is running.

Quite easy to get a Y connector and feed cold to both hot and cold inputs.

There is no market for second hand boilers, much of the cost for most people is the cost of installation. The might have some slight value to a DIY installer.

[Terry Casey]

We are away at the moment but I will check it when we return in early June

I have no way of checking this unless there is some simple method I could use

No supply problems in the year we've been there - although not all the time!

The electric shower heater should suffice for washing and the dishwasher and washing machine would take care of most other essentials. The kettle will, hopefully, cater for any more demand.

As the quote we are most interested in offers the option of a like for like replacement instead of the combi, I was assuming that the cylinder would remain gravity fed but will check.

[ARW]

You don't have to stand there and watch the bath fill up!

[Terry Casey]

We've been discussing this and, whilst we haven't come to a decision yet, we are thinking of just having the system flushed at the moment plus changing one or two TRVs to see what the effect is.

We have been told that there is water being pumped into the header tank and there is certainly a dripping noise when it is very quiet. My wife finds this disturbing and can hear it if she has an early night before the heating turns off and when it comes on again in the morning. If the system is flushed I'm hoping the improved flow will get rid of this.

Another thing we've noticed is the number of times the boiler turns on for a short period, then shuts down again when the heating is on which, again, I'm assuming is caused by the restricted flow.

Bath in the bathroom and en suite electric shower

There is very limited easily accessible storage room and my wife would love to regain the space that the cylinder takes up in the airing cupboard!

I will check this when we return next month

Improving gas flow included in combi quote

[ChrisK]

Probably not, it is quite a tortuous route the 15mm feed follows to join the upstairs HW pipework which is all 22mm. Not bad enough to do anything about it as it would involve taking up a lot of flooring.

CK

[ChrisK]

Of course, I knew that :-(, must engage brain before touching keyboard... It is running at about 1.5 bar.

Agree, it's not a problem, just a noticeable side effect & the 15mm pipe run is quite long with a lot of bends in it.

CK

[The Natural Philosopher]

A system boiler of 8-15kW, plus pressurised hot water tank with the added advantage of an immersion heater if you don't want to run the boiler, is around £1200 in total.

A 45Kw combi to deliver the same hot water flow rate is similar, or more costly, and is certainly bigger in the boiler room

(You can site a pressurised cylinder wherever you like)

If you have an unpressurised tank you need pumped showers.

(Interestingly, in the two new builds going up here, with no mains gas, they are using air source heat pumps, UFH, and a pressurised tank plus the mandatory immersion heater to bring the stored temp up to 60C to kill the bugs. Air sourced heat pumps simply can't deliver the peak output, and indeed the insulation level installed to allow them to even heat the house at all is phenomenal)

To heat 15 l/min (an average single shower) from 5C to say 45C takes 42KW.

To fill a bath with water at 45C in 4 minutes is around 20l/min or 55KW.

For sure you can add a heat store to a combi at EVEN MORE EXPENSE to give better figures than this, but that is pretty much what a DPHW system IS.

My point is simple. Heating a house requires a constant low to medium heat output over a long period of time. A boiler designed to do that efficiently does not have the capability for a massive output over 4 minutes of the day when you want to fill a bath. Attempts to rectify this with high power but modulating boilers etc simply make the boiler more complex and costly.

The simplest solution is a heat store. Mutatis mutandis a decent sized heatbank on a combi is no different to a system boiler with a DPHW tank attached.

The more complex solution is a thwacking great boiler.

Prices are similar, but even then, if you install the monster combi, you can STILL only cope with one shower or bath at a time and you cannot open a hot tap elsewhere.

The worst solution is to enforce a pathetic rate of hot water flow on the house-owner. Because you didn't give a shit when you built the poxy little house you sold him.

Actually no, the worst solution is a gravity fed unpressurised tank. In a bedroom, so no upstairs hot water pressure exists at all, and you have to pump.

In short:

Installing a system boiler geared to the CH requirements and a DPHW tank to provide peak flow of piping hot water gets you the optimal solution to both problems, and is no more expensive than a small combi with a large heat bank, or a large combi with no heat bank.

And even a large combi can't meet the same peak demands that stored hot water can.

The only cheap option is a pathetically limited small combi with litle or no heat bank.

Or a gravity fed cheap water tank with a system boiler.

I have experienced both. They suck, big time,.

[John Rumm]

Yup, having eco mode turned off makes it far more usable IME - with faster hot water delivery, especially when all you want is a small amount. Eco mode on will save some gas, but I would not expect it to make a significant difference to consumption in most households.

Often with combi you won't be adding any cold to the mix - i.e. you select a fill rate that delivers the hot water at final use temperature. When that is the case, the boiler is running flat out anyway.

Unlike a stored water system, you don't have the option to deliver the hot water at a much higher temperature than you need so that you can augment it with extra cold water (or at least there is no advantage in doing so, since to get it hotter, you need to have it slower).

