What size heat-only boiler?

[gerbils]

Finally decided to replace our non-condensing boiler and decided to stick with a standard heat-only boiler and maybe get a new hot water cylinder with it. The question we now have is what size to get? We are 4 Bed Detached with 13 Rads (old & single panel).

Our total heat loss is 7.76 kW and total energy demand is 19616.70 kWh.

I would like faster water heat-up time. Not sure if this is best done via a new cyclinder or more powerful boiler.

 

[MNW67]

We keep toying with something very similar and have also been wondering how to convert heat loss into an appropriate size for a boiler.

Just as a preliminary, how confident are you on the heat loss?

One common recommendation is to add 2KW or 3KW for the hot water cylinder. Then I've heard others say to add the whole output of the cylinder coil, which can easily be 15KW or more. Neither actually sound completely sensible to me. One of our installers quoted for 18KW and the other 30KW!! Our heat loss is about 11KW by calculation.

 

[desktop987456]

I upgraded my non condensing boiler to a condensing one this winter.
- I had 40kw boilers (2) to heat the house and the property had no insulation (these were the days when gas was really cheap; I am not in UK, outside temperature in my area remains in single digit)
- I did room by room calculation and heatloss was ~11kw for 12 rads (big and small). I don't use all the rads (for guest beds etc) but 4 big rads, 3 bath rads and a fan coil unit runs all the time.
- I oversized rads since I wanted lowest flow temperature on boiler that I could go with. (to run boiler efficiently with low return temperature)
- I opted for Viessmann 13kw heat only boiler and is doing the job perfectly. Flow temp on boiler is 50DegC and I keep room stat at 19-20DegC.
- I run CH from 0800-1100 and 1600-2130 daily, Gas consumption ranges between 5-8 kw. If I run it all day it consumes 8kwh.
- For HW I have separate arrangement but Pros say to add 3kw for HW when selecting a boiler.
- I have observed that my heatloss calculations were correct for upstairs but for downstairs they were skewed on higher side since downstairs rooms are overheated (probably since I have heated rooms directly above them on first floor).

Hope this helps. Go with an appropriate sized boiler for your property between 12-15kw should do the job based on the information you have given. I am not a pro but just a home owner like yourselves.

 

[Johntheo5]

The table is a bit confusing, the sum of all the heat losses comes to 11.8kw, then shouldn't the T50 sized rads outputs sum to the same and not 16.58kw??, the same applies to the T25 sizing, the oversizing factor for each rad should be (50/25)^1.3, 2.462?, and the sum of all these should come to 11.8kw when run at 50C flowtemp and not 8.1kw.
At a room temp of 20C a T50 rad will give 100% output with flowtemp/returntemp/dT/flowrate of 75C/65C/10C/1.43LPM (per kw rad output). the same rad(s) oversized by a factor of 2.462 (say 2.5) will give the same output with flowtemp/returntemp/dT/flowrate of 50C/40C/10C/1.43LPM.

What boiler return temperature are you getting?.

 

[desktop987456]

sum of all the heat losses comes to 11.8kw

Correct.

T50 sized rads outputs sum to the same and not 16.58kw?

My rads oversized and I have stated the name plate or stated output of the rad in this column.
The next column is oversized factor which is 'Heatloss divided by stated output'

oversizing factor for each rad should be (50/25)^1.3, 2.462?, and the sum of all these should come to 11.8kw

Correct. I don't know the return temp to boiler but flow temp of boiler is 49.5DegC. Rooms get to the temperature after an hour of boiler startup. A normal thermometer tells mean water temperature of 37DegC in a radiator so I am guessing my rads are 30DegC. what do you think?

What boiler return temperature are you getting

I have felt the return pipe on boiler which seems cooler; my best guess is it in early 40DegC but it is just a guess have not measured it. I have collected condensate from the drain pipe and it measured 1.5 litres per hour of condensate at this flow rate which tells me that it is condensing.

I am trying to get the efficiency numbers but they are all over the place as it is difficult to get the true gas usage for CH only due to stove and hot water usage.

 

[desktop987456]

For rads output at different flow rate I use the attach table which is roughly the same as you stated in your post. So in my case I just multiply the T50 rad output with Rad temperature rating e.g. 51.5 in my case hence the 8.1kw number I am getting in my spreadsheet against 11.8kw of heatloss. I hope it makes sense?

 

[Johntheo5]

I can't post files on here but you can easily build your own spreadsheet(s) which will be 100% correct.
As you probably know you just configure the first row and then copy and paste for any required extra rows/calculations required.
I have showed the effect of doubling and halving the flowrates on a "T25" rad with a 50C flow temperature.

 

[gerbils]

We keep toying with something very similar and have also been wondering how to convert heat loss into an appropriate size for a boiler.

Just as a preliminary, how confident are you on the heat loss?

One common recommendation is to add 2KW or 3KW for the hot water cylinder. Then I've heard others say to add the whole output of the cylinder coil, which can easily be 15KW or more. Neither actually sound completely sensible to me. One of our installers quoted for 18KW and the other 30KW!! Our heat loss is about 11KW by calculation.

We paid someone to do it so I assume it's decent.

They say "your maximum heat requirement on a very cold day (-4.6°C outside) is approx. 8kW" but this would be undersized according to every quote we've had.

 

[fixitflav]

I would like faster water heat-up time. Not sure if this is best done via a new cylinder or more powerful boiler.

