Boiler confusion

GDJTAM

I have a 20 year old gravity system Baxi boiler that is working fine even though I have never had the boiler serviced. We have a cylinder tank that is also well lagged.


I live in a 4 bedroom house with 1 bathroom and 1 ensuite. 4 adults live in my house, we all work different shifts so the water can be used from 5am to 9pm for showers.


My wife is fed up of saying we have no hot water and given the age of the boiler we believe it's not economical. So my new found problem as a result of looking to upgrade.....


We have had 3 quotes now to change my gravity boiler and all the advice is conflicting apart from each engineer say the house has great water flow.


I have been told that a 28-34KW Combi boiler is best for my needs given the different times we shower and also it would free up cupboard space with the removal of the cylinder.


I have also been told by an engineer that a system boiler with an unvented cylinder would be best. The engineer said that If I had a Combi boiler we couldn't have 2 showers running at the same time? I questioned this given my excellent water flow rate but was told the system wouldn't cope??


I have been quoted £2400-£2800 to have either boiler installed with power flushes, condesate discharge waste pipe into a soak-away etc.


I'm sure the engineers are giving me advise on what's best for them rather than for my household, I'm happy for the engineer to make a living but would also like to be sure that I also save money and we have an efficient running system that serves my family needs.


Any advice on what system is best and what to avoid would be most welcome.

matelodave

Personally I'd have a Combi - hot water on demand all day everyday rather than heating a tank and then either having to keep it hot and then running out.

It's true that a combi might struggle with two showers at the same time however it's not that difficult to sort yourselves out so that it doesn't happen - there are 1440minutes in a day and a shower should take only 5 minutes or so, giving you 288 slots where you could have a shower without interfering with anyone else.

We had a combi and both my daughters have had them fitted to replace their system boilers (one being a Baxi gravity) and none of us would want to go back to a system boiler/tank.

That said I'm now all electric and consequently have a tank - we don't run out because there are only two of us and it's a big 200litre (44 gallon) one - it heats for just over an hour a day (from our airsource heatpump) and gives us all the hot water we need.

GDJTAM

Thanks for the tip re time in the day but I would rather have advice on the systems and limitations.

matelodave

I'm not sure what you are looking for in terms of information.

Both systems will do the job of heating your house and producing hot water. Everyone has their own opinion on what is better or worse.

Possibly you might be able to improve your existing system by putting decent controls in - tank stat, motorised valves, programmable stat and TRV's.
I did that to my gravity system 30 years ago and it made a tremendous improvement to the running costs. When the boiler finally died I swapped to a combi and reduced my energy cost by over 30%

Storing hot water in a big tank means you've got a finite amount and might run out and it will leak heat into the surrounding area when it's hot.

Heating it when you need it using a combi seems to be more economical and you've got continuous hot water for 24 hours if you want it.

A combi allows you to get rid of the hot water tank and all the tanks in the loft thus reducing the possibility of freezing.

Nowadays all systems have to be condensing and have decent controls so in the end it comes down to what you might prefer, you pays your money and takes your choice.

HHarry

One issue with a combi is that if it goes wrong you have no backup plan for hot water. With a tank in the airing cupboard you can have an immersion heater as a back up - although I appreciate that you may never need to use it.
Another issue is that they are specified to raise the temperature of the incoming water by a certain amount. Therefore if you have a really cold winter your hot water might not be as warm as during the summer, as the incoming water is colder. Again this may not be an issue, and you can compensate by fitting a larger boiler.

There probably isn't really a wrong choice. You know the pro's & con's of both. My preference is for a traditional system, as you can see others prefer Combi's.

GDJTAM

Thanks guys, it sounds like a personal choice of having steak rare, medium or well done. Whatever my choice it's not a wrong choice.


I must say having extra storage is appealing so the Combi wins on that score. I am concerned that I would be limited to 1 shower user at a time though.


Having got an excellent water flow rate, would a 28-35KW combi give me enough oomph to power 2 showers at the same time?

matelodave

Generally a combi is rated at a temperature rise/flow rate, so the lower the flow the hotter the water will be. Likewise a lower input temperature will correspond to a lower output temperature for the same flow rate.
Lower flow showers, say 5-7lpm would therefore be able to cope better than say two power or high flow showers at 15lpm each.

ChumpusRex

The issue with combi boilers is that they heat the water on demand, and therefore must be powerful enough to handle the maximum plausible demand.

In general, a combi will be fine for most cases, but may struggle when high hot water flow rates are needed - 2 simultaneous showers or filling a bath. Depending on the design of combi, the water will either run cold, or the flow will be throttled back (making showers run cold).

In Summer, heating demands for water are less - the water tends to arrive at the house warmer, and you tend to want colder showers. You may find that 8 kW per shower is adequate, and the boiler will cope adequately with 2 simultaneous showers.

However, in Winter, you may be looking at 28 kW for a 10 litre/minute shower (a reasonable "power shower"). However, try to run 2 showers simultaneously, and you'll be disappointed. Similarly, if you want to fill a bath in Winter, you may find the fill time is extended as the boiler throttles back the water to cope.

The advantage of an system boiler and stored hot water system is that you have a reserve of hot water to get you over periods when a sudden burst of hot water is required. A 70 litre hot water tank will sustain two 10 l/m showers for probably 10-15 minutes before they run cold. If you get a "fast recovery" tank, then the system boiler should be able to recharge it as fast as a single shower can take, so a single shower shouldn't drain it.

The other advantage a stored hot water system is that you have a backup source of hot water via an immersion heater, in the event of a boiler breakdown. With a combi boiler, a boiler breakdown is a big problem because there is no way to provide temporary hot water, expect by boiling kettles or pots of water on the hob.

shortchanged_2

I have had experience of both and personally once I got used to it I prefer the cylinder option. Most modern hot water cylinders these days have a quick heat recovery from the boiler. I currently have a 150 litre tank, there are 5 of us in the house. There's enough capacity for a bath and shower at the same time. However we are running a pump for the shower as we have a gravity fed system but it provides a pretty good power shower.
I used to find it would take ages to fill a bath with the combi boiler. And the other problem is for us is that we would need a very powerful combi boiler for the size of our house.

Pincher

Four working adults, two shower rooms. It's a morning peak demand to design for.

Even if the combi can heat enough hot water INSTANTANEOUSLY for two showers, it will not be heating the house when doing so.
It is expecting a donkey to carry a camel's load.

It is also unlikely that your mains water supply will be sufficient to run both showers simultaneously. Remember, you are supplying BOTH hot and cold for two showers.

Try this twin system approach. Keep the existing hot water cylinder with loft cold water tank, and supply the main bathroom only.
The combi boiler supplies hot water to the second bathroom and kitchen, but heats the hot water cylinder at 4am. The combi boiler heats the house up from 5am.

Comes 6am, the house is warm, and you can have both barrels blasting. The mains water only has to supply one shower, and the combi is only heating water for one shower.

It's a very simple: "I made this hot water earlier."

GDJTAM

Well, Im really chuffed with the replies. I had it drummed into me that my 20 year old Baxi needed to go and replaced by a Combi. It sounds like I need to just get a good service done and carry on with my system boiler. I also like the twin system option.


More to ponder over but I now have reassurance.


Many thanks