Changing to a combi boiler - is this realistic?

housebuyer143

RainbowLaura said:

I am thinking of replacing my 25 year old system boiler & hot water cylinder with a combi. The main reason for this is ongoing problems with both the boiler and the cylinder - a new boiler wouldn't solve the cylinder issues and I'd quite like some extra space where the cylinder is located.

However. Although my house is fairly small (about 1000sq ft), it is a 3-storey with 14 radiators. I also like a bath, which I understand can be a sticking point with combis when they are not powerful enough. The boiler would need to go in a kitchen cupboard.

Is this combination of needs realistic? (No pun intended there!) It seems the only combis that would fit inside the cupboard would not be powerful enough. Is anyone able to kindly offer me some advice please? I really want a combi, I know there are down sides but I really don't want to deal with hot water cylinders anymore.

I personally chose to keep my system when they both went wrong. 
I like the idea of future proofing the property so you can get solar to heat your water, heat pump etc.

I also wanted to run showers at the same time and maintain hot water and pressure which I didn't feel a combi could achieve. No regrets.

scottishblondie

We had a 3 storey house, which also had 14 rads, 3 showers and 1 bath.  We had what I believe would be called an unvented system boiler and very large hot water tank - we got mains pressure hot water and could run 2 showers at the same time without issue. The boiler and tank were in a cupboard in the middle floor and we got hot water almost immediately at all taps. 

The house we have now is all on 1 level and has a 32kw combi. I really miss the system at the old house. It takes an age to get hot water anywhere other than the taps right next to the boiler. You can only really run one tap at a time - if you are running a bath and someone turns on the kitchen tap you get miserable flow in both. We have 4 showers and 2 baths here - 3 of the showers are electric (which I also hate) because of this issue. We did have an electric one in our en-suite but we changed it out to run off of the boiler - it’s fab as long as it’s the only thing the boiler is supplying!

My view is clearly coloured by my own experiences, but I wouldn’t be changing to a combi. If I could I would fit a new system here as we had at the old house. 

benson1980

ThisIsWeird said:

benson1980 said:

We had a new (at the time) combi in our last house, which was probably a similar number of radiators to yourself, and two bathrooms but around 1800sq ft. It was 30kw- we didn’t install it but I remember making enquiries when we were buying the house as to whether it was appropriately sized….and it seemed fine.

Anyway, it was ok. Hot water pressure in the upstairs bathroom was an issue. Shower wasn’t great, and we live in an area where mains pressure is very high. We could live with it though.

We have a heat only 17 year old boiler and new unvented cylinder in our current house (similar size), with the cylinder in the loft to free up space in the airing cupboard. There’s no doubt our current setup is a far superior system in terms of hot water pressure to the bath and all the showers. 

Plus we have potential solar options with the cylinder. I also like the fact that the boiler itself is simpler and less to go wrong as it’s heat only.

Running costs in terms of hot water also seem to be a bit cheaper with the cylinder (perhaps because the boiler isn’t firing up sporadically each time there’s demand), and none of us have short showers. To answer your question though if you want a combi (which my wife actually preferred as the whole system was quieter) it should be fine.

Agree with your summary, although I don't understand why you'd have a pressure issue upstairs, if your incoming was good. We have three 'bathrooms' - one has a shower, and the other two have baths with bath/shower mixers. The pressure in the upstairs appears to be just as good as the two downstairs.

The 30kW jobbie gives a great shower, but, yes, only one at a time.

Thinking about it again, yep maybe there was another issue going on there. The pressure wasn't amazing downstairs either...but not terrible by any stretch. 

BUFF

Installing a new combi will require more replumbing work than simply replacing your system boiler.
Even the latest combis are less efficient for DHW generation than a modern tanked system.
This may not be of concern to you depending upon your future plans but the next replacement cycle will almost certainly require stored DHW i.e. replacing all that piping & tank that you stripped out for your combi ...

Miser1964

Most houses are OK with heat input of well under 15kW to get them to target temp when -2C outside, so heating with a combi in the 24kW-30kW range shouldn't be problem. But a good installer will calculate the heat loss based on # of windows, wall insulation etc. rather than counting rads.

As has been said, combis are fine for one shower/bath but not so clever running two at the same time. 

If you decide to keep a HW cylinder (with electric immersion backup) and system boiler, then the modern design is 'Priority Domestic Hot Water' (PDHW) which allows the CH and cylinder feeds to be different temps for condensing boiler efficiency. 

Also a new install should certainly be using Weather Compensation on the heating and OpenTherm control rather than a simple on/off thermostat.

You'll need a power flush before installing any new boiler.  

FreeBear

Miser1964 said: You'll may need a power flush before installing any new boiler. 

A chemical flush might be all that is required. It will depend on how much sludge is in the system.