Heat pump radiators

[Clark Kent]

Had a heat loss survey with a Heat Geek, when it came to the topic of radiators the guy pooh-poohed column rads. He suggested they weren’t efficient and wants to use type 21 or 22.

 

I’m not after using Victorian cast iron ones, just something slightly more aesthetically pleasing.

 

I know that heat pumps run at a low temp so you have to have a decent surface area but I would have thought BTU’s were BTU’s…..

[Dillsue]

You are right that BTUs are BTUs and watts are watts. No matter what the style it's the output that matters, but..... The output of the rads needed as the flow temperature drops increases quite considerably. Manufacturers supply correction factors for their rads at lower flow temps so once you have the loss for a room you can size the rad for your planned flow temp. I'm designing at 35deg C and have just finished installing the last of the replacement/additional rads and there's some big uns gone in!!

 

Unless a room has a very low heat loss the fancy tall rads without any convectors will likely give a very low output at HP low temps. Do the calcs and see for yourself.

[nod]

We have eight SS in the bedrooms landing etc We are unlikely to use them But when we have tested them They don’t run hot from a HP The same radiators ran very hot from a gas boiler on our previous build 

You would have to use them like UFH 

man’s let them heat up slowly and let the thermostat keep the temperature within a couple of degrees of what you want 

 

Our upstairs tracks the ground floor UFH by about 1 degree less Same as our previous 

[Clark Kent]

Thanks for your replies.

 

I’ve done a fair bit of research on heat pumps and am quite aware that the rads will probably feel like they aren’t even on. We’re only putting them upstairs as we’re having UFH downstairs.

 

So basically without the convector's the column radiator isn’t as efficient and will need to be even bigger to work with the low flow temps?

[SteamyTea]

47 minutes ago, Clark Kent said:

BTU’s were BTU’s

 

32 minutes ago, Dillsue said:

You are right that BTUs are BTUs and watts are watts

Watts (W) are not British Thermal Units (BTU).

1 W = 3.412141633 BTU.h^-1 

The h = hour (3600 s).

It is a very important distinction.

A BTU is an energy unit, as is a watt-hour (Wh).

BTU.h^-1 and watt are power units.

 

If you hear an electrician, plumber or energy 'specialist' say killa wot per our, start to run away.

 

[Dillsue]

1 hour ago, Clark Kent said:

I’ve done a fair bit of research on heat pumps and am quite aware that the rads will probably feel like they aren’t even on. We’re only putting them upstairs as we’re having UFH downstairs.

At 35deg C+ you'll know they are on but they won't feel toasty hot like a typical boiler fed rad.

 

1 hour ago, Clark Kent said:

So basically without the convector's the column radiator isn’t as efficient and will need to be even bigger to work with the low flow temps?

Do the calcs. Your heat geek calcs should give you loss per room so you know the output you need. Use the manufacturers figures to give you the adjusted output you need at low temps and you can then pick your rad/rads.

 

Any rad without a convector is going to be less efficient than the same rad with a convector but you won't know what you need without doing the calcs

[JamesPa]

To give an example of the calculation:

 

Radiator output is normally (but not universally) quoted for a average deltaT (rad to room) of 50C.

 

Convector rad output is approximately proportional to deltaT^1.3 (manufacturers do quote exponents very slightly different to 1.3, but they are almost always close to 1.3). 

 

So a convector radiator operating at ft 35 and DT (flow to return) of 5, target room temp 20, will have an average deltaT of 35-5/2-20 = 12.5.  The output will therefore be (12.5/50) ^ 1.3 = 0.16 times the output at deltaT 50.  Thus a radiator quoted as 1kW at deltaT 50 will emit only 160W at deltaT 12.5.

 

You can invert this to get the 'oversize factor', in this case 1/0.16 = 6.25, the figure by which you must multiply the calculated loss to get the radiator output (at deltaT50).  For any given type of radiator (unless it is very short or very tall) the oversize factor is a constant so this is a very convenient way to work out which radiator from a range you need

 

Column radiators may have a different exponent than 1.3, or even a different basic formula (although the latter is unlikely); you need to check with the manufacturer.  There is, however, a fair likelihood that the same formula will apply.

 

Incidentally if you could pump a bit harder and reduce the DT to say 3, the oversize factor reduces to 5.25.

Edited Wednesday at 19:48 by JamesPa

[Clark Kent]

Thanks for your replies 👍

[marshian]

5 hours ago, Clark Kent said:

Thanks for your replies.

 

I’ve done a fair bit of research on heat pumps and am quite aware that the rads will probably feel like they aren’t even on. We’re only putting them upstairs as we’re having UFH downstairs.

 

So basically without the convector's the column radiator isn’t as efficient and will need to be even bigger to work with the low flow temps?

 

So size the rads to work on the same low flow temp as the UFH based on the heat loss of the upstairs rooms (allow a bigger oversize for bathrooms because ventilation losses tend to be larger - extract fans / opening windows etc)

[sharpener]

16 hours ago, marshian said:

 

So size the rads to work on the same low flow temp as the UFH based on the heat loss of the upstairs rooms (allow a bigger oversize for bathrooms because ventilation losses tend to be larger - extract fans / opening windows etc)

 

This may well result in infeasibly big rads whatever their construction. But the fancy column radiators do not have as much emitting surface area per unit frontal area as a conventional Type 22 or Type 33.

 

Our installers designed for a flow temp of 45 and used a (conventional) rad size uplift factor of 1.9. This is a compromise with the partial ground floor UFH set to 35C, but a mixed system is going to have very big rads indeed unless you sacrifice some efficiency. As were were migrating from an oil system we are happy enough if it costs no more to run.

[marshian]

50 minutes ago, sharpener said:

 

This may well result in infeasibly big rads whatever their construction. But the fancy column radiators do not have as much emitting surface area per unit frontal area as a conventional Type 22 or Type 33.

 

Our installers designed for a flow temp of 45 and used a (conventional) rad size uplift factor of 1.9. This is a compromise with the partial ground floor UFH set to 35C, but a mixed system is going to have very big rads indeed unless you sacrifice some efficiency. As were were migrating from an oil system we are happy enough if it costs no more to run.

Yes and Yes - very house and heat loss specific - I effectively went for same size T22 to replace T11 - only one rad needed to go from T11 to T22 and then T33 (cos it got extended) I didn’t design the system to run at 25 to 35 deg flow temps it just happens to be the range that heating 23.5/7 with WC works (30 mins for HW daily) 

 

I have 3 column rads in the house and whilst they are also an equivilent % upgrade 2 of them are single column and they really could do with being double column rather than single because they only just cope - I think the claims for heat output are based on T50 and when run at lower temps they aren’t as effective

[S2D2]

I looked at this for my install, anything without convectors just has a lower heat output so has to be much larger. I couldn't make it work without them being silly big so had T21/T22 fitted.

 

The thing to watch out for is most of the designer radiator sites will state the output @65C without telling you they're doing so, so dont assume its @50 and make sure you know what you're actually getting at your design temp. Adam (heat geek) actually fell foul of this for the install at his own house so its easily done if you're not fact checking what the sales blurb claims.