Has anyone fitted a hybrid ASHP/solar thermal/unvented cylinder system with gas boiler back up?

[Jock Spanners]

Good Afternoon All,

This is my first time on the renewables forum so please forgive my ignorance. I've been interested in ASHPs for about 15 years but have always doubted their economic viability - especially for retrofits. Now that there will be no gas connected to new builds in 4 years I am having a rethink. I've done some online training with Vaillant. As the first tentative step I am considering doing my own house especially now that my Duo-Tec is on the way out. The house is modern, very well insulated and faces south. It has concrete floors precluding the use of UFH. I am considering installing a solar thermal panel and connecting it to a twin coil, unvented cylinder. This would be heated from a hybrid gas boiler ASHP system. Theoretically I believe this is all possible but understand that linking the AHSP to the boiler for back-up would require the boiler to input at the same temperature as the ASHP. I am also aware that the control system required will be complex. My main worry is sizing the rads and pipes as this is crucial. The heating system currently has 22mm circuits feeding 15mm spurs. Will this be enough? I could easily double the size of most of the rads but would that be enough. Any advice would be welcome.

Jock.

 

[Ric2013]

If you're in Scotland, I would advise extreme caution in the use of ASHP. So it's good to see you asking these questions and I think you are on the right track in getting as much information as you can and considering larger radiators and unconventional controls.

While the Centre for Alternative Technology in Machynlleth, Wales, does (officially) recognise them as valid, a conversation I had with Adam Tyler of the centre's engineering department a few years ago told of a more complex picture. Anyway, Mr. Tyler mentioned that the one at the centre had been measured as using more electricity than conventional resistive heating. The reason for this is that the COPs for the machine provided by the manufacturer did not reflect real-world (or Welsh, anyway) temperatures and humidity. The outcome was that the evaporator tended to freeze over externally and the unit then powered up a defrost coil to (protect the unit and?) restore fuctionality. Mr. Tyler was not generally a fan of heat pumps as they were then being massively over-sold (think double-glazing in the 1980s) but admitted thay could work well in specific conditions.

Ringing the CAT advice line for a chat may prove useful, and the chat would be free. It's part of the service the charity provides. CAT might also suggest specialist engineers who could run some calculations for you or they may have staff who could do the same for a fee (depends on who works there these days). Possibly less of a vested interest in these avenues than product manufacturers will have.

 

[king of pipes]

I've been asked to quote to fit a air source heat pump to replace a Valliant eco Tec 30 kw heat only boiler I fitted 14 years ago, the house is a 4 bed detached property near the sea front it's been well extended and is big has 15 radiator's, I personally don't think the property is suitable for this sort of system without major work, it has cavity wall , double glazing , loft is well insulated but it's still not up to a standard for a renewable heat source especially where it is , when we had the beast from the east bad weather with a -3 wind chill for days they found it hard to raise the temperature above 20°c in the lounge, I've fitted a couple of these before but into new builds what your views on the subject chaps ?? Regards kop

 

[ShaunCorbs]

Tbh needs to be a passive house to be given any chance, most of our homes leak like anything even new build

 

[John.g]

Good Afternoon All,

This is my first time on the renewables forum so please forgive my ignorance. I've been interested in ASHPs for about 15 years but have always doubted their economic viability - especially for retrofits. Now that there will be no gas connected to new builds in 4 years I am having a rethink. I've done some online training with Vaillant. As the first tentative step I am considering doing my own house especially now that my Duo-Tec is on the way out. The house is modern, very well insulated and faces south. It has concrete floors precluding the use of UFH. I am considering installing a solar thermal panel and connecting it to a twin coil, unvented cylinder. This would be heated from a hybrid gas boiler ASHP system. Theoretically I believe this is all possible but understand that linking the AHSP to the boiler for back-up would require the boiler to input at the same temperature as the ASHP. I am also aware that the control system required will be complex. My main worry is sizing the rads and pipes as this is crucial. The heating system currently has 22mm circuits feeding 15mm spurs. Will this be enough? I could easily double the size of most of the rads but would that be enough. Any advice would be welcome.

Jock.

If shutting down overnight then I would think that one should run the rads/boiler at normal high flow temps to get the house up to temperature as quickly as possible, then switch off the gas boiler and use the HP to maintain the temperatures. It is also far cheaper to heat your cylinder with the gas boiler as HP need to run with flow temperatures as low as 35/40C to achieve a reasonable COP. I would agree that the rad areas should be increased by a factor of 2/2.5.
The screenshot below gives a idea of rad outputs with flow temperatures as low as 35C.

 

[king of pipes]

Tbh needs to be a passive house to be given any chance, most of our homes leak like anything even new build

That was my thoughts Shaun thank you 👍

 

[John.g]

The more insulation and air tightness the better obviously. My (cavity BLOCK) house requires ~ 5/6kw once up to temperature with a ambient of 5/7C, I ran my oil fired SE boiler at 40C last January (pipestat in series with the control stat) and it maintained 20C easily enough with the present (20kw) rads but I certainly wouldn't care to try to heat up a cold house in the morning from those conditions.

 

[SJB060685]

The type of system you propose is entirely possible but as you say the controls system and sequencing will be quite complicated and not something I personally have any experience with just yet.
As mentioned above your biggest friend here will be the dwellings level of insulation.
In terms of radiator sizing you really want to maximise the emitter sizes. Once you've allocated a reference room try and fit the largest emitter as possible and use this radiator as a reference to find the correction factor for the remaining radiators. This will provide nice even temperatures throughout the dwelling, whilst allowing you to run the system at as lower temperatures as possible.

 

[king of pipes]

You would be better to use low temperature fan convectors this enables you to keep the heat emmiter smaller there's nothing worse than a triple panel radiator, from a installers view big heavy and ugly and nigh on impossible to fix safely to some walls, these have more than doubled in price recently the price of materials is going up every week ☹️. Kop

 

[SJB060685]

You would be better to use low temperature fan convectors this enables you to keep the heat emmiter smaller there's nothing worse than a triple panel radiator, from a installers view big heavy and ugly and nigh on impossible to fix safely to some walls, these have more than doubled in price recently the price of materials is going up every week ☹️. Kop

Fan convectors are a good option yes but they're not without their cons.
. They're quite bulky.
. They require an electrical supply.
. They can be noisy.
. They releases higher amounts of conduction heat, rather than radiant and can increase allergens.