Radiator size and return to boiler temperature

[garden4ork]

Afternoon,

Recently had a new boiler fitted with new radiators, old radiators were the old steel type. The installer recommended reducing the size as the new panels are more efficient, which we were happy to do.

Now, I believe that for the output of the boiler the radiators are too small/not enough in the house. We're in a 70s detached. It barely gets to 20 degrees after 5 hours from around 12 degree start (5 degree outside) but radiators are very hot to the touch. Up in the attic next to the boiler there is what I assume to be the 'in' and 'out' of the central heating pipe. While there is a difference, it is quite minimal.

Had our first gas bill through and the amount of kWh used is way above average (62kWh) with boiler running around 8-9 hours a day

My question is if I were to increase the size of the radiators, this would obviously make the house heat up faster and get to temperature, but would it also be more efficient use of gas? (Particularly if it reaches the set temp on thermostat and goes off) The water going back into th boiler is reheated but what if it's too hot... If that makes sense?

Thank you for listening to my ramblings.

 

[ShaunCorbs]

Did he do a Heatloss Calc as you size the rads to each room Heatloss ?

 

[garden4ork]

Did he do a Heatloss Calc as you size the rads to each room Heatloss ?

I can ask, but I don't know for sure. I get the feeling he went for the smallest he could get away with as the original quote included boiler+rads - So smaller rads, cheaper price, more profit. I've had a look at the BTU calculators and they should be there or thereabouts but only just.

 

[SJB060685]

Radiators have a set output at a designed MWT (mean water temperature). To say modern radiators are more efficient and smaller ones are enough is not strictly true.

 

[ShaunCorbs]

I can ask, but I don't know for sure. I get the feeling he went for the smallest he could get away with as the original quote included boiler+rads - So smaller rads, cheaper price, more profit. I've had a look at the BTU calculators and they should be there or thereabouts but only just.

Get him back to replace them if there undersized at his cost

 

[Chuck]

"I were to increase the size of the radiators, this would obviously make the house heat up faster and get to temperature, but would it also be more efficient use of gas?"

Probably not to a worthwhile extent in practice.

Only if you were to increase the sizes to the point that the system could operate with low enough return temperatures that the boiler would stay well into the 'condensing region' of its operating range, say flow=65°C return=45°C. In practice, this would probably give you a reduction of 5% for a given thermostat setpoint.

But beware! When you reduce the temperature of the 'radiators' as a result of upsizing them you reduce the proportion of heat emitted by radiation (as opposed to convection). In my experience, makes the occupants feel colder so they want to turn the the thermostat up a degree or so, which wipes out the benefits.

If your house drops from 20° to 12°C overnight when it's 5°C outside it must be rather poorly insulated. Have you got double-glazing, loft insulation, etc.?

 

[garden4ork]

"I were to increase the size of the radiators, this would obviously make the house heat up faster and get to temperature, but would it also be more efficient use of gas?"

Probably not to a worthwhile extent in practice.

Only if you were to increase the sizes to the point that the system could operate with low enough return temperatures that the boiler would stay well into the 'condensing region' of its operating range, say flow=65°C return=45°C. In practice, this would probably give you a reduction of 5% for a given thermostat setpoint.

But beware! When you reduce the temperature of the 'radiators' as a result of upsizing them you reduce the proportion of heat emitted by radiation (as opposed to convection). In my experience, makes the occupants feel colder so they want to turn the the thermostat up a degree or so, which wipes out the benefits.

If your house drops from 20° to 12°C overnight when it's 5°C outside it must be rather poorly insulated. Have you got double-glazing, loft insulation, etc.?

Double glazed, loft is insulated with 200mm on the front, 50mm at the back. Not sure why, only been in the house a few months so was prev owners choice. I am looking to increase to 300mm all over.
Not sure on cavity wall insulation, surveyor only mentioned cavity walls - Not the insulation part.
I'd say it reduces to 12 degrees from 20 as it turns off at roughly 10pm and back on at 4pm with the boiler only on for two hours in the morning, 6-8am, which takes the chill out. So it's at it's coldest around 2-3pm because the morning boost doesn't do a huge amount due to the small size (or so I believe)

Boiler says that the rad temp is 82 degrees, installation paperwork mentions a weather controller.

 

[Gasmk1]

If you have weather comp then it’s not working as boiler should be on all day and runs at low rate. One of your problems is heating isn’t on long enough to warm up the house.

 

[Chuck]

Double glazed, loft is insulated with 200mm on the front, 50mm at the back.

IMO, 50mm is definitely worth increasing to 200mm. The benefits of changing from 200mm to 300mm are more marginal.

Not sure on cavity wall insulation, surveyor only mentioned cavity walls - Not the insulation part.

I am not a fan of retrofitted cavity-wall insulation for pretty much the same reasons that Jeff Howell gives in this article

Cavity Wall Insulation Problems

Jeff Howell discusses the cavity wall insulation problems they don't want to you know! An honest and factual report that will make you think twice about it.

www.askjeff.co.uk

This is particularly applicable in my region, the southwest, which has a warm and wet climate.

I'd say it reduces to 12 degrees from 20 as it turns off at roughly 10pm and back on at 4pm with the boiler only on for two hours in the morning, 6-8am, which takes the chill out. So it's at it's coldest around 2-3pm because the morning boost doesn't do a huge amount due to the small size (or so I believe)

Boiler says that the rad temp is 82 degrees, installation paperwork mentions a weather controller.

As @Gasmk1 says, the weather compensation needs to be set up correctly. In all but the coldest weather it should reduce the the flow temperature significantly and let the heating run for longer at lower outputs.

IMO, a house should be kept at a minimum of 16°C. Swinging the temperature between 20°C and 12°C as you are currently doing is a recipe for creating damp / condensation / mould problems.