If it's an old cylinder the heat-up time is limited by the design and by how badly furred it is. More powerful boiler makes no difference. Mine's an old one and I estimate heat input ~ 3kW, same as an immersion. Takes 30 mins to reheat and the boiler cycles on/off 3-4 times (old non-modulating boiler, I'd guess yours is the same as it's non-condensing).
Modern cylinders have higher heat input, link https://electricaldealsdirect.co.uk/advicecentre/kw-ratings-coils-indirect-cylinders.html
but I don't know how long they stay at that performance!
What's the rating of your existing boiler? 7.76kw sounds a bit sparse for your house.

 

[ericmark]

I have a 19 kW non condensing oil boiler for a 5 bedroom detached house with three floors, it can maintain the house with no problem, but heating from cold is another story.

So the work around was not to heat all rooms together, we have programmable TRV heads in most rooms, and we like the house to cool over night, and then warm up through the day, the hall is kept cooler than the living room, and if we leave the house the main thermostat auto turns down the heating. (Geofencing)

However on return reheating can take a couple of hours, unlike our old gas fired system, this one uses micro bore piping and although temporary turning down rooms not in use will speed up heating of used rooms, we find the boiler then starts to cycle, where with the old house we had no idea to if the boiler was modulating, it was running but no idea if at 10 kW or 28 kW there was nothing on the boiler to tell you. (Worcester Bosch)

But the point is the losses are nothing when compared with re-heat time. OK now retired so rarely leave the house so geofencing not used much, and using times can work out far better as you can set kitchen, then dinning room, then living room, and after a large delay bedrooms, so all rooms not re-heated together, and this worked with old house, but not with this house to same extent as the radiators in main rooms are not big enough to sink all the energy the boiler can produce.

But unless you live in a care home, it is unlikely you want the same temperature 24/7 in all rooms, so looking more at re-heat time to what losses there are.

As to domestic hot water that is likely part of my problem, as I can't control how much energy goes to heat it, using C Plan and no valve of any sort to turn it off, summer it is heated with solar, using an iboost+ it uses solar once everything else is satisfied, so no longer run boiler in the summer.

I think my problem can be cured with a second thermostat in the living room in parallel to one in the hall, but wife deciding she wanted kittens has messed up the central heating, as they scratch at closed doors, and leaving doors open upsets the balance between rooms.

 

[MNW67]

We paid someone to do it so I assume it's decent.

They say "your maximum heat requirement on a very cold day (-4.6°C outside) is approx. 8kW" but this would be undersized according to every quote we've had.

Is maximum heat requirement the same as heat loss. Is there any more detail. Does it say whether that includes an adjustment for heating the cylinder.

If 8KW includes everything, are they suggesting that this would be the ideal size for your boiler? It's hypothetical because I believe the smallest boiler would be 12KW. But say you could get an 8KW boiler, I think that would present problems in practice for the way in which many people actually live their lives. It means that on a very cold day, the heating would have to be on 24/7 at maximum output just to stop the house temperature falling. Some people can't sleep if they can hear the heating is on so the house would go cold overnight and then couldn't be reheated quickly. Lots of people simply need a house which is considerably warmer than 21C. Sometimes people just feel cold and need to quickly boost the temperature. If you go on holiday and return in a cold spell and don't have smart controls, it could take days to bring the house back up to temperature. Most people don't have the first clue how to run a heating system in any event. For these reasons, and others I am sure I haven't thought of, I believe it is sensible to build in contingency. The question would be how much.

 

[Mottie]

18Kw. That should do. We have a 3 bed semi with 10 rads and an 18Kw boiler. I have set it to 15Kw max output and it’s been fine but it’s good to know I have some reserve if it got really cold or added an extra room. Remember, you can always set the max output lower but if it’s too small, you can’t set it higher.

OP, what is the output of your existing boiler and is it sufficient for your needs?

 

[Johntheo5]

I would suggest +25%, if your heat loss has been calculated to perfection and is 100% correct it must be remembered that rads will emit far more than their designed T50 rating if the room temperature is low on a very cold morning after being off overnight. It wouldn't be unusual to see a room temperature of say 15C after being off overnight at a -5C OT, if the flowtemperature can be raised to 75C then the T50 rated rad (T50 rated at 20C room temperature) will emit almost 12% more heat, if you have been away for a few days then the room temps can be down to 10C, in which case the rads, with a flowtemp of 75c will emit 24% more heat, if you "oversize" the boiler by say 25% then that ensures that the boiler will reach 75C reasonably rapidly from a cold start and heat the room(s) far more rapidly, it will also take care of the HW demand IMO as this can be programmed to suit.
The only problem with specifying a still bigger gas boiler is that even with range rating, the minimum output might be say 6.5kw instead of 4.5kw which is very important now with zoning, WC and all that jazz as gas boilers fire up at ~ 65% of their max output which is a challenge for some to get away without numerous cycles.
Gas boilers have been well behind the curve IMO in providing low minimum outputs at ~ 2.5/3.0kw, Viessmann do make one down to 1.9kw but its not cheap.

 

[MNW67]

Remember, you can always set the max output lower

Some boilers still don't let you do that including, I believe, the two biggest sellers.

 

[Mottie]

Some boilers still don't let you do that including, I believe, the two biggest sellers.

Pick one that does then. (Who are they?)