Will I regret getting a combi?

ThisIsWeird

FreeBear said:

kittennose said: Not many locals want to do a standard boiler, either 40KW combi or system boiler and unvented.

With a flow rate of just under 12l/min, a 40KW combi is way over the top. I have a 10l/min flow, and a ~25KW combi was the recommended size. Opted for a (nominal) 30KW as I wanted a bit of spare capacity for when I fit a booster pump. This particular boiler is good for 12l/min according to the manufacturer's spec sheet. But I suspect a 32-35KW would be better.

I suspect a 'slip'. I doubt any plumber was recommending a 40kW combi.
Kitten's house must be pretty large, tho', as the heat-only/system options are themselves 30kW - that is BIG!.

Good point about going for a slightly larger combi than what would 'suit' their current pitiful water supply, tho', should Kit go the combi route. A 24kW jobbie is just pitiful, and doesn't leave any margin for improvement if the mains water is somehow boosted.

Kitten, are the plumbers really recommending a 30kW Heat-Only boiler? That's big.

housebuyer143

ThisIsWeird said:

kittennose said:

ThisIsWeird said:

"Keeping what we have is an option but clearly a harder one as we spend £3,500 and gain nothing in return other than maybe 20% off our gas bills." What would cost £3.5k in this scenario?

A regular boiler replacement + clean and a new central heating pump.

Thanks. I was afraid of that. 
Who the heck is charging £3.5k for a replacement heat-only boiler?
Is there anything unusual in this installation? In the middle of the house, so tortuous flue route?
A friend had a new H-O boiler fitted, ok, two years ago, which necessitated a condensate pipe route. £1.7k.
I, perhaps, wouldn't grumble at even £2.5k IF it included a FULL power-flush, but £3.5?! Why?
Who quoted for this, and who else has quoted?

I have had a few quotes for a standard boiler and honestly, I can't find one for less than £3.5k with power flush. The boiler is only £1300 so £2200 for probably two days labour feels like I'm being taken for a ride. 
British gas were cheaper than the local engineers! 

FreeBear

ThisIsWeird said: Kitten, are the plumbers really recommending a 30kW Heat-Only boiler? That's big.

My old Baxi back boiler was rated at 13KW, and even that was oversized for this 3 bed semi. A 30KW heat only boiler would be fine for an industrial unit, but waaaay too big for most domestic properties. With the (new) combi boiler that I have, it fires up at around 25-30% of capacity (24KW for CH), and then drops down to 10-15% for much of the time. That equates to 3KW up to 7KW of actual heating.

Some plumbers just don't have a clue when it comes to boiler sizing.

Postik

Section62 said:

Postik said:

I do find the idea of heating a huge tank of water very wasteful.

The problem with the above is people imagining the heat from a hot water cylinder vanishing into thin air at a rapid rate.  The reality is a well-insulated hot water cylinder kept in a space like an airing cupboard is not losing heat at anything like the rate people might imagine.

Yeah I get that.  My plumber pointed out the modern tanks are incredibly efficient and lose the heat very, very slowly.  However we aren't showering every day and putting on the hot water every day.  The longer we go without, the more hot water gets wasted over time.  This feels like it's compounded by the fact that we have no clue how long to actually heat the water for, given our usage.

We try to make a best guess - for example having it on for 20 minutes is fine if one person has a shower.  Could we get away with putting it on for just 10 minutes - possibly in the summer if your shower is short, but not if you have a longer shower in the winter.  So you're left sticking it on for 20 minutes any time of year and then giving it an extra 5 minutes "for good measure".

I think if you had 4 people who all took at least one shower a day then you'd just put it on for an hour or two every day and what isn't used up would remain hot until the next day when it's turned on again.

Perhaps one way or another the difference in cost is negligible compared to a combi.  It's just with a combi you know without question it only heats the water that you need.

