New member with an ASHP question

[KBW]

Hi

I really appreciate all the advice and information on this forum so have finally registered with a question of my own.

I have bought a house with no heating. We are going to install an ASHP behind an outbuilding and have already put in a heat main (twin 50mm pipes) from the outbuilding to the house, a distance of about 20m. What size pipe should we step this down to for routing through the house to the DHW cylinder and UFH gubbins? Is 28mm big enough?

Thanks

[epsilonGreedy]

There is a sweet spot for ASHP happiness and your situation might be a long way from that point.

 

ASHPs work best in highly insulated new houses with under floor heating, a larger than average hot water tank helps. Does your un heated house tick these boxes?

[joe90]

22 minutes ago, KBW said:

I have bought a house with no heating.

You really need to do a “heat requirement “ calculation. Part of this calculation is the insulation level and airtightness. Do you have access to mains gas as this might be a more realistic running price wise for a “standard” house.

[PeterW]

42 minutes ago, KBW said:

Hi

I really appreciate all the advice and information on this forum so have finally registered with a question of my own.

I have bought a house with no heating. We are going to install an ASHP behind an outbuilding and have already put in a heat main (twin 50mm pipes) from the outbuilding to the house, a distance of about 20m. What size pipe should we step this down to for routing through the house to the DHW cylinder and UFH gubbins? Is 28mm big enough?

Thanks

OK so let’s start from the beginning. 
 

How big is the house..?

How old / insulated / windows (DG..?)

How big is the heat pump..?

How big is the hot water tank ..?

 

need to understand all these to start with, and then you can start the sizing conversation. 
 

One thing I would say is twin 50mm is very large bore and you may struggle with flow rates with standard pumps - this could cause problems. If the 50mm is already in and buried you may find it being used as a very expensive conduit for a smaller pipe. 

[KBW]

Hi.

Thanks for your replies.

The house is 2 storey brick with cavity wall insulation. We have demolished the old single storey kitchen and bathroom which leaves a building with footprint of approx 55sq m. It has old double glazing which we plan to replace. We are building an open plan SIP extension - 2 storey with footprint of approx 80sqm. This is block and beam floor and will have UFH throughout. The size of the heat pump is TBC but we are planning on a unvented DHW tank of minimum 300ltrs. We were advised to put in the large bore heat main because of the siting of the ASHP away from the house. Installing mains gas is not an option due to distance from the nearest main.

[SteamyTea]

Do you know the exterior wall area, the floor and roof area, windows and door areas, then we can make a stab at the heat loss.

55m² is just slightly larger than my place and I wish I could add an 80m² extension.

Do you know what level of insulation you are going to get with the new B&B floor?  This is important to know with UFH.

[KBW]

I hoped our SAP report would have arrived to answer these questions but still waiting. We will have 100mm K103 Kingspan insulation on the block and beam.

[jack]

Welcome.

 

50mm is bigger than you need, but the other issue is insulation. 50mm pipe has nearly twice the surface area of a 28mm pipe. Water will also be moving at a third the speed, so more time for it to lose heat. Have you insulated the pipe well?

[SteamyTea]

2 minutes ago, jack said:

50mm pipe has nearly twice the surface area of a 28mm pipe

Or 3.188776 times.

[scottishjohn]

On 03/02/2021 at 22:27, PeterW said:

One thing I would say is twin 50mm is very large bore and you may struggle with flow rates with standard pumps - this could cause problems. If the 50mm is already in and buried you may find it being used as a very expensive conduit for a smaller pipe. 

and for sure it needs to very well insulated  

like this 

https://www.uponor.co.uk/products/heat-and-water-distribution/preinsulated-pipe-for-heat-networks

 over £100 per metre for right stuff

Edited February 11, 2021 by scottishjohn

[jack]

8 minutes ago, SteamyTea said:

Or 3.188776 times.

 

You sure?

[SteamyTea]

2 minutes ago, jack said:

 

You sure?

No

 

[jack]

You're calculating cross sectional area, but the surface area of a pipe is circumference x length. 

