Feasibility of DHW only mini ASHP with switchable air path.

[diarmidR]

Hi all,

 

We are planning a new-build that will approach passivhaus standards, and I feel that going for the typical ASHP+cylinder+UFH is engineering and financial overkill, especially since we like to have the house around 18-19C and cooler at night.

 

Instead I would like to have minimal electric radiators (or even just electric blankets) and something like the Mixergy iHP* to provide hot water at COP > 1. I would like to set it up so that it has two inlet/outlet paths according to external weather:

• If solar gain > heat loss: extract from top of vaulted glazed area and return to bottom to provide cooling
• If solar gain < heat loss: extract and return from/to external environment

 

The second case is to avoid having to run the radiators to provide hot water on days with no sun, which tend to be wet and not sub-zero where we are building.

 

Does this sound feasible? We will be getting an MVHR, but I was hoping that by having only internal/internal or external/external config we would avoid interfering with it's operation.

Are there any products out there that would allow a compact solution for changing the flow path? It doesn't have to be automatic or anything, but I want to avoid a mess of flexible tubing.

 

Cheers,

 

Diarmid 

 

* other products are available, I just like the look of the warranty and modularity of this one. 

 

[Gone West]

I designed and built a PH that was heated by an electric towel rail in each of the three bathrooms, 4 m2 electric UFH in the kitchen. We also had a Genvex Combi 185 which had an EASHP that heated the DHW and provided warm air through the integrated MVHR ducting if required. We kept the whole house at 23C without a wet heating system.

[ProDave]

Don't re invent the wheel.  Look at Exhaust air Heat Pumps that are usually built and packaged with a hot water system to do exactly what you describe.

 

Where in the UK are you?  I am skeptical that any UK house can really run all year with no heating,

[diarmidR]

Thanks both.

 

My issue with the exhaust air heat pump is that I've read elsewhere on this forum that the optimal air flow for the MVHR heat exchanger is too low to provide a good flow of heat, and alsowe wouldn't get to use the excess heat in the summer. Also, ProDave you commented on October 2nd on a different thread that you don't like the all-eggs-in-one-basket aspect of an EAHP.

 

ProDave, I don't know if the comment about heating was directed to me or Gone West, but I'm not expecting to do without heating, just that it will be a small enough amount for resistance heating to make sense. We're in the highlands like you!

 

[JohnMo]

I am in the same mind set as @ProDave skeptical about claims we have no heat and we only xyz heaters, i.e. they have heating.

 

Back to topic. Not sure blowing damp air around the house is the best thing, as the cold air you propose to use is likely to be below its dew point being sub cooled.

 

KISS, UVC for hot water, UFH for heat, reversible ASHP to drive it. 5 to 6kW will do most house sizes, have a 100mm screed heat during off peak, cool in summer either off peak or via solar PV. You will need MVHR under Scottish regs anyway.

[diarmidR]

JohnMo, that's a good point about the dew point. I'm guessing there must be a water collection pot somewhere in the mini HP unit though? Will check this! 

 

And again, I'm not saying I wouldn't have heating, just that I estimate the std. ASHP+UFH would cost me at least £4k more (not including the £9k from the generous taxpayer) and that would take a long time to recover via lower running costs compared to electric rads. Might still go for the UFH though, and it's exactly because it's the well-trodden path.

 

[JohnMo]

9 minutes ago, diarmidR said:

I'm not saying I wouldn't have heating, just that I estimate the std. ASHP+UFH would cost me at least £4k more (not including the £9k from the generous taxpayer) and that would take a long time to recover via lower running costs compared to electric rads

All depends how you buy really. £4k would get you an ASHP with cylinder and UFH. Without a grant, if you want.

 

You could do most/all your heating off peak, you would struggle to do that with electric heaters. Off peak nearly half the price of full cost ASHP will have SCoP of 4, so 4x2, 8x cheaper than direct electric heaters to run.

[diarmidR]

Cheers JohnMo, how would I go about getting a system for £4k?

 

I take your point about the off-peak heating, I hadn't thought about that.

[JohnMo]

City plumbing - Ideal heat pump cylinder 210L £805 (includes 3 way valve)

Samsung 5kW ASHP £1681 (you may need to add a controller)

 

You will need, Flexi pipes, Flexi feet, and pipes to join ASHP to cylinder and UFH £500

 

Above is about £3k (leaves you £1k for the UFH and electric towel rails in bathrooms) the prices are plus vat, but a new build can claim all that back. 

 

There are other options for ASHP, I paid £1300 delivered incl Vat.

 

Do it single zone one thermostat required £50. Passivhaus house heat demands are exempt having multiple room zones.

 

Above needs to add labour.

Edited October 27, 2023 by JohnMo
Labour

[Conor]

No reason why a small number of storage rads and a small DHW cylinder all running off-peak wouldn't do the job.

 

BUT be mindful that for mortgage valuation and resale, a house with no central heating can still be an issue for some.

 

AND you say "approach" passive house standards. Quite different from hitting them. We are the same and even exceed the min U values in most places... but would not be without our ASHP & UFH. But we did successfully heat the house in its first couple months with a couple dehumidifiers and a fan heaters. Still, can't beat the convenience of UFH that costs pennies to run.

[Beelbeebub]

2 hours ago, JohnMo said:

City plumbing - Ideal heat pump cylinder 210L £805 (includes 3 way valve)

Samsung 5kW ASHP £1681 (you may need to add a controller)

 

You will need, Flexi pipes, Flexi feet, and pipes to join ASHP to cylinder and UFH £500

 

Above is about £3k (leaves you £1k for the UFH and electric towel rails in bathrooms) the prices are plus vat, but a new build can claim all that back. 

 

There are other options for ASHP, I paid £1300 delivered incl Vat.

 

Do it single zone one thermostat required £50. Passivhaus house heat demands are exempt having multiple room zones.

 

Above needs to add labour.

I'd point out you *will* need a controller for the Samsung, and it's about £500.  Samsung seem to be a little cheeky in that their "gen6" models are cheap because a) they are superseded so I guess they are having a sale and B) they don't have a pump or controller in them. You have to supply them which adds a bit to the cost. 

 

Vaillant do (I think) the smallest air to water monoblock at 3.5kw nominal which might do. It's a bit pricer but does have the pump etc included and uses the newer r290 fluid which gets you higher temps (for legionella cycles) and possibly more importantly goes down to -25 external temp. They're supposed to be very quiet too. 

[diarmidR]

Thanks for the comments folks, we've decided to go for ASHP+UFH. Now we can start the battle to get one installed that isn't massively oversized!

[JohnMo]

Any plumber can do the UFH, hot water cylinder and the plumbing between ASHP. Our electrician did the ASHP connections. Just a matter of finding someone prepared to read a manual.

 

Government may be offering loads of cash, but off over inflated prices, where you don't what you ask for. That's why I went DIY, learnt loads on the way.

[diarmidR]

I am tempted by your approach John, would rather put a bit of DIY into an interesting job like this than something dull like taping and filling!

[Tim731]

Just to add to Gone west post

 

Genvex now have the combi 165 (Now on the UK site ) which is an upgrade on the combi 185

[Gone West]

2 hours ago, Tim731 said:

Genvex now have the combi 165 (Now on the UK site ) which is an upgrade on the combi 185

I haven't been able to find it. Have you got a link? I wonder if that means the DHW tank is now 165l rather than 185l.

[Tim731]

in my answer I should have written not on the UK site :

 

https://www.genvex.com/produkter/ventilation-varmt-vand-og-opvarmning