Any downside to installing larger radiators to allow for lower boiler flow temperature ?

Inigo_Montoya

We are going to be replacing our boiler (15 years old) and also the old style radiators (40+ years old)
- its a fairly large house with about 14 radiators that need replacing

We are going to install a new gas boiler rather than a Air Sourced Heat Pump

The existing radiators are mostly old style double panel rads with no fins but also a couple of single panel rads with no fins

Since modern rads with fins have a higher BTU output compared to the same sized old style rads with no fins
we are wondering if we can install these (same sized) higher BTU type 22 modern finned rads and then run the boiler at a lower temperature ?

Another benefit is that we would be future proofing the rads if we later have to install an ASHP which would 
have to run at a lower temp with larger rads

Also apparently a condenser boiler is more efficient when run at a lower temperature ?

For reference I did a quick check on a couple of the rooms & it is looking like the (same sized) modern rads will produce about
80 to 90% higher BTU than the current old style rads

Is there any downside to this plan ?

I am also aware there are online tools to calculate the required BTU output for a given room but have not explored this yet
eg https://www.plumbnation.co.uk/heating-calculator/
- these tools are not ideal in my planned scenario as they assume that the boiler will be running at quite a high flow temp (eg delta-T = 50 degrees)

when you do google searches about oversized rads there seems to be quite a lot of discouragement about doing it with
the advise saying you will end up wasting energy - however I dont see this would be an issue if you both run the boiler
at a lower temperature & have TRV valves on the rads so they will turn off when the required room temp is reached ?

BUFF

 a new replacement system design should now be for a max. radiator flow temp of 55C (your old one was probably for 80c, delta T 50C) so when checking outputs/sizes make sure to use the appropriate output values (probably delta T 30) for your needs. https://www.stelrad.ie/home/stelrad-correction-factor/  but as a rough rule of thumb doubling the output of your existing rads should roughly give you the correct sizing to maintain output whilst dropping flow temp from delta T 50 to delta T 30. If you can increase it even more then you can drop the flow temp even lower.
Yes, the lower that you can run flow temp the more efficient a condensing boiler will become (probably down to ~40C/delta T 20 & heat pumps will like it even lower again for their best COP.

Modern controls will also help, not only will you will have to have TRVs on all rads (bar possibly a bypass or the one where a room stat is also installed) but allowing for advanced modulation (load/weather compensation) between the boiler & controls will also further lower energy use.

Inigo_Montoya

BUFF said:

 a new replacement system design should now be for a max. radiator flow temp of 55C (your old one was probably for 80c, delta T 50C) so when checking outputs/sizes make sure to use the appropriate output values (probably delta T 30) for your needs. https://www.stelrad.ie/home/stelrad-correction-factor/  but as a rough rule of thumb doubling the output of your existing rads should roughly give you the correct sizing to maintain output whilst dropping flow temp from delta T 50 to delta T 30. If you can increase it even more then you can drop the flow temp even lower.
Yes, the lower that you can run flow temp the more efficient a condensing boiler will become (probably down to ~40C/delta T 20 & heat pumps will like it even lower again for their best COP.

Modern controls will also help, not only will you will have to have TRVs on all rads (bar possibly a bypass or the one where a room stat is also installed) but allowing for advanced modulation (load/weather compensation) between the boiler & controls will also further lower energy use.

thanks thats clarified my thinking a bit 

I also found this web page that suggests the heat output at delta 30 is about half that at delta 50 -

I assume that means if you know the BTU of a given rad at delta 50 then you half that figure to give the heat output at delta 30 i.e. you are looking a for a delta 50 rad with double the BTU you actually want

https://heating-style.co.uk/pages/delta-t-conversion

this seems to fit in reasonably well of my idea of replacing the double panel old rads with type 22 ones of the same size as my calculations on a couple of the rooms were that the new rad would produce about 80 to 90 percent more heat output

ThisIsWeird

You are doing the right thing. The only downsides are the initial cost and the physically larger sizes of the new rads. Everything else should be a plus.

gm0

To really make this worthwhile for the ASHP later you need to seriously tackle matching heat load for (almost) peak with distribution heat capacity.  And then add size (and maybe size and switch to K3 triples) until you get back to the right distribution power output at the desired circulating operating temperature - which should be as low as possible.    Yes that DeltaT20 number.

Peak lopping max demand from worst of winter and most heatpump hostile conditions.  A plan for worst case supplementary heating in the overall design will allow the main heat pump system in future to be a smaller less power hungry heat pump than it would otherwise have to be to cope at peak while defrosting itself etc. Taking DHW off (under those conditions) to the immersion and just let it get on with CH and defrosting itself now and then

You want the heatpump in its output vs peak power sweet spot.  And at a CoP efficiency sweetspot for heat out for electricity in.   And that means low low water temp in the radiators. And that means a really big radiator area for a given kW to room

It doesn't matter what heats the water. Emitter physics is just hot water (delta), materials and area.

So could be 2x to 3x bigger than a hot running gas system - depending upon the comparator prior.
But less if you increase target temperature - which reduces efficiency and increases future running costs.

I haven't done my full sizing yet but I have a large existing kero double radiator system without UFH. 
So I looked at the Stelrad K3 data sheets for lower temperature operation on triples. 

Sizing up doubles at 40C would be rapidly scary big to be confident of low temperature ASHP operation in my application.  So remodelling what space is available for bigger/extra with K3. 

Within the constraints of not causing corrosion (boiler data sheet) you can of course benchmark an existing system simply by reducing the flow temperature and seeing what happens in room on existing rads.  Testing better than estimating.

ASHP are certainly improving with higher temperatures with different coolant technologies and compressors and electronics - and this may well continue further for a while.  If it does - the required size upgrade may reduce with the higher temperature (for a given running cost).  Which would help in larger applications or where space constraints make the radiator upgrade undesirable.

I don't like the additional power cabling requirement, noise, additional component failure modes, attendant on fan boosted convectors.  But some people are fine with them.

Good luck

FreeBear

gm0 said: To really make this worthwhile for the ASHP later you need to seriously tackle matching heat load for (almost) peak with distribution heat capacity.

And to reduce the max heat load required, insulate as much as you can. 300mm of fibreglass/rockwool in the loft. Wall insulation where practical, and under the floors if possible. Plug those cold draughts at source. Draught proofing and loft insulation are both relatively low cost and well within the capabilities of most able bodied people.

Insulating the walls can be done on a DIY basis if you have the confidence (keeps the costs down), else it is an expensive proposition - I'm slowly doing the solid brick walls here. Messy work, but the material costs are well under £500.

RonRamsgate

Well done for not going down the heat pump route, maybe when your new gas boiler gives up. Then consider heat pumps.
Fitting larger radiators needs the existing water pipes supplying them to be of sufficient diameter. Maybe they are already.
Metal panel radiators have been around for 50+ years and not much has really changed, have fitted die cast aluminium radiators. Their design offers better radiant & convection. They are used abroad but not so popular in uk. Nordic Epsilon, Faral tropical, Fondital calidor are Ok die cast aluminium radiators.
Consider rearranging the flow pipe into a top connection on the radiator, can be made to look ok and gives around 10% more output.
Would choose a Baxi gas boiler as first choice. linked to an 'opentherm' thermostat or evohome controls Its all doable.

Just needs installers to think outside of the box't

tacpot12

Only other dwonside I can see if that the larger radiators will be heavier. You might find you need better/more fixings to mount them solidly to the walls.