[TonyT]

Is the 50mm not a duct to pass the 28mm through later?

[jack]

3 minutes ago, TonyT said:

Is the 50mm not a duct to pass the 28mm through later?

 

I initially read it as that, but talk of stepping it down suggests it's an actual pipe.

[scottishjohn]

I trust this is what you have fitted for the piping 

https://www.underfloorstore.co.uk/product/uponor-ecoflex-thermo-twin-pre-insulated-pipe

 and buried it very deep 

Edited February 11, 2021 by scottishjohn

[Bramco]

On 03/02/2021 at 21:41, KBW said:

We are going to install an ASHP behind an outbuilding and have already put in a heat main (twin 50mm pipes) from the outbuilding to the house, a distance of about 20m

 

We looked at installing the ASHP away from the plant room but after some thought decided not to do it.  If you intend to use the heat pump in reverse for cooling in the summer, then you'd have a bit of a problem. Assuming it's a monobloc ASHP that is.  If you are cooling then the pipes from the heat pump to the UFH manifold will be full of cool water. When you switch to wanting to heat the DHW, all that cool water will circulate through the heat exchanger in your DHW tank before any hot water comes through to heat the tank. At least AFAIK!

 

If you have 20M of 50mm pipe that's a LOT of cooling of the DHW tank that's going to happen before it starts heating.

 

I know from experience that using fatter pipe seems like a great idea in pressure terms but can cause issues. When we built our extension, I put a 22mm pipe from the new boiler room to the kitchen sink - it takes twice as long as it should to get the hot water in the sink and of course there's quite a lot of wasted heat after you've turned the tap off.

 

Simon

[scottishjohn]

On 03/02/2021 at 22:27, PeterW said:

One thing I would say is twin 50mm is very large bore and you may struggle with flow rates with standard pumps - this could cause problems. If the 50mm is already in and buried you may find it being used as a very expensive conduit for a smaller pipe. 

if he steps it down at house end ,then normal pump will run it -as the flow rate will be what the smaller pipe can flow --I think you will  have more heat loss in the buried though due it holding more volume 

 could be a good idea to put smaller size one up the big one  if you feel it is a problem 

Edited February 11, 2021 by scottishjohn

[SteamyTea]

37 minutes ago, jack said:

You're calculating cross sectional area, but the surface area of a pipe is circumference x length. 

Ah right  see what you mean.

57 minutes ago, jack said:

Water will also be moving at a third the speed, so more time for it to lose heat.

This confused me as the flow speed was near enough correct.

[jack]

7 minutes ago, SteamyTea said:

Ah right  see what you mean.

 

It's kWh.

Oh, wrong thread.

[SteamyTea]

2 minutes ago, jack said:

 

It's kWh.

Oh, wrong thread.

It is, but there is total surface area and area of just the ends.

I seem to remember that there is a size where the losses can be greater if you add insulation because the tube has a larger area than the uninsulated pipe.

[Pocster]

4 hours ago, SteamyTea said:

It is, but there is total surface area and area of just the ends.

I seem to remember that there is a size where the losses can be greater if you add insulation because the tube has a larger area than the uninsulated pipe.

Look Mum !

 

Look what I can see !

 

It’s a bored man ?

[A_L]

@SteamyTea , @jack , @pocster et al, Steamy is however correct, the insulation of a cylinder (pipe) is subject to the 'Critical Radius of Insulation' below which insulation actually increases heat loss. e.g.

 

https://en.wikipedia.org/wiki/Thermal_insulation - 'look  for insulation of cylinders' about 1/3rd down page

[Pocster]

15 minutes ago, A_L said:

@SteamyTea , @jack , @pocster et al, Steamy is however correct, the insulation of a cylinder (pipe) is subject to the 'Critical Radius of Insulation' below which insulation actually increases heat loss. e.g.

 

https://en.wikipedia.org/wiki/Thermal_insulation - 'look  for insulation of cylinders' about 1/3rd down page

He's still a boring (expletive deleted) wit

[SteamyTea]

2 hours ago, pocster said:

He's still a boring (expletive deleted) wit

Is that a euphemism again.