Hello,
I'm a newbie member but long time browser of the forum. I have a solution to our DHW needs with an air to air heat pump that I'd appreciated some hive mind input on.
We are retrofitting a 1960's bungalow and the PHPP is predicting a heating load of just 2kw. Because of this, we're thinking a mini-split air to air heat pump would be a good choice, supplemented by perhaps a towel rail or direct electrical heating if and when needed. I looked at the Daikin multi plus, but the output is way more than we'll need for heating and hot water. I'm reckoning on an extra 500W load for the hot water as there are just two of us and we're not big users of hot water.
The system that looks a better pairing with our air to air heat pump is a hot water heat pump where the heat pump is integrated on top of the water cylinder and the whole thing lives inside. Something like the Tesy AquaThermica Compact or the Dimplex Edel model.
Initially I was going to connect the inlet and outlet to the outside air and then uncouple the pipes in summer to get some useful cooling as the heat pump ran on internal air. However, I'm now wondering whether to run it on internal air all year round. This is a common set up in the States it seems where the units are often located in basements. However ours would be in a study right inside the thermal envelope. Obviously it will be sucking warm air in from the house and releasing cool air so it may seem a bit mad. But here's why I think it could work:
1. It could piggy back on the very efficient air to air heat pump heat - we'd just run that unit longer / harder
2. Doing it this way avoids two holes in the wall and the heat losses associated with that
3. We could charge up the DHW over night on a cheap tariff which would mean that excess air movement created by the HWHP wouldn't be bothersome.
4, As the ingoing air would be relatively warm, the HWHP shouldn't need too much energy to move the heat.
I haven't been able to find anyone who's done something like this. Does anyone here know of someone trying this? Or is there something I'm missing and this would be a mad thing to do and I should just vent to the external air.
For context, we are doing a whole house retrofit in the coming months. Location is south coast of England. We will have a central MVHR system. Floor area around 100m2 with a loft extension but only 74m2 for PHPP because of loft eaves. Battery and PV to be installed.

Thanks for the response, djh.

I'll be sure to model the set up as far as I can before going ahead, but good to know someone else has been mulling this. Noise is certainly something to consider, but in our situation shouldn't be an issue.

We are close to EnerPHit but are running out of cost effective ways to hit it so may just be pragmatic and let that one go.

If it looks like a goer on paper, I'd be tempted to try this configuration. At worst we could put in external ducting later if it's not working out.

I'll share where we go with this one and how it works out if we do go ahead.

Hi Peter in Hungary,

Thanks for your suggestion. I have considered an immersion heater and I think in our situation, even with higher running costs, an immersion tank on an overnight tariff might well be a cheaper solution and the simplicity IS appealing. However, we've got enough budget to try to find a lower energy solution, so I'm tempted by the HWHP. It does have a back up immersion heater so if the heat pump fails or is too noisy or drafty we can revert to using that. The Tesy isn't madly expensive (£999 on sale) though I expect installation will be more pricey than a standard immersion tank. I've found a plumber willing to install and should get a quote from them next week.

Posted By: PeterStarckThe problem I would have with the Tesy is that it has an anode which would need replacing at intervals depending on the water supply.

That sounds like a feature rather than a problem. If you feed hard water into plumbing, you have to deal with it somehow.

Our mains pressure immersion heated DHW tanks have sacrificial anodes (about 1" dia and 12" long) and with continual daily use of the tank they last about 5 years. Of course YMMV as local conditions may change the life span.

Posted By: WillInAberdeenWould be interested whether a cheaper steel cylinder is UK compliant, and if so then why aren't there lots more cheap imports available at merchants?

Over here a 120 ltr mains pressure DHW immersion tank is about 280 GBP (that’s the most common size). A 200 ltr tank is about 460 GBP and a 10 year guarantee is usual. There is no mandatory testing required and replacements if needed is often done DIY as is any maintenance due to limescale build up or anode replacement.

I think ordinary steel has been tried in the UK and found wanting. Rust and other corrosion, too heavy, various other failure modes and maintenance of anodes etc. Even for enamelled steel. Stainless OTOH has taken over for cylinders. Or so a quick dive tells me.

My guess would be there are no large enameling plants/kilns in the UK. by comparison, copper and SS can fairly easily be fabricated.

Posted By: WillInAberdeenOnly reason I can think of, is that we have some regulation that they aren't allowed here for some reason. But I can't find it anywhere I looked.

There's a document titled WRAS Material Guidance that might have some relevance. Para 7.6 says:

"Ceramic & vitreous enamel based products

"Any of these products which do not contain organic ingredients, either because they are
not present in the formulation or would have be lost due to firing or sintering should be
tested for conformity with the requirements of clause 8 of BS 6920-1:2000/2014 in
accordance with BS 6920-2.6 (extraction of metals). No other tests are required."

I have no idea what the BS say, or whether the para is actually relevant?

There's an interesting page at https://www.flexiheatuk.com/domestic-hot-water-storage-tank/ by a UK company that makes both enamelled and stainless water tanks.
edit to add: there's an Australian page at https://www.energymatters.com.au/solar-hot-water/stainless-glass-tanks/ that discusses the relative merits of various types of tanks used with solar heating.

"About Us: Flexiheat UK Ltd,.. [are] the sole distributor for Kroll Energy products... The company’s head office is based 30 km northeast of Stuttgart."


BS 6920 says how to test nonmetal plumbing components to check they don't leach chemicals or odours into the water. Would apply to vitreous coatings and also to plastic pipes and push fit fittings etc.

No reason why enamel coatings shouldn't pass this test, though maybe the EU manufs can't be bothered with passing UK tests if the market is small here?