Separate heat sources for HW and CH?

[NorfolkNumpty]

Hi, I'm an newbie, please could I have some help with the right choice of boiler(s)?

I live in a bungalow and I'm looking to switch from a 1970s oil-fired open-vented CH & HW system to a gas-fired sealed (combi?) system in the loft. Here's the objectives:

1. Save space in the house by eliminating the boiler and hot water tank in a big cupboard on the ground floor.
2. Save space in the garden by eliminating the oil tank.
3. Improve the use of space in the loft by having a wall-mounted boiler (boilers?) on the gable wall at one end, eliminating the cold water tank and F&E tank stuck right in the middle of the loft.
4. Achieve a shower flow rate at least equal to the pumped power shower we have at the moment.
5. Increase system efficiency to reduce running costs.

My mains water supply is 35 litres/minute at 3.0 bar. The power shower currently outputs 13 litres/minute at the shower head. According to the calculation tool on the Energy Saving Trust website, the heating requirement of this semi-detached 2-bedroom 1-bathroom bungalow is in the order of 9-10 kW.

I had been working on the theory of fitting a big combi in order to get a good enough hot water flow for the shower. The Worcester Bosch 42CDi seems to meet that part of the spec. It's HW output of 17 l/m for a 35°C increase would probably just give me the 13 l/m shower output I would like, even in winter. That's assuming the restrictions of a thermostatic mixer and the shower head, and the need for it to maintain that rate at a 40°C increase at this time of year. Trouble is, a 42CDi would be massive overkill on the CH side, because it only modulates down to about 9.2 kW.

OK, so I need to choose a much smaller heat source for the central heating. What's the best soltion that will still allow me to have the really good shower flow I've got now?

Would an unvented HW cylinder do the job with a smaller combi? If so, what size would I need? I don't think this is really an option though, because there is relatively little height available in the loft (internally about 1.4m at the ridge). I know that some companies make horizontal ones, but from the reviews I've read they sound very disappointing.

Here's what I'm really considering — two boilers. For the central heating only, a smallish sealed boiler like a Valliant EcoTEC Plus 415. In addition to that, for the hot water only, a gas-fired instantaneous water heater like a Rinnai 26i. The Valliant can modulate between 5.o and 15 kW, which sounds fine. The Rinnai outputs 19 l/m at 33°C increase, which should work well for the shower.

Is two boilers the answer, or am I completely insane?

Thanks in advance for any advice supplied.

 

[fitz1]

the 13ltr for the shower is a mix of hot and cold , so you dont need 13ltr of hotwater.i would contemplate something like a 33kw.im fitting (after a couple of years of others testing and sorting the new boiler syndrome problems out) the baxi 28 , 33 , and 40kw boilers ,i think their performance are great and no problems as of yet.price is really good aswell. other than them i fit vaillant which are far more expensive and a far worse manufacturer contact and backup system.

 

[AfterburN003]

If hot water isyour main concern maybeyou should consider fitting a HE System boiler and a Megaflow. This will give you ur hot water u require andstill have theefficiency u are looking for. Downside is thesize of Megaflow , but this could possibly go in loft with boiler??

 

[NorfolkNumpty]

consider fitting a HE System boiler and a Megaflow

Thanks for responding, but I really don't think there's room for a Megaflow. I know they can work well — my mum and dad have got one with their combi. Unfortunately the tallest part of the loft is only just going to be tall enough for the boiler itself. That means that the only place that even the smallest unvented cylinder could go would be right in front of the boiler, which is unacceptable as far as access goes. Putting a Megaflow in the ground floor cupboard would defeat the primary objective of the project.

Surely in an era of invigorating showers and efficient insulation I can't be alone in needing more hot water than heating?

 

[NorfolkNumpty]

the 13ltr for the shower is a mix of hot and cold , so you dont need 13ltr of hotwater.i would contemplate something like a 33kw.im fitting (after a couple of years of others testing and sorting the new boiler syndrome problems out) the baxi 28 , 33 , and 40kw boilers ,i think their performance are great and no problems as of yet.price is really good aswell. other than them i fit vaillant which are far more expensive and a far worse manufacturer contact and backup system.

The heating output reqirement is something I'm having trouble understanding. If the heating need is greater than the minimum output of the boiler, does that mean the boiler is on for the whole time the CH is switched on, and it just modulates the output down as required?

Conversely, if the heating requirement is less than the minimum output of the boiler (as in the scenario above), then does it mean the boiler keeps switching on and off? That's what my current boiler does, so surely I'd be no worse off?