over sizing radiators

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So.... Was i right or wrong..... I've highlighted your contradictions.

They are not contradictions.

You still need to calculations and are not going to throw a 3300W (@Delta 50), 1800 x 900mm behemoth into a small 1m x 1.2m ensuite in the centre of a house that has a 100w heat loss at -2C external temp.

Spec them too big and you'll pull too much heat out of them and some rads wont get as warm.

This is what's wrong.

The system simply needs balancing correctly for the heat loss and flow/return of the boiler. You won't "pull too much heat out of them", whatever that means.

Spec them correctly. Your boiler needs a drop in return temp in order for it to run efficiently .

Also not correct, your boiler needs to drop the flow temperature and subsequent return temperature (due to correct system balancing) to ensure it runs in the condensing zone for as long as possible.

Worchester 24i it's nearly 10 ys old attic has 2ft of insulation right to the walls .. no draughts to speak of .. i'll keep in mind what your saying replaced the bathroom rad today ..with a towel rail £66 .. put it on for 10 mins far better than the old one .. thx

You attic shouldn't have an draft from the internal part of the house, E.g. loft hatch. Ensure that is tight fitting or has a strip sleaing it well. Ideally get some insualtion board on that also. You don't want warm dry air escaping up into the cool loft space as that could lead to consdensation, damp etc.

The cold loft space should have good cross ventiliation, ideally coming in throught the soffits and possibly even some ventilated ridge tiles.

 

[dirtychinchilla]

Also not correct, your boiler needs to drop the flow temperature and subsequent return temperature (due to correct system balancing) to ensure it runs in the condensing zone for as long as possible.

His point was that the dT should be maximised, and you're saying minimise your flow temperature at the same time. Both together are important.

 

[jpaul]

is the differential ever important for efficiency - return temperature needs to be low and the flow is whatever is enough to provide heat, to heat the house.


I'd wondered if there was an online simulator looks so

eQUEST

https://www.njcleanenergy.com/files/file/Pay%20for%20Performance%20Contractor%20Portal/FY15%20Updates/Technical%20Topic-%20Pay%20for%20Performance%20-%20Modeling%20Condensing%20Boiler%20Measures%206-24-2015.pdf

much more fun than GTA

 

[dirtychinchilla]

It's very important because it dictates how low your flow temperature will be, which determines, in part, the efficiency of your system.

 

[mattyg]

They are not contradictions.

You still need to calculations and are not going to throw a 3300W (@Delta 50), 1800 x 900mm behemoth into a small 1m x 1.2m ensuite in the centre of a house that has a 100w heat loss at -2C external temp.



This is what's wrong.

The system simply needs balancing correctly for the heat loss and flow/return of the boiler. You won't "pull too much heat out of them", whatever that means.

Also not correct, your boiler needs to drop the flow temperature and subsequent return temperature (due to correct system balancing) to ensure it runs in the condensing zone for as long as possible.


You attic shouldn't have an draft from the internal part of the house, E.g. loft hatch. Ensure that is tight fitting or has a strip sleaing it well. Ideally get some insualtion board on that also. You don't want warm dry air escaping up into the cool loft space as that could lead to consdensation, damp etc.

The cold loft space should have good cross ventiliation, ideally coming in throught the soffits and possibly even some ventilated ridge tiles.

Why do I have 2 big 30KW boilers in my house? Its a big house but lets assume it was a small house with 1 30KW boiler but every radiator was twice the size and the same number, But the heat loss the same..... Would I need the 2nd boiler

And how does a boiler drop the flow temperature. Surely a boiler adds heat.
Isn't it the Radiators that take the heat out of the water, Usually about 10degrees if I remember.

 

[LuckyBenski]

You don't want warm dry air escaping up into the cool loft space as that could lead to consdensation, damp etc.

Sorry to nitpick but it's warm moist air that will lead to condensation... When it hits cold surfaces.

 

[jpaul]

It's very important because it dictates how low your flow temperature will be, which determines, in part, the efficiency of your system.

can see from eg. https://www.heatgeek.com/what-is-boiler-modulation/
that boiler efficiency is just(first order) a function of return temp , not outflow temp.

e: the low efficiency loss if you boiler were only cyclling slowly shown there is interesting too,
although seems Nest may not be able to control that versus other brands

Reddit - Dive into anything

www.reddit.com

 

[dirtychinchilla]

That's true for boilers but for heat pumps, your flow temperature will be a big dictator of your COP

 

[jpaul]

a heatpump delta t discussion -
discussing that it needs to be small to help heat-pump efficiency giving an even heat transfer inside its heat exchanger, so need a sufficiently high flow/pump rate to do that
point about modern radiators producing more even heat distribution (baffling ...) across radiator surface is interesting

 

[PishY]

Interesting about the Heatpumps, I’ve installed a few in new builds of our own now, due to the SAPS we can’t install combi boilers

We double the rad sizes due to the lower flow temps and it works well, but they work so much better with under floor heating if you have it!

I agree to spec the radiators to the size of the room for the ultimate efficiency with the correct flow temps though!

 

[-Ad-]

Why do I have 2 big 30KW boilers in my house? Its a big house but lets assume it was a small house with 1 30KW boiler but every radiator was twice the size and the same number, But the heat loss the same..... Would I need the 2nd boiler

And how does a boiler drop the flow temperature. Surely a boiler adds heat.
Isn't it the Radiators that take the heat out of the water, Usually about 10degrees if I remember.

