Love or Hate your Combi Boiler - advice appreciated

aelitaman

Cardew wrote: »

Your solution brings us to the classic dilemma!

With modern condensing boilers it is apparently a big mistake to over-size boilers. All of the technical advice I have read states that the boiler output should be matched to heating demand to keep the boiler in condensing mode.





So it seems that getting a oversize boiler(in output) to satisfy hot water demand, will lead to lower efficiencies when heating.

I mentioned in the post that it needs to be sized.

Mine is a max output of 37.5KWh but it modulates down to 7.5Kw.

All of the boiler sizing in the article you linked to requires a boiler of at least 19.5Kwh, so modulating down to 7.5Kwh means this boiler is suitable for most houses.

stan5001

I hated having a tank, you'd have to wait 30 minutes before having a bath, or 10 if you wanted to do the dishes.

If the boiler failed in winter, you'd have at most 12 hours hot water, so I don't see the argument that they're better if they fail, especially if you have no hot water to keep!

With the combi the water is there 24 hours on demand. OK, so it takes slightly longer to fill a bath, but you don't have to wait half an hour beforehand so you still save time.

Cardew

stan5001 wrote: »

I hated having a tank, you'd have to wait 30 minutes before having a bath, or 10 if you wanted to do the dishes.

If the boiler failed in winter, you'd have at most 12 hours hot water, so I don't see the argument that they're better if they fail, especially if you have no hot water to keep!

With the combi the water is there 24 hours on demand. OK, so it takes slightly longer to fill a bath, but you don't have to wait half an hour beforehand so you still save time.

The reasoning is that if the boiler failed you would heat the Hot Water tank with the immersion heater.

Most of us have hot water on demand from a HW tank - why would we wait 30 minutes? We set our timers so water heats before it is required.

Modern HW tanks with proper insulation(normally a foam jacket) lose very little heat. They are tested to a British Standard and typically with the tank full of water at 65C they will lose between 1kWh and 2kWh in 24 hours.

In practice the tank would not be at 65C for 24 hours, so losses would be a few pence a day at the most. In any case that heat is not 'lost' as it heats the fabric of the house - and keeps the airing cupboard warm.

alleycat`

stan5001 wrote: »

I hated having a tank, you'd have to wait 30 minutes before having a bath, or 10 if you wanted to do the dishes.

If the boiler failed in winter, you'd have at most 12 hours hot water, so I don't see the argument that they're better if they fail, especially if you have no hot water to keep!

With the combi the water is there 24 hours on demand. OK, so it takes slightly longer to fill a bath, but you don't have to wait half an hour beforehand so you still save time.

I don't understand this?
We have a tank, the hot water is on 40 to 50 minutes in the morning (5am time) and again for the same length of time at 4pm (via the boiler).

We have full hot water all day around, including having a bath for the wife or my kids.

If the boiler fails, turn on the immersion switch and after about 40 minutes the tank is fully warmed up again for the next 12 hours.

The tank is fully lagged and insulated so short of a full power cut we always have access to hot water even when the boiler goes pop.

The same doesn't apply to combi's and since we have the space i prefer the tank / airing cupboard arrangement.

I understand why people like combis with regards to space and "almost instant" hot water but having a tank doesn't have the problems you describe if run correctly.

edit:- cross posted cardew. Basically what he said :-)

Cardew

aelitaman wrote: »

I mentioned in the post that it needs to be sized.

Mine is a max output of 37.5KWh but it modulates down to 7.5Kw.

All of the boiler sizing in the article you linked to requires a boiler of at least 19.5Kwh, so modulating down to 7.5Kwh means this boiler is suitable for most houses.

Firstly a disclaimer - I am no heating engineer.

However all modern boilers modulate their output. Therefore your contention(as I understand it) that if the boiler output modulates down, it means that a large output boiler(37.5kW) is suitable for all houses.

That contention doesn't square with any of the advice I have read. That advice is that fitting a large over-spec boiler for the heating load will lead to lower efficiency.

I had always assumed this is because boilers are more efficient when operating closer to their maximum output.

stan5001

alleycat` wrote: »

I don't understand this?
We have a tank, the hot water is on 40 to 50 minutes in the morning (5am time) and again for the same length of time at 4pm (via the boiler).

We have full hot water all day around, including having a bath for the wife or my kids.

If the boiler fails, turn on the immersion switch and after about 40 minutes the tank is fully warmed up again for the next 12 hours.

The tank is fully lagged and insulated so short of a full power cut we always have access to hot water even when the boiler goes pop.

The same doesn't apply to combi's and since we have the space i prefer the tank / airing cupboard arrangement.

I understand why people like combis with regards to space and "almost instant" hot water but having a tank doesn't have the problems you describe if run correctly.

edit:- cross posted cardew. Basically what he said :-)

We both had showers in the morning then maybe washing up dishes in the evening. But if you did the washing up in the morning, then we had no need for hot water in the evening.

But every so often we'd want a shower or bath in the evening. So if we had it on like you suggested, there would be an awful lot of wasted hot water...

stan5001

Cardew wrote: »

The reasoning is that if the boiler failed you would heat the Hot Water tank with the immersion heater.

Most of us have hot water on demand from a HW tank - why would we wait 30 minutes? We set our timers so water heats before it is required.

Modern HW tanks with proper insulation(normally a foam jacket) lose very little heat. They are tested to a British Standard and typically with the tank full of water at 65C they will lose between 1kWh and 2kWh in 24 hours.

In practice the tank would not be at 65C for 24 hours, so losses would be a few pence a day at the most. In any case that heat is not 'lost' as it heats the fabric of the house - and keeps the airing cupboard warm.

Firstly, the cupboard was too small to keep anything else in, and secondly, since we had the heating on only for three months a year, the other nine months was wasted. And if you work erratic hours, the timer is useless.

Cardew

stan5001 wrote: »

That's what I mean; we had to have the boiler running for 2 hours for one bath, one shower and some washing up! And if we went out for dinner on a whim, we'd come back to a tank of hot water...which would be wasted, and then have to be heated up again the next morning!

Ditto if you didn't need a shower or bath. Or just wanted a shower.

Well you must have had a faulty boiler and a faulty or leaking Hot water system.

A tank of Hot water will stay hot for days. As I said above, with water at a very hot 65C a tank will only lose a tiny amount of heat in 24 hours.

A huge number of people heat their water on Economy 7; so it stops heating the tank very early in the morning and is still hot at night.

As for two hours to heat a tank - that is just not the case for normal systems. A gas boiler produces 30kW or more to heat hot water in a tank. In 30 minutes even from cold there is enough for a bath.

If 30kW from a combi is sufficient to run a bath, then 30kW supplying hot water to a tank will heat it quickly.

Cardew

stan5001 wrote: »

Firstly, the cupboard was too small to keep anything else in, and secondly, since we had the heating on only for three months a year, the other nine months was wasted. And if you work erratic hours, the timer is useless.

Heating only on 3 months in the year? - sorry I thought this discussion was about UK - not Barbados;)

We have already stated that a big advantage of a combi is the saving of space; but you have given some unrealistic conditions to justify a combi e.g. water going cold overnight, 2 hours to heat a tank and only 3 months heating in a year.

Those conditions might apply to you, but this is a general discussion on the pros and cons of the two systems, and your situation is atypical.

Owain_Moneysaver

If the incoming water supply is suitable, and as it sounds like you'll be doing fairly major works to get it all in, an unvented system boiler and a 'megaflow' unvented cylinder, in the loft if possible.

Also get the heating zoned onto separate zones for upstairs and downstairs, only costs a zone valve if done at the time of installation, but gives more flexibility with control.