Its sometimes instructive to do the sums for various inlet and outlet temperatures to see what rates you can expect. If you assume that a kg of water needs 4200 joules of energy for every C lift in temperature, and the combi can provide 24000 x 60 joules per minute, then you can get some numbers. So for example, if the incoming water is at 10 deg C, and you want a bath at 45 deg C, then that's a 35 degree lift. 24000 x 60 /

35 / 4200 = just under 10 lpm. If the inlet water is colder, say 5 deg, and you want water at 50, then it becomes 24000 x 60 / 45 / 4200 = bit over 7.5 lpm.

Yup mostly. Depending on the boiler it might be motorised, or some have a kind of solenoid activated one. However the basic principle is common to many combis; when they detect DHW flow, they fire up the main burner, and activate the diversion valve. This re-routes the primary flow so that it only goes through the secondary plate heat exchanger for the hot water and not the rads. If this valve were not fully diverting the flow, then you would expect the rads to get warm, and the ability to heat the incoming cold water would be impaired.

The latter usually alas, unless you can flog it to someone who will self install.

Note however, that you *can* have a hot water cylinder heated with a combi. Just because it can also do hot water on its own, does not mean you have to use that capability. There is nothing stopping you from adding external zone valves and controls to divert the CH water through a cylinder in the normal way.

In some cases this can be a system worth installing from new. if you have a conventional gravity fed HW cylinder. You get fast bath filling, but also have mains pressure (wholesome) hot water available direct from the combi side for either a shower or kitchen tap. With a mains (i.e. unvented) cylinder, then there is less benefit in this approach since the cylinder will also feed a good shower. However if you already have the boiler, it saves the cost of replacing that - and 24kW is more than adequate for a fast cylinder recharge.

[John Rumm]

Yup, time filling a bucket of known volume from your fastest tap.

You can get a pressure testing kit from a plumbers merchant. Normally about 10 to 15 quid. Includes a gauge and selection of adaptors to attach it to different types of tap.

Yup, that's a resonable backup plan.

You can also use a combi to heat a cylinder, but then pipe its DHW feed to somewhere where it will be of advantage. Say feeding a shower (high pressure without a pump), or a kitchen hot tap - e.g. save time by filling a saucepan with pre-heated water prior to boiler etc.

[John Rumm]

Vaillant ecoTec 612 £916 Vaillant unistor 120L £605

Vs

Vaillant ecoTec 838 combi £1497

Odd definition of "a lot more"...

[ARW]

The eco mode IMHO should be on a timer.

[mechanic]

Well there's no substitute for horsepower as they say. WB supply nice 40kW machines that should be enough for any medium house.

[Andy Burns]

IRTA "I've stayed friends with relatives who have a combi" and thought boiler choice is hardly a falling-out issue :-)

[Roger Mills]

LOL!

[DJC]

I found the pre-heat function useless, so prefer 'eco mode' anyway. Without pre-heat (eco mode): turn on tab, wait for cool water to clear dead leg, wait a bit longer for water to heat. Turn off tap, turn it on to top up a while later; warm water in dead leg, then shot of too-hot water that has been sitting in boiler, then cold as fresh water enters the boiler before it has fired, wait for water to reach full tempreture.

With pre-heat there is always that slug of overheated water sitting in the boiler.

[Vir Campestris]

My son has a combi in his flat.

When you turn the tap on it takes an _age_ to get hot water - the boiler has to heat up first, then hot water starts running down the pipe, then you get hot. But not very - looks like peak power on the boiler won't feed the kitchen tap on full. Likely a fault.

And you'll need a pretty powerful one to get bath flow rates out of a combi. Showers are good though.

We have a tank.

When our boiler goes wrong we'll put the immersion heater on to have a hot bath or shower. There can be no immersion in a combi.

Our shower is fine - gravity fed, and downstairs. Upstairs we'd need a pump. And we can run a bath full as fast as the tap will go.

Andy

[Tim Watts]

Many WBs have a preheat mode which keeps everything upto the plate exchanger hot.

Thought of that - I'll keep my inline heater with valving to isolate the boiler and enable the heater. It's 10kW but we've run off that for some time now. Fills a bath in 20-30 minutes (summer vs winter). Needs a little forethought, but quite workable.

[Muddymike]

We changed from storage tanks to instant combi at our least house. Never again. Despite water pressure being good the supply of hot water was never as good. Then the mains in the road burst cutting off our supply for 36 hours!

How we wished we still had that big tank of water in the loft!

Mike

[Scott]

I was persuaded to install Combisave. It appears to cut down the amount of water wasted but whether it saves energy I don't know. Any ideas?

[newshound]

Agree with all you say, apart from the final point; gravity fed water tank is OK *as long as you have a shower pump, and (perhaps) one for the other DHW*. This does add £300 to the cost: don't buy a cheap shower pump, get a Stuart Turner.