How long does your central heating take to heat the house?

TanDiy

macman said:

As above: assuming the heating was on the day before and went off at 10pm or thereabouts, it should not be dropping as low as 14.5C in about 8 hours.
You have told us lots about your boiler and rads, but absolutely zilch about the insulation. Do you have double glazing, cavity wall insulation, adequate loft insulation, draughtproofing?
Bear in mind that a combi cannot provide CH and DHW simultaneously, so if baths or showers were being taken between 6.30am and 10.15am then there would be no heating input during that time.
A combi would not be my choice for a large 4 b/r detached property with 2 or more showers or bathrooms.

Hi macman, thanks for coming back. The house is not particularly big. It is double glazed, thick loft insulation, no cavity wall insulation. Spent some time considering combi or unvented cylinder but pleased with the combi. Only one bathroom with a shower and a small cloakroom. Very impressed with the water. Seems pretty high pressure even when two taps are open and we have very short runs as the boiler is directly under the bathroom in the utility room next to the kitchen.

macman

I have a frost protection function on our programmer: it's set to 14C, but I've been away during cold weather with the CH otherwise off for a week or more, but it's never yet kicked in.
Once you get down to that temp, the structure of the house has lost a lot of heat and so getting the room temp back up will take quite a while, as the structure is rapidly absorbing the input of heat.
More info would be helpful, but I'd guess that the insulation is inadequate. It's quite possible that the older warm air system had a higher output rate, designed for the low insulation standards of the 1970's. You were getting the same heat loss of course, and the same high fuel usage, but less noticeable in terms of air temp at start-up.

FreeBear

TanDiy said: The only real problem we have is that the bathroom is not very warm but it is north facing and we had to put the rad on an internal wall rather than one of the two external walls. Need to rethink that one!

In a modern, well insulated house, there is no real need to put the radiator on an external wall or under a window. The reasoning of old for putting a radiator under the window was to combat the heat loss through the window which gave rise to cold draughts - With a decent double (or triple) glazed window, the heat loss is not that great.

David28

You should be using a lot less gas now.
I used to have warm air heating, I used between 18,000 kWh and 24,000  kWh of gas a year with it.
Five years years ago I replaced it with a combi and 8 radiators and the maximum gas usage in a year has been 14,600 kWh.
I find it does take longer to get up to the set temperature than with warm air.
My bathroom also seems colder but then there isn't warm air being blown in from the ceiling all the time.
It takes my 3 bed semi about 2 hours to reach 21 deg C with the outside temp below freezing

Jeepers_Creepers

Hi Tan.

Lots of good info there. I don't think you need to worry. This is happening at the coldest of times, so it's a temporary situation that you just need to cater for.

During the day and evenings your system manages to keep your house cosy enough - yes? Cool. So it's only that sluggish start in the morning that's really the issue?

Yes, to get the house up from 14-odd degrees is going to take some time. You could fit significantly oversized rads so that this will happen more quickly, but that seems a little silly when it's only for a few months of the year. 14oC is pretty chilly, so I think an obvious first step would be to set your overnight temp to 16 or 17oC - it'll be nicer for your house, and also give you a better start in the morn.

Do you have a PROG stat? I'd hope so. Try reprogramming it for that.

I do not agree with another poster who finds keeping the overnight temp at ~20oC is the way forward. Sure, that will give you a guaranteed cosy house to wake up to, but at the expense of larger energy bills; gawd made duvet covers for a reason. Let's get one thing straight - keeping a house at an unnecessarily-high temp might 'feel' like a good and comfy thing to do (and, if you are elderly, it might be) but it WILL cost more. It will. Heat loss from your house is directly proportional to the temp DIFFERENCE betwixt the inside and outside temps. The more you keep that difference high, the more heat that will be lost. Yes, a cold house will take longer to get up to temp than one that's been kept warm, but the latter will still be at the cost of extra energy. That is a fact.

You'll likely find that the boiler will only come on during the night on rare occasions even when set to 16/17oC, and - when it does come on - it'll only be for short intervals. The boiler and rads will find it easy to maintain this acceptable ambient temp, and it'll be more ready to get your house up to full temp when the demand is there.

