Which boiler to use for hot water only?

[oldbuffer]

Whatever type of heating you use will need to provide sufficient heat into the building to overcome the heat loss from it through roof, walls, windows, floor etc. to atmosphere.

Assuming that the losses (the element of fuel usage which does not go towards balancing the heat loss) in a heating system are roughly equivalent between gas and electric heating, then the running costs are directly proportional to the fuel cost per unit of heat (normally kilowatt). Normal rate electricity is 2 1/2 to 3 time the cost of gas, so heating by electricity would be, say, 2.5 time the running cost of gas heating the same building.

It is probably true to say that electrical heating is a bit more efficient than gas in terms of losses, so maybe the cost is 2.25 times.

It may also be possible to get night rate electricity, which alters the equation, but may well cause lifestyle / timing difficulties.

 

[propertydev83]

I've definitely decided against electric UFH. From what I perceive as more reliable evidence found, the running costs of electric UFH will be significantly higher than wet UFH or central heating in order to achieve the same level of heat.

I probably will also not opt for wet UFH for this particular property. According to what I've found the running cost savings will not be significant, and in my mind, if it will cost higher for installation than a central heating system with radiators, then the mild energy bill savings do not warrant the extra cost.

What I'm still unsure of is determining the power needs of the combi boiler for a central heating system with radiators? A couple of bits of information which I found significant, and mentioned in a previous post:

Flow Rate / Boiler Power Output...

If you’re after a really powerful shower option you will need to look for a combination boiler that is capable of supplying hot water at a flow rate of at least 12 litres a minute. Obviously the bigger the shower head the more pressure will be needed. If you do have a combination boiler your best bet is to go for a thermostatic shower mixer valve, to even out any temperature changes.

You like a decent shower. So, you need a powerful combi (38kW-ish)

However, you firstly need a plumber to come out and test your cold mains supply for pressure and flow. If these are decent, you can get a very good shower from a powerful combi. If they aren't, you'll need to store water.

Booster pump...

A booster pump can be fitted; a single pump or a dual/twin pump. Which ever is chosen it must be connected into the power supply in order to operate.

A single pump: This type must be fitted in a position between the spray and the mixer control. This will boost the supply of water from the control to the spray itself giving the desired effect.

A dual/twin pump: This must be fitted to the supply pipes. It increases the pressure by boosting both the hot and cold water supplies before they reach the mixer.

A couple of seemingly contradictory comments:

The combi however is good, but as said before, if someone opens up another tap, it really is just a little trickle that you'd get.

Thermostatic mixer:

It's a good choice because...
The controlled temperature makes it extremely convenient. Also the thermostat means that the temperature and flow of water should not be affected when water is being used elsewhere in the house.

If going with gas central heating, the hot water needs would be:
- 6 x radiators, 1 x heated towel rail
- 2 x bath taps
- 1 x thermostatic mixer shower
- 1 x mixer tap for bathroom sink
- 1 x mixer tap for downstairs WC sink
- 1 x mixer tap for kitchen sink

How is the power needs of the combi determined given these needs (and the additional uncertainties in my mind raised by what I've read in those quoted pieces)?

 

[Dan Robinson]

For a com I you need to look at mains water capacity vs. Lowest heat capacity to heating. Max ch output is irrelevant.



No point having a 42kw com I if mains supply is only 12l/min

In fact 42kw combis aare pretty pointless IMHO.

A 28kw combi I will fill a bath in circa 12 - 15 mins. No com I will give two decent (by my standards anyway ) showers simultaneously .

Which brings me back straight to my original post

 

[propertydev83]

Thanks for the reply.

No com I will give two decent (by my standards anyway ) showers simultaneously .

If we're talking about having only one shower, does this also mean that you will not get a decent shower if any taps are open in the house?

 

[Dan Robinson]

That will depend on the shower and your careful use of flow restrictors on wc's sinks and basins.

 

[ALEC1]

the wet ufh manufacturers make a killing in controls, so keep it simple, a weather compensated boiler with its own mixing valves if its got rads as well, or just a boiler working with compensation controls and the flow temp capped...

 

[D_Hailsham]

Flow Rate / Boiler Power Output

You firstly need a plumber to come out and test your cold mains supply for pressure and flow. If these are decent, you can get a very good shower from a powerful combi. If they aren't, you'll need to store water.

Correct. As I said earlier, you must realize that the incoming cold mains supplies all water to the house - hot and cold. If the flow rate and pressure is not adequate a simple thing like flushing the toilet will be noticed by someone having a shower.

Booster pumps
I don't think pumps are allowed if a combi boiler is used.

The combi however is good, but as said before, if someone opens up another tap, it really is just a little trickle that you'd get.

It all depends on the incoming flow rate and pressure

Thermostatic mixer

It's a good choice because...
The controlled temperature makes it extremely convenient. Also the thermostat means that the temperature and flow of water should not be affected when water is being used elsewhere in the house.

Agreed that the temperature will not be affected, but the flow will be.

If going with gas central heating, the hot water needs would be:
- 6 x radiators, 1 x heated towel rail

They are not part of the hot water needs.

2 x bath taps do you mean One bath with two taps, or two baths?

How is the power needs of the combi determined given these needs

That's putting the cart before the horse. First you have to check that the incoming flow rate and pressure is sufficient. Obviously one would not expect all outlets to be running at the same time, but it's quite feasible that someone could be having a bath at the same time as another person is having a shower. A minimum incoming flow rate of 20 litre/minute is suggested at a pressure of about 1 bar. This would supply two showers.

 

[propertydev83]

Thanks for the replies everyone. It would seem a still have a fair bit to learn here, and it's all too easy to get incorrect information from the internet (not referring to this site, BTW). Luckily, I have some really good literature from my father which covers central heating amongst a million other things to DIY, and hopefully it should be invaluable. So far I've only read as far as calculating heat loss but it goes into immense detail of every aspect and hopefully I should gain some expertise on the subject.

But I'll report back here later anyway at some point...