Air Source Cylinder to power both hot water and underfloor?

[cee]

Hello, this is my first post here so please be kind!

I have just bought a house that requires full renovation. One of the topics I've been passionate about since I was a kid is green energy, and I am really excited to see the new technologies emerging like ground source and air source as well as more established technologies improving such as insulation and solar panels.

I am considering switching my hot water to an air source cylinder and installing underfloor heating powered by the same cylinder. I estimate I need about 200l hot water a day for showers etc and about 60l of water to power about 100sqm of underfloor heating. I am really new to this so I have a couple of questions I hope you lovely people can answer.

1. is 60l about right for 100sqm of underfloor?

2. can I run both my washing water and my underfloor water off the same cylinder and not get ill from the water going around the system a bit?

3. Is this a good idea overall since it seems scarily cheap compared to other options (eg ground source pumps).

4. Is the bathroom a good place to put the cylinder, my theory being that the bathroom will get hot during showers and baths and the cylinder can pick back up that heat more efficiently than in a cold room?

Thanks in advance!!!

 

[JohnMo]

Hi there

A couple of things, your stored water temp will be around 47 degs, so 200l capacity will need to be looked at bearing that in mind.

 

Your cylinder does not power the UFH, your heat pump does, it basically supplies two temperatures via a diverter valve either to UFH or cylinder.

 

To get CoP (lowest running costs) from the heat pump it needs to be run at as lower temperature possible. Hense running two temperatures.

 

You possibly need to do quite a bit of reading in the heat pump sections on here and UFH so you do that correctly also.

[sharpener]

I think @cee is talking about the hw cylinders that have a small integral HP supplied by air ducted from/to the outside.

 

This won't IMO be suitable for supplying underfloor heating as well. You need a small conventional ASHP to supply both hot water and the UFH (at two different temperatures). With this in mind my detailed answers would be:

 

1. is 60l about right for 100sqm of underfloor? - not really the right question, you need to size the HP according to the heat loss of the heated room(s)

2. can I run both my washing water and my underfloor water off the same cylinder and not get ill from the water going around the system a bit? No, you need an indirect cylinder with a big coil suitable for HPs

3. Is this a good idea overall since it seems scarily cheap compared to other options (eg ground source pumps). No, it is scarily cheap because it is not a good technical solution.

4. Is the bathroom a good place to put the cylinder, my theory being that the bathroom will get hot during showers and baths and the cylinder can pick back up that heat more efficiently than in a cold room? If you are tempted to take the input air from inside the bathroom - even for a hot water only HP - be aware that the house will take in make-up air from outside by infiltration if no overt pathway and this will have to be heated somehow or the house will get steadily colder.

[ReedRichards]

8 hours ago, cee said:

2. can I run both my washing water and my underfloor water off the same cylinder and not get ill from the water going around the system a bit?

Whether or not you actually get ill, regulations require that all components that come into contact with potable water (which includes the hot water you will wash with) are WRAS Approved for use as such.

 

For example, I have a recirculated hot water system that takes hot water from my cylinder round a loop so that the hot taps run hot quickly.  This requires a special circulation pump that is WRAS Approved for potable water (and uses brass components in contact with the water).  I would quite like to adapt this system using a motorised valve.  Motorised valves are commonplace items in central heating systems but I have not been able to find one approved for use with potable water (at any reasonable cost).    

[SteamyTea]

Welcome.

 

As a rule, before you start to think about replacement heating and water system, of any kind, you really need to establish what your requirements are.

If you are living in the house, this is pretty easy to establish.  Monitor you usage, either by manual meter readings or getting a data logger (not that hard to make one for electricity, but harder for gas).

Then draw up your house and calculate wat difference any changes should make i.e. change a wall from U-value 0.8 down to 0.5.

 

If the roof is good, and at suitable angles, get a basic PV system up there, this will always be useful.

[DanDee]

11 hours ago, cee said:

I am considering switching my hot water to an air source cylinder and installing underfloor heating powered by the same cylinder.

Air source cylinders do not have enough power to supply the energy required for hot water and heating.

Edited April 15, 2023 by DanDee

[SteamyTea]

19 minutes ago, DanDee said:

Air source cylinders do not have enough power to supply the energy required for hot water and heating.

That all depends on the amount of stored energy, plus the power and time the ASHP is running, and the power the house needs to keep itself at the desired temperature.

Though it isn't coming to work too well with a small Ecocent.

[sharpener]

3 hours ago, ReedRichards said:

Motorised valves are commonplace items in central heating systems but I have not been able to find one approved for use with potable water (at any reasonable cost). 

Personally I would just look very carefully at the materials used but IIRC they are brass, s/steel and various polymers. The only problem might be de-zincification if you have very soft water, but you may be able to get a DZR valve more easily than a potable water one. I guess that is why the pumps are actually bronze.

[Nickfromwales]

6 hours ago, ReedRichards said:

Whether or not you actually get ill, regulations require that all components that come into contact with potable water (which includes the hot water you will wash with) are WRAS Approved for use as such.

 

For example, I have a recirculated hot water system that takes hot water from my cylinder round a loop so that the hot taps run hot quickly.  This requires a special circulation pump that is WRAS Approved for potable water (and uses brass components in contact with the water).  I would quite like to adapt this system using a motorised valve.  Motorised valves are commonplace items in central heating systems but I have not been able to find one approved for use with potable water (at any reasonable cost).    

These are largely brass units with composite 'paddles'? Odd that they don't attain WRAS.

 

Why the need for a motorised valve?

 

Change of £85 :) ?

 

EDIT: Goes up to £100 if you choose 230v / other requisite "options".