FreeBear

Postik said: We try to make a best guess - for example having it on for 20 minutes is fine if one person has a shower.  Could we get away with putting it on for just 10 minutes - possibly in the summer if your shower is short, but not if you have a longer shower in the winter.  So you're left sticking it on for 20 minutes any time of year and then giving it an extra 5 minutes "for good measure".

One should have a thermostat on the tank. That will remove any guesswork as you just turn the boiler on, and the thermostat will turn it off once the water is up to temperature.

Albermarle

FreeBear said:

Postik said: We try to make a best guess - for example having it on for 20 minutes is fine if one person has a shower.  Could we get away with putting it on for just 10 minutes - possibly in the summer if your shower is short, but not if you have a longer shower in the winter.  So you're left sticking it on for 20 minutes any time of year and then giving it an extra 5 minutes "for good measure".

One should have a thermostat on the tank. That will remove any guesswork as you just turn the boiler on, and the thermostat will turn it off once the water is up to temperature.

Will not the thermostat only turn off when the whole of the water in the tank has reached that temperature?
Most houses do not need a full tank of water each day ( especially if you have a decent sized tank).
I thought that if you just put the water heating on for 20 mins ( for example), there would be enough hot water in the top of the tank for a shower, without having to heat the whole tank?
Seems a waste for the OP ( or anybody ) to heat the whole tank for one short shower a day.

Lorian

An oversize boiler is not so much of an issue if it has a high modulation ratio and weather or load compensation. I see initial heats in the morning at about half the maximum and generally runs at 6kW/h once warmed up and modulated down.

This is on a system boiler  there is a setting to range rate it too but I don't need to use it.

Section62

Postik said:

Perhaps one way or another the difference in cost is negligible compared to a combi.  It's just with a combi you know without question it only heats the water that you need.

Except that statement is open to question.

There will be (1) water in the 'hot' pipe between boiler and the last tap/appliance used, and (2) unless set up for economy, many combi boilers will cycle on periodically to maintain a store of already heated water (pre-heat).

1) goes back to the point FreeBear made about the location of the boiler relative to the hot taps that are used the most.  If the boiler is close to where most hot water is used then the losses will be lower than where the boiler is on the opposite side of the house to the most-used hot taps.  If it is impractical to have the boiler close to the most commonly used hot taps then the efficiency of a combi boiler system will be lower - it may be the heat loss from a hot water cylinder is less than the heat lost in long runs of pipe from a combi boiler to where the hot water is used.

2) If 'pre-heat' can be turned off then more water will need to be run off before hot water comes through, and the boiler and pipework will still contain hot water after the hot tap is closed.

The simple answer is that there is no simple answer - the most efficient system will depend on the circumstances of the building and the people living in it.

FreeBear

Section62 said:

Postik said:

Perhaps one way or another the difference in cost is negligible compared to a combi.  It's just with a combi you know without question it only heats the water that you need.

There will be (1) water in the 'hot' pipe between boiler and the last tap/appliance used, and (2) unless set up for economy, many combi boilers will cycle on periodically to maintain a store of already heated water (pre-heat).

Most combi boilers also have a small quantity of water inside that must be heated up first - With my boiler, this is about 6l.

About the only boiler that doesn't have this problem is the Intergas range as the heat exchanger heats both DHW and CH water. Also no diverter valve or secondary heat exchanger.

ThisIsWeird

In essence, Kitten, if you don't do anything to boost your incoming cold mains, then all the alternatives to your current system will almost certainly be worse, and costly.
At least, with a vented system as you have, you have a large supply of stored water at your disposal, and it's then pretty straight forward to give that a useful boost to provide everything you could want.
How many rads do you have? Have your plumbers given you an assessment of your total kW needs with a H-O or system boiler? Is it really 30kW? That's a hell of a lot.
Most folk, with a stored hot water system, would manage with 20kW or a bit above, and I can't see why this should cost much more than £2.5k, and then add a couple of Grundfos boosters.
Gush a gogo.