Low flow temperature radiators

waqasahmed

If you buy these, for gas boilers, could you then switch them to electric quite easily down the line?

Right now, the house I'm buying, has a brand new gas boiler largely because their old one broke. Doesn't then seem worth doing much until 2035, but if I can slowly switch over by having better radiators for heat pumps that'd be decent 

Reed_Richards

Please understand that there isn't really any such thing as a low temperature radiator, it's just a radiator with a much larger surface area than the one you would need at high temperatures.  The consequence of using such a radiator with an ordinary boiler is that the room would warm up very quickly; you would probably need a Thermostatic Radiator Valve to get the temperature you want and to stop the room getting too hot.  It would not work with a room thermostat unless you had a similar low temperature radiator in every room in the house.

waqasahmed

Reed_Richards said:

Please understand that there isn't really any such thing as a low temperature radiator, it's just a radiator with a much larger surface area than the one you would need at high temperatures.  The consequence of using such a radiator with an ordinary boiler is that the room would warm up very quickly; you would probably need a Thermostatic Radiator Valve to get the temperature you want and to stop the room getting too hot.  It would not work with a room thermostat unless you had a similar low temperature radiator in every room in the house.

Ahhh thanks for that. Say I replaced all radiators with a large surface area one (gas), would that still work if I used electric heating? 

Reed_Richards

Yes.

Most radiator outputs are specified for Delta T = 50 C.  For a heat pump you would want to use a Delta T of 27 C or less.  So if you want a radiator output of (say) 500 W with your heat pump, you would look for a radiator with a specified output around 1250 W.  That is x 2.4 to x 2.5 what your requirement is.

matelodave

This explains DeltaT and has a conversion chart  in 5degree steps and a calculator  which shows how the radiator heat output varies with DeltaT.

https://www.clyderadiators.co.uk/delta-t-conversion?msclkid=c2ea5b55bd5411ec9f9c9a0557810170

This shows it as a graph. with the standard Delta T of 50 degrees (ie  average rad temp of 70 and room temp of 20)  equals 1 kw

With a lower flow temp of say 40 degrees (DeltaT = 40-20 = 20 then the rad will only deliver 400 watts. As said above you need to adjust the radiator spec to get the desired heat output at your proposed DeltaT

As a rough guide if you reduce your Delta T from 50 to 30 degrees you'll need to specify  a radiator with double the output to get the same amount of heat from it.

1961Nick

There are heat pumps under development with a flow temperature in excess of 60C & possibly closer to 70C. By the time your new gas boiler needs replacing, you probably won’t need to fit higher output panels.

Reed_Richards

“Ye cannae change the laws of physics”.  It may well be possible to develop heat pumps capable of higher flow temperatures but they will be less efficient and so more costly to run than heat pumps that operate at lower flow temperatures.

matelodave

1961Nick said:

There are heat pumps under development with a flow temperature in excess of 60C & possibly closer to 70C. By the time your new gas boiler needs replacing, you probably won’t need to fit higher output panels.

Have a shufti at this - https://www.theecoexperts.co.uk/air-source-heat-pumps/high-temperature#:~:text=What%20is%20a%20high%20temperature%20heat%20pump%3F%20A,the%20same%20speed%20%E2%80%93%20as%20a%20gas%20boiler.

I know Daikin have been making HT heatpumps for at least the last 12 years as I investigated them when we installed our low temperature unit. They were more expensive, less efficient and according to my installer, less reliable.

That's probably changed a bit in the past dozen years but as RR says, you cant change the laws of physics so you will get a lower COP than with a low temperature unit and they are still probably still more expensive as well. Even running an LT unit at its maximum output will get you a significantly lower COP than running it optimally

As in all these things you have to do some very careful sums to work out what is or isn't worth doing and paying a bit extra for a larger radiator (or one with more fins on the back) may well be more cost effective than paying extra for an HT heatpump that cost more to run than an LT one.

1961Nick

Even with a COP of 3, a HT heat pump would be at parity with the current cost of gas heating. A TOU tariff could make it cheaper to run than a gas boiler?

A modulating HT heat pump with weather compensation & a sophisticated control system would achieve a COP in excess of 4 most of the time.

Regarding cost, the domestic ASHP boiler is still very much a cottage industry & has a long way to go to see the benefits of scale. Achieving a flow temperature capability of 60C, would enable it to be plumbed into an existing wet system without any costly property modifications.

matelodave

If gas is around 8p/kwh and leccy around 28p then you need a COP of at least 3.5 to break even on running costs and, unless you heat your house overnight when leccy is cheaper, the higher cost of a TOU tariff during the day and evening  could cost even more to run.

I'm not saying that it couldn't work but at the moment gas is still quite a lot cheaper and more flexible if that's what  you've already got rather than pulling it out and expecting to save money with a heatpump.

As it's been said many a time and often - saving energy is not the same as saving money, although a heatpump is energy efficient it uses energy that costs 3-4 times as much as less efficient types of heating.

My heatpump is significantly less efficient when it's freezing cold outside than when it's moderately cold (say 7-8 degrres rather than -5). It still works and it still heats and we are still warm and toasty but the cost of running it increases dramatically the colder it gets.

1961Nick

matelodave said:

If gas is around 8p/kwh and leccy around 28p then you need a COP of at least 3.5 to break even on running costs and, unless you heat your house overnight when leccy is cheaper, the higher cost of a TOU tariff during the day and evening  could cost even more to run....

The break even is probably closer to 3 when you factor in the efficiency of a gas boiler of 85%. I know they can get into the 90's but that's probably the exception rather than the rule.

Once a HT heat pump had achieved the set point & any DHW needs, it would start to modulate & the COP would improve to over 4.

Smart controls could make use of tariffs like Octopus agile to ensure consumption is kept to a minimum during the surge pricing periods.