That's crazy having x2 30kW boilers. Have you done a heat looks calc to work out your actual loss on the design temp for the coldest winter day (say -2).

Boilers have generally just been thrown in by plumbers with no regard to proper heat loss calcs and design.

Is it an older solid walled and what sort of sqm you looking at?

I'm at circa 240sqm, with solid walls downstairs, better walls upstairs with the extension to 1st floor. I'm at about 13.5kW heat loss at -2degsC and the boiler is sized at 35kW, so I'm range rating it downto 15kW for testing soon.

Sorry to nitpick but it's warm moist air that will lead to condensation... When it hits cold surfaces.

Lol, I know how it works, I used to work as a Mech Engi building services design

My loft had some issues due to insulation blocking up the ventilation gap from the soffit are to the loft, so I'm rectifying that before winter really hits. Plus the loft hatch needs sealing up and insualting.

Worchester 24i it's nearly 10 ys old attic has 2ft of insulation right to the walls .. no draughts to speak of .. i'll keep in mind what your saying replaced the bathroom rad today ..with a towel rail £66 .. put it on for 10 mins far better than the old one .. thx

Have a read up on Heat Geek for some good info on tweaking your boiler and balancing your heating system. They have good youtube videos also. Some knowledge and tweaks are free and can make the most difference, without having to spend anything.

 

[mattyg]

That's crazy having x2 30kW boilers. Have you done a heat looks calc to work out your actual loss on the design temp for the coldest winter day (say -2).

Why is it crazy?

 

[robj20]

They are not contradictions.

You still need to calculations and are not going to throw a 3300W (@Delta 50), 1800 x 900mm behemoth into a small 1m x 1.2m ensuite in the centre of a house that has a 100w heat loss at -2C external temp.



This is what's wrong.

The system simply needs balancing correctly for the heat loss and flow/return of the boiler. You won't "pull too much heat out of them", whatever that means.

Also not correct, your boiler needs to drop the flow temperature and subsequent return temperature (due to correct system balancing) to ensure it runs in the condensing zone for as long as possible.


You attic shouldn't have an draft from the internal part of the house, E.g. loft hatch. Ensure that is tight fitting or has a strip sleaing it well. Ideally get some insualtion board on that also. You don't want warm dry air escaping up into the cool loft space as that could lead to consdensation, damp etc.

The cold loft space should have good cross ventiliation, ideally coming in throught the soffits and possibly even some ventilated ridge tiles.

I had to add more ventilation after I redid all the insulation. I just put small bits of foam between the sheets of membrane to allow more air flow. Fixed the issue overnight.

I still have my spreadsheet for doing the central heating I sized my radiators to a delta t35.
So for instance the calculations for the master bedroom said I needed 1363W, after adjusting it was a 2164W radiator I needed and I got a type 21, 600x1800 at 2295W.

 

[-Ad-]

Why is it crazy?

What's the calculated heat loss of the house for winter design temp? This is what matters.

Unless you have them heating a swimming pool or huge annex, then it's either a country manor or has the the most un-insulated construction.

 

[Cyber-Mav]

we got oversized rads, works good, takes bit longer for initial heat to build but its good when its going. boiler set to 60c, know someone who had 80c+ boiler temp for rads and one time she fainted in bathroom and landed on rad pipe and got mega burns needing skin graft. 60c rad temp would not have done that

 

[Deleted User 298457]

I'm finding flow temps of 50 on my "kind of oversized" rads to be a game changer. The cost impact, despite moving from Tracker (hovering around ~5p) to Fixed (6.78p/kwh) and using heating every morning, I don't seem to be attracting super high bills (maybe an extra £1.40/day). The house actually hits 18 degrees too. Super happy with my insulation efforts!

 

[robj20]

Anyone doing all this should also check if your boiler supports Opentherm and if so use it.

 

[Deleted User 298457]

Anyone doing all this should also check if your boiler supports Opentherm and if so use it.

Do you think I'd recoup my costs? I have a Hive but have been tempted to switch to something that does OpenTherm...

 

[robj20]

Do you think I'd recoup my costs? I have a Hive but have been tempted to switch to something that does OpenTherm...

It's difficult to workout. I think I've saved a fortune since having Evohome. But I was fitting a new system anyway. But it often flows down to low 20c.
You could experiment and try to see how much you use at different flow temps to see if it makes enough difference to recover the cost.

It might not be worth a major investment but if it's something you already have but it's not wired up then changing it would be worth it.

 

[thebandit]

Just a comment on the oversizing of rads, I scan read through and don't believe anyone has mentioned the fundimental ... balancing. Minimum rad size is important, maximum not so much. Bascially you need a rad sized at least big enough to heat the room, if its bigger than the recommended its no issue. The most important thing to do with the heating system is balance the radiators correctly. This is what ensures all rooms heat well and the return to the boiler is low so it condenses and runs efficiently. There are loads of guides across the internet but basically you have to start at the first rad on the circuit, with the thermostatic valve fully open (removed ideally) , adjust the lockshield until you get ~12 - 18°c drop across the flow and return pipe on the rad. Move along the rads in the heating circuit until you have done all of them. You'll be surpirsed how little the lockshield needs to be open on the first rads in the circuit, often only a quater of a turn from fully closed. So many people have no clue about this, they think having everything fully open will be better, but whats happening in the background is the first rad with a fully open lockshield is pushing very hot water straight back to the boiler preventing it condensing and also stealing heat from the rads further along the heating circuit.