It's also really not ideal to allow houses to swing hugely in temp like yours currently appears to; the warm 21oC house in the evening - full of moist air - is going to find lots of surfaces to condense out on when the temp plummets to 14oC for a start...

As you say, 80oC boiler temp is higher than you'd ideally want because the way a condensing boiler saves energy is by having its return temp as cool as possible. So, as soon as you can without losing comfort, tweak that down to as close to - or below - 70oC as you can.

So, two solutions: (1) is to keep the overnight temp to one that's comfy and not excessively low - I'd suggest 16-17oC as a good balance, or (2) set your Prog Stat to come on a half-hour or so earlier. I'd suggest the former as it's just not 'good' to allow your house to become too cold overnight.

Possibly your rads have been 'undersized' a bit. A clued-up plumber should know to 'oversize' rads to suit modern condensing boilers, the idea being that they can run on cooler temps whilst still giving out the same heat as 'traditional' systems (a large cool rad's output will equal a tiny hot one!). However, just bear in mind that this is an issue for a few months of the year, so you can sort the issue with some tweaks for that time; a slightly warmer overnight temp, and/or an earlier kick-in time. Once the weather improves, turn that boiler stat down :-)

TELLIT01

We have a 1930's 3 bed semi with cavity wall insulation.  Overnight it only dropped by about 2 degrees C from 20 to 18, and was back up to 20 within an hour.  Snow on the ground outside so guessing the outside temp as at or below zero.  Outside temperature is currently zero!

TanDiy

David28 said:

You should be using a lot less gas now.
I used to have warm air heating, I used between 18,000 kWh and 24,000  kWh of gas a year with it.
Five years years ago I replaced it with a combi and 8 radiators and the maximum gas usage in a year has been 14,600 kWh.
I find it does take longer to get up to the set temperature than with warm air.
My bathroom also seems colder but then there isn't warm air being blown in from the ceiling all the time.
It takes my 3 bed semi about 2 hours to reach 21 deg C with the outside temp below freezing

Hi David
Thanks for coming back. We have only had this for a couple of weeks but the gas usage, according to the thermostat (Bosch Easy Control) shows that we are using around two thirds of the gas we were using with the warm air. Only problem now is removing all the ducts from the loft and repairing the holes in the ceiling! 

TanDiy

Thanks Jeepers Creepers, really appreciate the valuable advice. I think we will give this a go, setting the overnight temp to around 16. 

ic

Have a dig in your settings on the bosch app for "optimum start" and enable that.  That way you set the programme for the time you want the room to be a temperature - say 7 am at 21 oC.  The thermostat will learn how quickly your system takes to heat up, and adjust the time it actually kicks in based on the weather and the temp inside the house, so on a very cold winter's night it might kick in at 4 or 5 am, whereas in the spring it may only turn the heating on 20 mins before hand as the house won't be so cold to start with.

Another thing to look for in the app is the setting to enable "weather compensation".  If available (depends on the your boiler model) it'll use the local weather conditions reported on the internet to adjust the flow temp of the water in the rads.  On a very cold day, it'll increase towards the max, whilst on a mild spring day the temp will be be much lower.  This is more efficient as your boiler will run more often in condensing mode, and should result in a more comfortable house.  Make sure the dial on the front of your boiler is set up towards max for the rads - as this is ultimately the maximum it'll run at.  (This basically automates what Jeepers_Creepers is talking about with tweaking the flow temp - let the thermostat do it for you second by second with the ever changing weather conditions).

dave23

We have a 4 bed detached built in 1997. Double glazed, good loft insulation and I presume from the build date cavity wall insulation. Conventional boiler 2 years old running 13 radiators.  Last night Hive tells me the temperature at 9.22 am was 15.1 C when the heating was switched and hit 21 at 12pm. The boiler would have been off for about 10 hours so the house does cool down when below freezing outside but does heat up quickly. In 2020 used 13600 kWh