[HughF]

Some exhaust air hot water cylinders are available that will also supply a heating circuit, although they are usually only 3.5-5kW to the heating side.

 

I wouldn’t fit one though. Too new and unknown. Stick with a standard ashp in the garden and a heat pump rated UVC

[cee]

Thank you for your replies, I am going to try to answer all of them to give the context.

 

I am talking about an air source water cylinder that goes inside the home and heats the water directly.

I found this one that appears to be suitable for both DHW and UFH together: https://www.modernheat.co.uk/product/2484 (I hope links are allowed?)

I have a space in my bathroom where the old hot water cylinder is to put it.

 

I am not living in the property, and can't until hot water is installed, therefore the energy needs of the home are not known. The boiler was from 1988 and does not work. The property has single glazed windows and very little insulation, so any heating system would be insufficient right now (I sit next to a little electric heater when I am there and I struggle to get the room to 12c). We are looking to replace all that with double glazing and insulate properly.  

 

The floors I am looking to use UFH on are concrete slab with parquet on top. The parquet needs sanding and re-laying, so it will have to be taken up anyway. The amount of water needed seems important as the cylinder should be able to cope with that AND baths/showers? The area we'll underfloor heat is about 100sqm with standard height ceilings. The other rooms will have electric heaters for when they are in use (which won't be often)

 

I currently run my electric water heater (at my rented accommodation) at about 45c, turning it down to this temperature actually saved me quite a bit of money. In summer I turn it down even more. This means I don't have to add cold water to a bath or shower. I've read that UFH can be run between 45c and 65c, so this seems about right to serve both? (I could set it higher for UFH and just add some cold when washing, most people do that anyway, and I wont need UFH in summer so then the temperature is irrelevant).

 

And re solar panels, we've been told we can get 12 on the roof and 6 on the flat roof, we just need to decide if we do the extra 6 or not.

 

It seems like the general consensus was that this plan isn't a good idea. Hopefully the extra context will help.

[SteamyTea]

16 minutes ago, cee said:

Hopefully the extra context will help.

It does.

With UFH you need to have a lot of insulation below it, or you are just heating up the earth.

Don't confuse temperature with power, or energy, they are all different.

UFH really needs to work at the lowest temperature possible to deliver the power needed. 40⁰C is really the upper limit, you would not want to stand, bare footed on that for too long.

 

[JohnMo]

36 minutes ago, cee said:

I've read that UFH can be run between 45c and 65c,

It could but I am not sure you would. The cylinder you are linking to only has a thermal power of 1.2kW, so would stand zero chance of heating much more than a very smallest of heating loads.

 

The extra context say don't do it.  The concept may be ok for hot tap water but not for heating 100m2 of floor.

 

UFH needs good insulation standards below it. In the order of 100mm of PIR insulation or 150mm polystyrene or more ideally.

[sharpener]

2 hours ago, cee said:

It seems like the general consensus was that this plan isn't a good idea. Hopefully the extra context will help.

 

The killer info is this <0.663 + 1.5 (e-heater) = 2.163 kW>. So the actual HP is rated at only 663W i.e. only 22% of a standard 3kW immersion heater. Hence to get a reasonable reheat time you will have to use the 1.5kW electric element, even then it will be slower than a standard immersion. So no HP power at all left over for the UFH.

 

Without seeing the property or doing heat loss calcs the best I can suggest is that from the cheat sheet here a complete renovation might need 30 - 65 W/m^2 so 3 to 6.5 kW for 100m^2. Many would disagree with this appoach, but it gives an order-of-magnitude indication of how much heat you might need and the combined ASHP is inadequate by a factor of 5 - 10.

 

Edited April 16, 2023 by sharpener

[JohnMo]

3 minutes ago, sharpener said:

The killer info is this <0.663 + 1.5 (e-heater) = 2.163 kW>. So the actual HP is rated at only 663W

 

Heat pump is rated at 1.2kW,

 

From the specification - HP maximum absorption 0.663 + 1.5 (e-heater) = 2.163 kW

 

Heat Pump Thermal Power yield (P rated) 1.2 kW

 

Average heat pump consumption [Condition EN16147:2017 A7/W55] 0.466 kW.

[sharpener]

Ok, misinterpretation on my part. Still a long way from what is needed.

[RichardL]

14 hours ago, cee said:

the cylinder should be able to cope with that AND baths/showers?

Are you saying you want to run the same water through your UFH as in your shower?

Generally, best practice, perhaps even laws, (I am not a plumber) - you don't mix heating water with anything you wash in or especially consume.

Traditionally your heating water ends up a nasty mix of radiator corrosion and inhibitor chemicals,
For UFH while corrosion isn't a problem additives to stop blocking the pipes etc I thin are a thing,

You really don't want to bathe or shower in it.

(If I misunderstood ignore the above)

[JohnMo]

2 minutes ago, RichardL said:

Are you saying you want to run the same water through your UFH as in your shower?

He is not, the cylinder has a separate coil within it to allow you run a pressurised heating system.

[cee]

Thank you all for your replies, and apologies for the delayed response.

I have decided a few things from this thread:

1. My water cylinder should just be for heating DHW, and I am going with a Stiebel Eltron 150, which is 50 bigger than what I have now in my rented flat. The cylinder I suggested seemed too good to be true, and it was.

2. Underfloor is a great idea but I'd need to take up all the parquet, insulate my existing concrete base, put back down the parquet, which would be horribly expensive

3. Focus will be on insulating the home and triple glazed windows, with electric heaters to heat. Again using the same brand as I have now which is Blyss.

4. The extension can have underfloor and the method (water/electric) is TBC.

Thank you again for all your replies.