Greener heating systems (sense at last!)

[Deleted member 317777]

I've just remembered something I read last year. Apparently, you could easily fix the winter problem just by fitting a bigger ASHP. But then it wouldn't run efficiently in the warmer months. There is a sweet spot for efficiency of about 50% output, IIRC.

Can I see that article please ?

I’ll use my house as a quick example. It’s got a 14kw heat loss, I’ve range rated the boiler to 18kw.

When it’s cold I can’t achieve the desired room temp unless I set the flow temp to 70 degrees.

That should highlight the real issue

 

[Notch7]

When it’s cold I can’t achieve the desired room temp unless I set the flow temp to 70 degrees

You need bigger radiators

 

[MNW67]

Can I see that article please ?

I’ll use my house as a quick example. It’s got a 14kw heat loss, I’ve range rated the boiler to 18kw.

When it’s cold I can’t achieve the desired room temp unless I set the flow temp to 70 degrees.

That should highlight the real issue

I don't have a link. Sorry. It was either on the green building forum or a second forum which I can't remember. I've not looked for almost a year. The posters on both really knew their stuff. I learnt lots about the science. The main issue now with heat pumps, I was told, is not that they don't produce enough heat. It's about getting them to work more efficiently at warmer temperatures. I suppose it's a bit like a modulating boiler. But the difference in efficiency vs output is much bigger with an ASHP than a boiler.

 

[Deleted member 317777]

I don't have a link. Sorry. It was either on the green building forum or a second forum which I can't remember. I've not looked for almost a year. The posters on both really knew their stuff. I learnt lots about the science. The main issue now with heat pumps, I was told, is not that they don't produce enough heat. It's about getting them to work more efficiently at warmer temperatures. I suppose it's a bit like a modulating boiler. But the difference in efficiency vs output is much bigger.

Maximum flow temperature is the issue in older buildings.

Flow temp on a ashp is 35-55 max. That wouldn’t touch a large open plan room, regardless of the radiator size.

It’s called heat loss for that exact reason.

It sounds like you’ve read up on it all.

My mate the Mitsubishi rep, it’s having his house converted to ashp. He’ll have to run it all the time to maintain the heat, but his total heat loss is 4kw. My front room needs that much

 

[MNW67]

Maximum flow temperature is the issue in older buildings.

Flow temp on a ashp is 35-55 max. That wouldn’t touch a large open plan room, regardless of the radiator size.

It’s called heat loss for that exact reason.

It sounds like you’ve read up on it all.

My mate the Mitsubishi rep, it’s having his house converted to ashp. He’ll have to run it all the time to maintain the heat, but his total heat loss is 4kw. My front room needs that much

ASHP efficiency is really complicated. I got my head around some of it last year but I have forgotten most of it. There is a sweet spot based on compressor speed where about 50% is best. And then there is efficiency based on the water flow temperature where lower is better. Before going on the forums I had it all arse about face.

Presumably it must be possible, in theory, to heat any room at a flow temperature of say 45C, as long as the radiators are big enough? They might have to be absolutely massive! But it would be possible?

 

[Notch7]

Think you facts to back up that claim

Why haven’t you asked Nwgs for facts to back up his claim?

Anyway it’s perfectly possible to use a DOT (design outside temp) of -5deg for a heat pump in many U.K. homes.

Heat pumps can work well even in relatively poorly insulated homes.

At the moment the Kw cost of electricity is much higher than gas, which is why heat pumps arent cost effective in many cases…..although solar + battery can tip the balance. But then the problem is capital investment and payback period.


Heat pumps can work out well for those people on heating oil

 

[Notch7]

ASHP efficiency is really complicated. I got my head around some of it last year but I have forgotten most of it. There is a sweet spot based on compressor speed where about 50% is best. And then there is efficiency based on the water flow temperature where lower is better. Before going on the forums I had it all arse about face.

Presumably it must be possible, in theory, to heat any room at a flow temperature of say 45C, as long as the radiators are big enough? They might have to be absolutely massive! But it would be possible?

The key factors as I understand it are:

Heat loss from the house
Radiator size
Volume of water in the heating system.

 

[Deleted member 265726]

I'm happy sticking with my gas central heating and boiler.

 

[MNW67]

I'm happy sticking with my gas central heating and boiler.

Me too!!

 

[Notch7]

I think it’s stupid that govt has been pushing heat pumps for the last few years.

The massive failure I think is the fact condensing boilers became mandatory for all new installations since 2005, but probably 90% + of those installations hardly condense at all. So what was the point of the regs?

The govt should’ve given grants to convert systems so they operate at lower flow temps.

Maybe grants could’ve been given to set up training colleges to train heating engineers and incentivise manufacturers to make boilers to work properly for weather comp, hot water priority etc.

 

[MNW67]

I think it’s stupid that govt has been pushing heat pumps for the last few years.

The massive failure I think is the fact condensing boilers became mandatory for all new installations since 2005, but probably 90% + of those installations hardly condense at all. So what was the point of the regs?

The govt should’ve given grants to convert systems so they operate at lower flow temps.

Maybe grants could’ve been given to set up training colleges to train heating engineers and incentivise manufacturers to make boilers to work properly for weather comp, hot water priority etc.

Most people will only ever save a couple of percent from condensing. The maximum theoretical saving is 11%, but you probably need to run the boiler at 25C.

 

[Deleted member 317777]

ASHP efficiency is really complicated. I got my head around some of it last year but I have forgotten most of it. There is a sweet spot based on compressor speed where about 50% is best. And then there is efficiency based on the water flow temperature where lower is better. Before going on the forums I had it all arse about face.

Presumably it must be possible, in theory, to heat any room at a flow temperature of say 45C, as long as the radiators are big enough? They might have to be absolutely massive! But it would be possible?

How many heat pumps would you need though ?

That defeats the purpose really.

 

[Deleted member 317777]

I'm happy sticking with my gas central heating and boiler.

Me too, mines set up to run low and slow.

It’s got imi valves to keep the system balanced, weather comp and open therm, but I’m led to believe it’s better to just have OT.

It’s mainly got a flow temperature of 60.

I’d need 2 heat pumps, probably 3 to keep it warm.

 

[Deleted member 317777]

Most people will only ever save a couple of percent from condensing. The maximum theoretical saving is 11%, but you probably need to run the boiler at 25C.

It is 2% free heat at optimal temperature.

Most people have their flow temp to high to condense.

Temperature set comes into it.

 

[MNW67]

How many heat pumps would you need though ?

That defeats the purpose really.

I can't remember. I found it all very hard, and they weren't the friendliest bunch for a newbie. But I don't think the heat output drops as much as you might think with outside temperature. It's more that it produces the same heat but less efficiently. That was what I had got wrong before I started asking questions last year. I think a single 18kW ASHP might be enough in your example. The key would be having massive radiators.