Heat demand too low for Vaillant Heat pump?

[SteamyTea]

2 hours ago, Ewan said:

He sounds quite knowledgable (to my untrained ear).

A same one has to listen to him.

And he talked about Thermal Mass.

 

 

[JamesPa]

2 hours ago, Ewan said:

One installer is suggesting we go direct with no buffer or volumiser, have the UFH & rads on one open circuit, but size the rads down slightly to avoid overheating bedrooms (at my request) and balance the whole system.

I honestly don't know, suggest you interrogate your installer and see if his/her answers are credible, or see what others here have to say.  The possible problems I can think of are:

 

UFH is commonly designed for a flow temp of 35C.  Radiators are commonly designed for a flow temp of 45C or above (historically around 70C but ASHPs aren't efficient at that temp, which is why they 'need bigger radiators'). 

 

Of course you can run radiators at 35C, but they would be mighty big.  That might just work well with undersizing the bedrooms, you'd need to do the calcs.

 

Im not sure if there would be any problem with balancing between the UFH and the radiators.  Your installer seems to be suggesting 'not' - and he might be right.

 

Unlike some on this forum I'm not an installer, just an amateur with a university degree in physics (which is good for understanding this stuff!), a general interest in how almost anything works and a specific interest in renewable technologies both at work and at home.  I am also a great fan of Occam's razor which is one reason I always ask 'why' when anybody proposes adding to a system a component for which there is no obvious function (buffer tanks in a suitably large system where no mixing is required being the case in point).

Edited January 18, 2023 by JamesPa

[Ewan]

 

4 minutes ago, JamesPa said:

I honestly don't know, suggest you interrogate your installer and see if his/her answers are credible, or see what others here have to say.  The possible problems I can think of are:

 

UFH is commonly designed for a flow temp of 35C.  Radiators are commonly designed for a flow temp of 45C or above (historically around 70C but ASHPs aren't efficient at that temp, which is why they 'need bigger radiators'). 

 

 

Ah I see where you were coming from now. According to heat punk and other heat loss calcs, we should be able to run everything at 35ºc flow without radiators being particularly large, IF we install the EWI.

 

Balancing UFH with rads would be the key then. We'd have plenty of volume then.

Thanks for all the input. I appreciate the disclaimer and won't be making any decisions based solely off your remarks.

 

 

27 minutes ago, SteamyTea said:

A same one has to listen to him.

And he talked about Thermal Mass.

 

 

 

 

Lost me there.

[ReedRichards]

3 hours ago, Ewan said:

One installer is suggesting we go direct with no buffer or volumiser, have the UFH & rads on one open circuit, but size the rads down slightly to avoid overheating bedrooms (at my request) and balance the whole system.

Does that sound achievable / sensible?

 

I don't know if it is achievable but I strongly suspect that the balancing would take longer than any installer would wish to spend or that you would wish to pay for.  So you might well end up doing that yourself.

[JamesPa]

1 hour ago, Ewan said:

 

 

Ah I see where you were coming from now. According to heat punk and other heat loss calcs, we should be able to run everything at 35ºc flow without radiators being particularly large, IF we install the EWI.

 

 

EWI =  external wall Insulation I presume.  If so then that makes sense, if the house is very well insulated then you don't need much heat input so the radiators aren't so big.  It sounds like you may have happened across a pragmatic and thoughtful installer who doesn't insist on putting in every component under the sun as an insurance policy ... or of course a complete chancer!  

[Ewan]

On 18/01/2023 at 14:51, JamesPa said:

EWI =  external wall Insulation I presume.  If so then that makes sense, if the house is very well insulated then you don't need much heat input so the radiators aren't so big.  It sounds like you may have happened across a pragmatic and thoughtful installer who doesn't insist on putting in every component under the sun as an insurance policy ... or of course a complete chancer!  

 

Seems to be the approach favoured by Heat Geeks, contacted the nearest "Heat Geek Elite".

 

On 18/01/2023 at 13:47, ReedRichards said:

 

I don't know if it is achievable but I strongly suspect that the balancing would take longer than any installer would wish to spend or that you would wish to pay for.  So you might well end up doing that yourself.

With some instruction, we can probably do that. I think it will take a long time with UFH in the slab, long heat up and cool down times.

Hopefully not impracticably long..
 

[DanDee]

On 23/12/2022 at 17:14, Ewan said:

In the absence of any specs from Vaillant, I've tried to calculate Vaillant 5kW (and 3kW) heatpumps minimum output. Looks like they can only modulate down to 2.3kW:

400l/hr ➗3600 = 0.111 L/sec x (4.2 SCH x 5 DT) = 2.33kw - is this accurate?

 

Our calculated heat demand for our retrofit (best case) is 3000w (20w/m2 * 150m2) at -2.5º. Suspect it will be somewhat higher but not sure how much.

 

This means that at anything between 10º - 15º outside temp our heat demand could be between 1500w - 1000w.

We don't want a buffer tank, not enough space.

Is this mismatch in minimum output vs heat demand going to be a problem? Some cycling is to be expected, but would this be excessive?

https://www.vaillant.cz/downloads/projek-n-podklady/kl-06-e2-verze-01-18012023-2564719.pdf

 

Capacity tables from page 34

[ringi]

Remember with large thermal mass UFH the thermal mass behaves as a buffer so it as if the UFH contained a lot more water then it does.   So cycling of heatpumps that have directly connected always open UFH will be less of an issue then with radiators.

 

Personally I would use fan converter radiators upstairs rather then having a mixer/pump to reduce UFH flow temperature so the complete system runs at the same flow temperature with a single pump.

 

Use the flow temperature to control the heat output from the UFH, with balancing flow rates to control individual rooms and the thermostat controled fans on upstairs fan convectors to control bedroom temperature.

Edited October 24 by ringi

[JohnMo]

8 hours ago, ringi said:

Personally I would use fan converter radiators upstairs rather then having a mixer/pump to reduce UFH flow temperature so the complete system runs at the same flow temperature with a single pump.

 

Use the flow temperature to control the heat output from the UFH, with balancing flow rates to control individual rooms and the thermostat controled fans on upstairs fan convectors to control bedroom temperature.

An example of how it looks, we have a fan coil in a heated summer house, with very different heating and heat loss profile to the house. Flow one flow temp, no mixers. This is overnight, orange line is the supply temp from the heat pump. The other two lines are temperature in the house and summer house. Outside temp dropped to 5 degs. The spike in the orange is starting to do DHW.