Investigating house heating methods for a new build. The building will have a very low space heating requirement so majority of heating is to produce hot water in a thermal store.

All the stuff I read about heat pumps and their COPs talks about how they work best with the lowest delta T possible (simple thermodynamics) generally providing 35degC to thermal stores for underfloor heating.

I am struggling to find out how they perform when producing domestic hot water (eg 80degrees to a thermal store – ie much larger delta T). Can anyone comment on this. Would the COP be almost 1 and therefore I’d be better off just using an induction heater?!

Should I just install a woodburning stove with a backboiler as our architect has suggested?

80 degrees for domestic hw?
Building regs recommend something like no hotter than 60 at the tap.
There are high temp heat pumps that will do 60 at a push, but they probably won’t qualify for rhi.

You’ll get a more geeky reply on here: http://www.navitron.org.uk/forum/

80degC?
A heat pump with modern refrigerant wouldnt even reach it, the COP is neither here nor there. I vaguely recall there being a cascade system which may be able to do it.

Daikin do cascade heat pump that they say can get to 80ºC. And they are one of the best makes IMO. geoffj to get 60ºC out of a calorifier/water cylinder you’ll be needing flow >60ºC so 80ºC would be about right.

http://www.daikin.co.uk/domestic/needs/heating/renovate-replace-your-oil-or-gas-burner/daikin-altherma-high-temperature/index.jsp

beaten to it!

DHW will be 60degC or less as you say, it’ll be 80degC at the top of the thermal store and my question is, “how do you generate that sort of temp if your only heat source is a heat pump?”

Cheers for the input.

Do you need a thermal store?

Dobbo – not necessarily but can a heat pump heat water for a shower AND the kitchen taps at the same time?

You’ll need a buffer tank.

You will need a thermal store, as no way can a domestic heat pump produce 80C water on demand!

The heat pump with 80ºC output could heat a standard hot water cylinder. That stores hot water (for taps/shower) but it’s not a thermal store in heating terms. A thermal store goes on the primary side of the heating system, i.e. to heat the water tank and the radiators/underfloor.

Some heat pumps use an accumulator setup that does hot water, some have an electric element in them to raise hot water to 60ºC.

The attraction of the thermal store is that it can provide mains pressure hot water, (or throttled down a bit, constant pressure hot water) with no pumps etc. Plus we could plumb in a solar thermal set in the future or a wood burning stove.

How does it not need any pumping arrangement? Have I missed something?

My mate has the biggest thermal store you’ve ever seen in his dungeon cellar. Several thousand litres of slightly tepid water, kept at a bacteria and algae-friendly C40-ish with a heat pump, evac-tube solar array and back-boilerd stove providing energy in. Last winter we very nearly bought him an immersion heater as a joke Christmas present, only to learn that he’d already fitted three.

@wrecker: You use the immense reservoir of heat in the store to quickly heat up the hot water via a coil that winds its way through it. Like a combi boiler, but the heat is provided by a body of water. The cold feed in is directly from the mains so effectively you end up with hot mains water.

When I say no pumps I’m only talking about the hot water supply for the house taps. Mains pressure cold water in, through coil in thermal store, then out to taps at mains pressure. I’d expect a pump would be needed to feed water to hot water taps from a hot water cylinder (unless it’s in the loft, but you’d still need to pump it to the loft).

Early days so excuse my ignorance!

Hot Fiat – sounds like your mate has been sold a few lemons there! I believe these sorts of systems work a lot better if you have a low energy demand house (high levels of insulation and good building standards to avoid cold bridging, have high airtightness, mechanical ventilation with heat recovery system).

I presume his thermal store is constantly running cool because his heat requirements are quite large? Sounds awful!

Just sounds like a huge shell and coil HEx to me. I’m struggling to see the advantage; heat loss from the store, huge footprint and L8 risks. And it still won’t be mains pressure once it has wound its’ way through XXm of coil. I assume these are for the collection of heat from various sources (solar, biomass ect)?

I will share some experience about heat pumps though; size it so that it can deliver the maximum amount of heat you will require on the coldest day of the year AND make sure you have 100% back up.

The store I am looking at is from Finland (where this technology is quite popular). It has mega insulation so a 1200l store is massive. The advantage is that say you use a wood burning stove to heat it up to 75degC, it’ll lose only around 0.5degC per day.

Obviously if your central heating comes on it’ll cool quicker but the point is that you can gain energy from many sources: wood burner, heat pump, solar thermal, traditional boiler etc and it can be stored there for use later.

Other advantage is high pressure hot water for skin stripping showers.

Navitron website is super-geeky so thanks for that link!

geoffj to get 60ºC out of a calorifier/water cylinder you’ll be needing flow >60ºC so 80ºC would be about right.

Not IME. We have a tiny 300l store fed by oil, solid fuel boiler, solar and immersion.
Losses between top of tank and tap/shower are 5 degrees at most on the tank side of the mixer valve. 4 bed detached house. Maybe my pipes are just well lagged.

geoffj I meant to get a hot water flow to the taps of 60ºC you’ll need a higher temp maybe up to 80ºC going in to the hot water tank heating coil, in your case from the oil, solid fuel boiler, solar and immersion. I didn’t make that too clear.

1200l is pretty big.

We run a 3000 sq ft barn and family of four off 700+250 litres fed by a 20kW biomass burner/furnace or 9kWp solar thermal array.

Personally I don’t like heat pumps as quoted cops are rarely achieved and even less so after seasonal adjustment. And you rarely beat net co2 production when compared to alternative fuels once you’ve allowed for source generation.

And you’re pegged to electricity cost rather than some kind of alternative,

Dobbo – aah, yes I see what you mean

Hi Stoner

What is the issue (apart from initial cost and space required for it)?
Another line of thinking is to use a wood burning stove with back boiler to feed the thermal store.

With low heat loss in the house (therefore little space heating requirement) I’m hoping to be able to run the stove in the morning or evening and have water for showers the next day.

Surely a larger stove means you’ll be capturing as much heat from the stove as possible and won’t need to dump heat so often?

Cheers!

An oversized store will have a number of negatives

1) at 1.16 wh/Kgk of water, 10 degree mains temp and an AVERAGE water temp in the tank (allowing for stratification) of say 60 degrees (don’t forget you want to maintain a cold temp at the bottom of the tank to make the most of any solar collection), then your tank would hold around 70kWh of energy. that’s quite a lot to be trying to take out of a back boiler even before replacing consumption. Back boiler likely to be rated no more than 2-5kW.

2) 1200 litres is a lot to try and get up to a usable temp in the top layers from solar.

3) that’s a lot of inhibitor to replace anytime you need to drain down

4) draining down will take a fortnight 🙂

Edit Bear is your man in here for sensible sizing advice.

I don’t think its realistic to expect a heatpump to deliver like a gas/oil combi without supplementary energy input. Sure its possible to do it, but the running costs aren’t going to be pretty however you manage it.